BIOLOGICAL OVERVIEW

FTZ-F1 was first identified as a protein that binds to the zebra element, a 740 base pair DNA sequence upstream of the fushi tarazu (ftz) transcriptional start site. The zebra element is responsible for regulating fushi tarazu expression in seven stripes of alternating segment (pair rule) periodicity in the early embryo. Mutational alteration in the FTZ-F1 binding site results in a lack of expression of ftz in stripe 1, weaker, but detectable expression in stripes 2, 3 and 6, and normal staining in stripes 4, 5 and 7.

FTZ-F1 is found in both early and late forms, corresponding to an early protein found during the first few hours of embryonic development and a late protein, migrating on gels at a faster rate, appearing between 16 and 19 hours of development. DNA binding specificity cannot be distinguished between early and late forms. Besides binding to several sites in the zebra element, FTZ-F1 binds to additional sites within the ftz gene (Ueda, 1989)

FTZ-F1 also influences molting. Chromosomal puffing in the late prepupal salivary gland polytene chromosomes allows for an assessment of the order of gene activation. Puffing is evidence of a 'loosening' of the chromatin holding a gene together, and is thought to accompany gene activation. FTZ-F1 is directly involved in the regulation of the gene activation hierarchy in salivary gland chromosomes. Antibodies directed against FTZ-F1 protein detect staining of 166 loci in the late prepupal salivary gland polytene chromosomes, suggesting that FTZ-F1 regulates transcription of many genes active in polytene chromosomes. It is presumed that FTZ-F1 functions to regulate expression of the gene puffs to which it binds. 51 of these loci represent ecdysone-regulated puffs. Of 33 puffs that show increased activity after the peak of the 75CD puff (responsible for FTZ-F1 synthesis), 17 show reproducable staining for FTZ-F1. These include two prominent late prepupal puffs (74EF and 75B) encoding respectively ets-related and steroid receptor superfamily DNA binding proteins. These late prepupal puffs occur in the latter part of the early phase of the puffing hierarchy. Both 74EF and 75B are induced directly by the late larval and prepupal pulses of ecdysone. These results suggest that FTZ-F1 contributes to a significant fraction of