YY1 interaction with the basal transcriptional apparatus

YY1 is a zinc finger transcription factor whose DNA-binding motif exhibits the properties of an initiator element. Only three factors are required to direct specific basal transcription on a supercoiled template DNA carrying the YY1 initiator: YY1, general transcription factor IIB, and RNA polymerase II. This minimal in vitro reaction does not require the TATA-binding protein (TBP). It is proposed that, under appropriate conditions, YY1 can function like TBP, as a factor that binds to the core promoter and recruits the polymerase to the initiation complex (Usheva, 1994).

Depending on promoter context, YY1 can activate or repress transcription, or provide a site for transcription initiation. Does the ability of YY1 to induce DNA bending influence its ability to activate and repress transcription? To investigate this question, simple synthetic promoters were constructed in which the YY1 binding site was inserted between the TATA box and either the (nuclear factor 1) NF1 or AP1 recognition sequences. In transient transfections of COS cells, the NF1YY1TATA and NF1RYY1TATA promoters exhibit a dramatic 15-20-fold increase in correctly initiated transcription. These promoters exhibited even larger 60-80-fold increases in transcription in HeLa cells. Neither multiple copies of the YY1 binding site alone, nor placement of a YY1 site upstream of the NF1 site activate transcription. Deletion of 4 bp between the NF1 and YY1 sites, which changes the phase of the DNA bends, abolishes the 16-fold activation of transcription by NF1YY1TATA. Insertion of the YY1 site between the AP1 site and the TATA box decreases transcription approximately 3-fold. Replacing the YY1 binding site with an intrinsic DNA bending sequence mimics this transcription repression. Sequences of similar length which do not bend DNA fail to repress AP1-mediated transcription. Gel mobility shift assays were used to show that binding of YY1 to its recognition sequence does not repress binding of AP1 to its recognition sequences. These data indicate that YY1-induced DNA bending may activate and repress transcription by changing the spatial relationships between transcription activators and components of the basal transcription apparatus (Kim, 1996).

The Ying-Yang 1 protein (YY1) DNA-binding site functions as an initiator element at which YY1, transcription factor IIB (TFIIB), and RNA polymerase II sponsor basal transcription from a supercoiled DNA template. TFIIB is shown to bind to YY1, stabilizing its interaction with DNA, and YY1 contacts the large subunit of polymerase II, directing it to the initiation site. YY1 directs initiation from linear DNA containing mismatched sequences within its binding site, leading to the inference that supercoiling facilitates the separation of DNA strands and suggesting that YY1 likely remains bound to the start site as DNA strands separate during initiation. These results provide a mechanistic basis for transcriptional initiation directed by YY1 in the absence of the TATA box-binding protein (Usheva, 1996).

YY1 interaction with DNA

YY1 is a ubiquitously expressed zinc finger DNA binding protein. It can act as a transcriptional repressor or activator and, when binding at the initiator element, as a component of the basal transcription complex. Binding sites for YY1 have been reported in a wide variety of promoters, exhibiting substantial diversity in their sequences. To better understand how YY1 interacts with DNA and to be able to predict the presence of YY1 sites in a more comprehensive fashion, YY1 binding sites have been selected from a random pool of oligonucleotides. The sites display considerable heterogeneity, but contain a conserved 5'-CAT-3' core flanked by variable regions, generating the consensus 5'-(C/g/a)(G/t)(G/t/a)CATN(T/a)(T/g/c)-3', where the upper case letters represent the preferred base. This high degree of flexibility in DNA recognition can be predicted by modeling the interaction of the four YY1 zinc fingers with DNA. A detailed model for this interaction is presented and discussed in this paper(Hyde-DeRuyscher, 1995).

Ying-Yang 1 protein (YY1) supports specific, unidirectional initiation of messenger RNA production by RNA polymerase II from two adjacent start sites in the adeno-associated virus P5 promoter, a process which is independent of the TATA box-binding protein (TBP). The 2.5-A resolution YY1-initiator element cocrystal structure reveals four zinc fingers recognizing a YY1-binding consensus sequence. Upstream of the transcription start sites protein-DNA contacts involve both strands; downstream, they are virtually restricted to the template strand, permitting access to the active center of RNA polymerase II and ensuring specificity and directionality. The observed pattern of protein-DNA contacts also explains YY1 binding to a preformed transcription bubble, and YY1 binding to a DNA/RNA hybrid analog of the P5 promoter region containing a nascent RNA transcript. A model is proposed for YY1-directed, TBP-independent transcription initiation (Houbaviy, 1996).

Autonomous replication of human chromosome fragments is stimulated by the presence of an 18 bp sequence, REE1, which exhibits transcriptional silencer activity. The REE1 sequence is partly homologous with the serum response element (SRE) required for expression of the human c- fos gene. Interaction of REE1 with human nuclear proteins has been examined using a gel retardation assay. One of the REE1-protein complexes formed shows almost the same mobility as the SRE-protein complex and complex formation is competitively inhibited by the SRE fragment. The protein complex with REE1 as well as that with SRE contains the transcription factor YY1, known to bind to the SRE. These results suggest that YY1 protein may participate in stimulation of replication through its interaction with REE1 (Obuse, 1998).

Interaction between YY1 and various transcription factors

The c-Myc oncoprotein has previously been shown to associate with transcription regulator YY1 and to inhibit its activity. Endogenous c-Myc and YY1 have been shown to associate in vivo and changes in c-Myc levels, which accompany mitogenic stimulation or differentiation of cultured cells, affect the ratio of free to c-Myc-associated YY1. The mechanism by which association with c-Myc inhibits YY1's ability to regulate transcription was investigated. c-Myc does not block binding of YY1 to DNA. However, protein association studies suggest that c-Myc interferes with the ability of YY1 to contact basal transcription proteins, TATA-binding protein and TFIIB (Shrivastava, 1996).

The human myeloid nuclear differentiation antigen, MNDA, is expressed only in myelomonocytic and a subset of B lymphoid hematopoietic cells. MNDA is uniformly distributed throughout the interphase cell nucleus and associates with chromatin, but does not bind specific DNA sequences. MNDA binds nucleolin and nucleophosmin/NPM/B23 and both of these nuclear proteins bind the ubiquitous zinc finger transcription factor YY1. Investigations of the possible effect of MNDA on the interaction between YY1 and NPM, show that MNDA bind YY1 directly under both in vitro and in vivo conditions. The MNDA-YY1 interaction enhances the affinity of YY1 for its target DNA and decreases its rate of dissociation. The N-terminal half (200 amino acids) of MNDA is sufficient for maximum enhancement of YY1 DNA binding: a portion of this sequence is responsible for binding YY1. MNDA participates in a ternary complex with YY1 and the YY1 target DNA element. The results show that MNDA affects the ability of YY1 to bind its target DNA sequence and that MNDA participates in a ternary complex, possibly acting as a cofactor to impart lineage specific features to YY1 function (Xie, 1998).

The human papillomavirus type 18 (HPV-18) upstream regulatory region (URR) controls viral gene transcription in a cell-type-specific manner. The HPV-18 URR is active in HeLa cells but inactive in HepG2 cells. The activating activity of YY1 in HeLa cells is dependent on its functional interactions with the switch region, which is critical for the HPV-18 URR activity in HeLa cells. A protein complex composed of C/EBP beta and YY1 binds the switch region, which is detected only in HeLa cells, not in HepG2 cells. Transfection of C/EBP beta into HepG2 cells restores the formation of the C/EBP beta-YY1-switch region complex, accompanied by increased transcription directed by the HPV-18 URR. Mutations in the switch region that abolish the complex formation also abrogate C/EBP beta-induced transcriptional activation. This provides a strong correlation between the binding of the C/EBP beta-YY1 complex to the switch region and cell-type-specific URR activity. Taken together, a novel C/EBP beta-YY1 complex has been identified that binds the switch region and contributes to cell-type-specific HPV-18 URR activity (Bauknecht, 1996).

The adenovirus 12S and 13S E1A proteins have been shown to relieve repression mediated by the cellular transcription factor YY1. The 13S E1A protein not only relieves repression but also activates transcription through YY1 binding sites. In this study, using a variety of in vivo and in vitro assays, it is demonstrated that both E1A proteins can bind to YY1, although the 13S E1A protein binds more efficiently than the 12S E1A protein. Two domains on the E1A proteins interact with YY1: an amino-terminal sequence (residues 15 to 35) that is present in both E1A proteins and a domain that includes at least a portion of conserved region 3 (residues 140 to 188) that is present in the 13S but not the 12S E1A protein. Two domains on YY1 interact with E1A proteins: one is contained within residues 54 to 260, and the other is contained within the carboxy-terminal domain of YY1 (residues 332 to 414). Cotransfection of a plasmid expressing carboxy-terminal amino acids 332 to 414 of YY1, fused to the GAL4 DNA-binding domain can inhibit expression from a reporter construct with GAL4 DNA binding sites in its promoter: inclusion of a third plasmid expressing E1A proteins can relieve the repression. Thus, a correlation is found between the ability of E1A to interact with the carboxy-terminal domain of YY1 and its ability to relieve repression caused by the carboxy-terminal domain of YY1. It is proposed that E1A proteins normally relieve YY1-mediated transcriptional repression by binding directly to the cellular transcription factor (Lewis, 1995).

Polycomb group (PcG) proteins form multimeric protein complexes that are involved in the heritable stable repression of genes. Two distinct human PcG protein complexes have been identified. The EED-EZH protein complex contains the EED and EZH2 PcG proteins, and the HPC-HPH PcG complex contains the HPC, HPH, BMI1, and RING1 PcG proteins. YY1, a homolog of the Drosophila PcG protein Pleiohomeotic (Pho), interacts specificially with the human PcG protein EED (Drosophila homolog: Extra sexcombs) but not with proteins of the HPC-HPH PcG complex. Since YY1 and Pho are DNA-binding proteins, the interaction between YY1 and EED provides a direct link between the chromatin-associated EED-EZH PcG complex and the DNA of target genes. To study the functional significance of the interaction, the Xenopus homologs of EED and YY1 were expressed in Xenopus embryos. Both Xeed and XYY1 induce an ectopic neural axis but do not induce mesodermal tissues. In contrast, members of the HPC-HPH PcG complex do not induce neural tissue. The exclusive, direct neuralizing activity of both the Xeed and XYY1 proteins underlines the significance of the interaction between the two proteins. These data also indicate a role for chromatin-associated proteins, such as PcG proteins, in Xenopus neural induction (Satijn, 2001).

Esc, the Drosophila homolog of EED, is distinguished from other PcG proteins in Drosophila in that it is primarily required only during embryogenesis. It has been speculated that deacetylation of histones by HDACs and the recruitment of EED to the HDAC proteins may be among the initial repressive events during embryogenesis that eventually lead to stable and heritable PcG-mediated repression of target genes. Now it has been found that YY1 is part of or associated with the EED-EZH PcG complex, which displays HDAC activity. Since Pho and YY1 display specific DNA-binding properties, this finding suggests a model in which YY1 directs the EED-EZH PcG complex to target genes. This first step is consistent with the early developmental role of the EED-EZH complex, as has been defined genetically. It is also consistent with a role for histone deacetylation, mediated by the HDACs, which are associated with both EED and YY1, as an early event by which PcG proteins set up stable repression of target genes (Satijn, 2001).

The existence of two distinct PcG protein complexes has also been observed in Drosophila. Using a two-hybrid analysis, the Esc and E(z) proteins have been found to interact. Furthermore, two distinct Drosophila PcG protein complexes have been characterized biochemically. One complex contains the Pc, Psc, and Ph proteins; the other contains the Esc and E(z) proteins. These findings are very similar to the observations in the human system. There are, however, significant differences between the two developmental systems. For instance, the Drosophila Pho protein lacks a domain that mediates histone deacetylation activity. This domain is present in the YY1 protein. It is possible that this constitutes a fundamental difference between the Drosophila and the vertebrate systems, indicating that histone deacetylation plays a less significant role in the Drosophila system. Further, the Pho protein has not been detected in the Pc-Ph complex. However, the Pho protein could not be detected in the biochemically purified ESC-E(Z) complex either. These puzzling findings may reflect a more transient nature of interactions between Pho and other proteins, which precludes biochemical purification as part of a stable protein complex. The observation that even an in vitro interaction between EED and the Drosophila Pho protein exists at least suggests a highly conserved nature of the interaction between EED and YY1. Also, the virtually identical phenotypes that are induced by Xeed, XYY1, and Pho in Xenopus embryos suggest that YY1 or Pho is either a stable component of or at least transiently associated with the EED-EZH PcG complex and not the HPC-HPH PcG complex (Satijn, 2001).

To study the functional significance of the interaction between EED and YY1, the expression levels of the Xenopus homologs of these proteins, Xeed and XYY1, were manipulated. Both proteins, but no other PcG proteins, induce an ectopic neural axis in Xenopus embryos, but neither Xeed nor XYY1 is able to induce mesodermal tissue, such as muscle or notochord. Importantly, the Drosophila Pho protein induces the same phenotype. The similarity of effects underlines the significance of the EED-YY1 interaction. The fact that Pho induces the same phenotype and neural tissue in ectoderm explants also substantiates the notion that YY1 is indeed a functional homolog of the Drosophila Pho protein (Satijn, 2001).

These data point towards an early developmental role for the EED-EZH complex. Also, in homozygous eed minus mice the earliest developmental decisions are affected, pointing towards an early role for EED in setting up vertebrate PcG-mediated repression. It may be significant that homozygous eed minus mice lack a node and, probably as a consequence of this, also neural tissue. Whereas the homozygous YY1 minus mutation is embryonic lethal, in heterozygote YY1/+ mice the formation of a proper neural tube is seriously hampered. Both phenotypes are complementary to the phenotypes observed after overexpression of both Xeed and XYY1 proteins in Xenopus embryos. Although a detailed comparison between the loss-of-function data in mice and overexpression of proteins in Xenopus is not possible, the opposing effects on neural tissue are compatible with each other. The results reinforce one another and both point towards an early role of these PcG proteins in developmental decisions, such as the induction of embryonic tissues (Satijn, 2001).

The following questions remain: which are the target genes of Xeed and XYY1, and how does the modulation of the activity of these target genes result in the induction of neural tissue? Since EED is a repressor of gene activity, it is likely that Xeed is also a repressor of gene activity. Furthermore, both XYY1 and XYY1-EnR directly induce neural tissue, and by virtue of the EnR domain, XYY1-EnR is a transcriptional repressor. It is, therefore, likely that the target genes of Xeed and XYY1 are repressed by these proteins and that this repression results in the induction of neural tissue. It will be of considerable interest to identify these target genes. Since the effects of Xeed and XYY1 occur early in development, these target genes may well represent a class of PcG target genes other than the known PcG target genes in Drosophila that are affected relatively late during development. Also, identification of such target genes may reveal pathways, distinct from the known ones, that are involved in mediating neural induction in Xenopus (Satijn, 2001).

To explore mechanisms for specificity of function within the family of E2F transcription factors, proteins that interact with individual E2F proteins have been identified. A two-hybrid screen identified RYBP (Ring1- and YY1-binding protein) as a protein that interacts specifically with the E2F2 and E2F3 family members, dependent on the marked box domain in these proteins. Previous work has demonstrated that the interaction of the adenovirus E4 ORF6/7 protein with E2F, mediated through the marked box domain, leads to a stimulation of E2F-dependent transcription. The Cdc6 promoter contains adjacent E2F- and YY1-binding sites, and both are required for promoter activity. In addition, YY1 and RYBP, in combination with either E2F2 or E2F3, can stimulate Cdc6 promoter activity synergistically, dependent on the marked box domain of E2F3. Using chromatin immunoprecipitation assays, it has been shown that both E2F2 and E2F3, as well as YY1 and RYBP, associate with the Cdc6 promoter at G1/S of the cell cycle. In contrast, no interaction of E2F1 with the Cdc6 promoter was detected. It is suggested that the ability of RYBP to mediate an interaction between E2F2 or E2F3 and YY1 is an important component of Cdc6 activation and provides a basis for specificity of E2F function (Schlisio, 2002).

The transcription factor hGABP/E4TF1 is a heterotetrameric complex composed of two DNA-binding subunits (hGABP alpha/E4TF1-60) and two transactivating subunits (hGABP beta/E4TF1-53). In order to understand the molecular mechanism of transcriptional regulation by hGABP, proteins that interact with the non-DNA-binding subunit, hGABP beta, were sought using yeast two-hybrid screening. A human cDNA was identified encoding a protein related to YAF-2 (YY1-associated factor 2), which has been isolated as an interacting partner of the Ying-Yang-1 (YY1) transcription factor. Reflecting this similarity, both YAF-2 and this novel protein (named YEAF1 for YY1- and E4TF1/hGABP-associated factor-1) interacts with hGABP beta and YY1 in vitro and in vivo, indicating that YEAF1 and YAF-2 constitute a cofactor family for these two structurally distinct transcription factors. By using yeast three-hybrid assay, hGABP beta and YY1 have been demonstrated to formed a complex only in the presence of YEAF1, indicating that YEAF1 is a bridging factor of these two transcription factors. These cofactors are functionally different in that YAF-2 positively regulates the transcriptional activity of hGABP but YEAF1 negatively regulates this activity. Also, YAF-2 mRNA is highly expressed in skeletal muscle, whereas YEAF1 mRNA is highly expressed in placenta. It is speculated that the transcriptional activity of hGABP is in part regulated by the expression levels of these tissue-specific cofactors. These results provide a novel mechanism of transcriptional regulation by functionally distinct cofactor family members (Sawa, 2002).

Smad proteins transduce transforming growth factor beta (TGF-beta) and bone morphogenetic protein (BMP) signals that regulate cell growth and differentiation. YY1, a transcription factor that positively or negatively regulates transcription of many genes, has been identified as a novel Smad-interacting protein. YY1 represses the induction of immediate-early genes to TGF-beta and BMP, such as the plasminogen activator inhibitor 1 gene (PAI-1) and the inhibitor of differentiation/inhibitor of DNA binding 1 gene (Id-1). YY1 inhibits binding of Smads to their cognate DNA elements in vitro and blocks Smad recruitment to the Smad-binding element-rich region of the PAI-1 promoter in vivo. YY1 interacts with the conserved N-terminal Mad homology 1 domain of Smad4 and to a lesser extent with Smad1, Smad2, and Smad3. The YY1 zinc finger domain mediates the association with Smads and is necessary for the repressive effect of YY1 on Smad transcriptional activity. Moreover, downregulation of endogenous YY1 by antisense and small interfering RNA strategies result in enhanced transcriptional responses to TGF-beta or BMP. Ectopic expression of YY1 inhibits, while knockdown of endogenous YY1 enhances, TGF-beta- and BMP-induced cell differentiation. In contrast, overexpression or knockdown of YY1 does not affect growth inhibition induced by TGF-beta or BMP. Accordingly, YY1 does not interfere with the regulation of immediate-early genes involved in the TGF-beta growth-inhibitory response, the cell cycle inhibitors p15 and p21, and the proto-oncogene c-myc. In conclusion, YY1 represses Smad transcriptional activities in a gene-specific manner and thus regulates cell differentiation induced by TGF-beta superfamily pathways (Kurisaki, 2003).

The distal regulatory region (DRR) of the mouse and human MyoD gene contains a conserved SRF binding CArG-like element. In electrophoretic mobility shift assays with myoblast nuclear extracts, this CArG sequence, although slightly divergent, binds two complexes containing, respectively, the transcription factor YY1 and SRF associated with the acetyltransferase CBP and members of C/EBP family. A single nucleotide mutation in the MyoD-CArG element suppresses binding of both SRF and YY1 complexes and abolishes DRR enhancer activity in stably transfected myoblasts. This MyoD-CArG sequence is active in modulating endogeneous MyoD gene expression because microinjection of oligonucleotides corresponding to the MyoD-CArG sequence specifically and rapidly suppress MyoD expression in myoblasts. In vivo, the expression of a transgenic construct comprising a minimal MyoD promoter fused to the DRR and beta-galactosidase is induced with the same kinetics as MyoD during mouse muscle regeneration. In contrast induction of this reporter is no longer seen in regenerating muscle from transgenic mice carrying a mutated DRR-CArG. These results show that an SRF binding CArG element present in MyoD gene DRR is involved in the control of MyoD gene expression in skeletal myoblasts and in mature muscle satellite cell activation during muscle regeneration (L'honore, 2003).

BMP signals act in concert with FGF8, WNT11 and WNT antagonists to induce the formation of cardiac tissue in the vertebrate embryo. In an effort to understand how these signaling pathways control the expression of key cardiac regulators, the cis-regulatory elements of the chick tinman homolog chick Nkx2.5 have been characterized. At least three distinct cardiac activating regions (CARs) of chick Nkx2.5 cooperate to regulate early expression in the cardiac crescent and later segmental expression in the developing heart. In this report, attention was focused on a 3' BMP-responsive enhancer, termed CAR3, which directs robust cardiac transgene expression. By systematic mutagenesis and gel shift analysis of this enhancer, it has been demonstrated that GATA4/5/6, YY1 and SMAD1/4 are all necessary for BMP-mediated induction and heart-specific expression of CAR3. Adjacent YY1 and SMAD-binding sites within CAR3 constitute a minimal BMP response element, and interaction of SMAD1/4 with the N terminus of YY1 is required for BMP-mediated induction of CAR3. These data suggest that BMP-mediated activation of this regulatory region reflects both the induction of GATA genes by BMP signals, as well as modulation of the transcriptional activity of YY1 by direct interaction of this transcription factor with BMP-activated SMADs (Lee, 2004).

How might the interaction of SMADs with YY1 modulate the activity of this transcription factor when bound to CAR3? Because YY1 can function as either a transcriptional activator or repressor, SMAD association with YY1 may serve to recruit co-activators that modulate the activity of this transcription factor to become an efficient transcriptional activator. Indeed, recruitment of co-activators such as p300 by TGFß activated SMADs is a well-characterized mechanism for SMAD target gene activation. Similarly, known interacting partners of YY1 also include several members of the histone deacetylase family as well as a histone H4 methylase, which have been implicated in either transcriptional repression or activation of YY1 regulated target genes, respectively. It will be interesting to determine if SMAD association with YY1 alters the interaction of this transcription factor with either of these families of histone modifying enzymes, and to what extent chromatin modification is responsible for appropriate regulation of Nkx2.5 (Lee, 2004).

SMAD-mediated modulation of YY1 activity adds an interesting new facet to the repertoire of functions of YY1 during heart development, which also includes direct recruitment of transcriptional co-activators to promote the expression of cardiac B-type natriuretic peptide, inhibition of the expression of the cardiac {alpha}-actin gene, and both activation and inhibition of the expression of the cardiac-specific Mlc2 gene. Clearly, the context within which YY1 functions is of great importance, and it is likely that transcription factors such as GATA and SMAD proteins, when bound to neighboring cognate binding sites, modulate either the association of co-factors with adjacently bound YY1 or the activity of such co-factors. In addition to the GATA, YY1- and SMAD-binding sites, linker scanning mutational analysis of the chick Nkx2.5 CAR3 BMPRE has revealed other sites yet to be characterized that also have a significant impact on the BMP response of this regulatory element. A complete understanding of complex enhancers such as Nkx2.5 CAR3 will require not only the identification of the transcription factors that regulate their expression but also elucidation of the transcriptional co-factors that are recruited to such regulatory elements in a combinatorial fashion (Lee, 2004).

YY1 is a transcription factor that plays an essential role in development. However, the full spectrum of YY1's functions and mechanism of action remains unclear. YY1 ablation results in p53 accumulation due to a reduction of p53 ubiquitination in vivo. Conversely, YY1 overexpression stimulates p53 ubiquitination and degradation. Significantly, recombinant YY1 is sufficient to induce Hdm2-mediated p53 polyubiquitination in vitro, suggesting that this function of YY1 is independent of its transcriptional activity. Direct physical interactions of YY1 with Hdm2 (the human ortholog of Mdm2) and p53 have been identified and the basis for YY1-regulating p53 ubiquitination has been shown to be its ability to facilitate Hdm2-p53 interaction. Importantly, the tumor suppressor and cyclin-dependent kinase inhibitor p14ARF compromises the Hdm2-YY1 interaction, which is important for YY1 regulation of p53. Taken together, these findings identify YY1 as a potential cofactor for Hdm2 in the regulation of p53 homeostasis and suggest a possible role for YY1 in tumorigenesis (Sui, 2004).

The tumor suppressor p53 regulates cell-cycle progression and apoptosis in response to genotoxic stress, and inactivation of p53 is a common feature of cancer cells. The levels and activity of p53 are tightly regulated by posttranslational modifications, including phosphorylation, ubiquitination, and acetylation. The transcription factor YY1 interacts with p53 and inhibits its transcriptional activity. YY1 disrupts the interaction between p53 and the coactivator p300, and expression of YY1 blocks p300-dependent acetylation and stabilization of p53. Furthermore, expression of YY1 inhibits the accumulation of p53 and the induction of p53 target genes in response to genotoxic stress. YY1 also interacts with Mdm2 and the expression of YY1 promotes the assembly of the p53-Mdm2 complex. Consequently, YY1 enhances Mdm2-mediated ubiquitination of p53. Inactivation of endogenous YY1 enhances the accumulation of p53 as well as the expression of p53 target genes in response to DNA damage, and it sensitizes cells to DNA damage-induced apoptosis. Hence, these results demonstrate that YY1 regulates the transcriptional activity, acetylation, ubiquitination, and stability of p53 by inhibiting its interaction with the coactivator p300 and by enhancing its interaction with the negative regulator Mdm2. YY1 may, therefore, be an important negative regulator of the p53 tumor suppressor in response to genotoxic stress (Gronroos, 2004).

Recruitment of histone H4-methyltransferase by YY1

Methylation of specific residues within the N-terminal histone tails plays a critical role in regulating eukaryotic gene expression. Although great advances have been made toward identifying histone methyltransferases (HMTs) and elucidating the consequences of histone methylation, little is known about the recruitment of HMTs to regulatory regions of chromatin. The sequence-specific DNA-binding transcription factor YY1 binds to and recruits the histone H4 (Arg 3)-specific methyltransferase, PRMT1, to a YY1-activated promoter. These data confirm that histone methylation does not occur randomly but rather is a targeted event and provides one mechanism by which HMTs can be recruited to chromatin to activate gene expression (Rezai-Zadeh, 2003).

YY1 is regulated by O-linked N-acetylglucosaminylation

YY1 regulates the expression of genes with important functions in DNA replication, protein synthesis, and cellular response to external stimuli during cell growth and differentiation. How YY1 accomplishes such a variety of functions is unknown. A subset of the nuclear YY1 appears to be O-GlcNAcylated regardless of the differentiation status of the cells. Glucose strongly stimulates O-linked N-acetylglucosaminylation (O-GlcNAcylation) on YY1. Glycosylated YY1 no longer binds the retinoblastoma protein (Rb). Upon dissociation from Rb, the glycosylated YY1 is free to bind DNA. The ability of the O-glycosylation on YY1 to disrupt the complex with Rb leads to a proposal that O-glycosylation might have a profound effect on cell cycle transitions that regulate the YY1-Rb heterodimerization and promote the activity of YY1. These observations provide strong evidence that YY1-regulated transcription is very likely connected to the pathway of glucose metabolism that culminates in the O-GlcNAcylation on YY1, changing its function in transcription (Hiromura, 2003).

Interaction of YY1 with the nuclear matrix

Continued: pleiohomeotic Evolutionary homologs
part 2/3 | part 3/3 |

pleiohomeotic: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 30 September 98

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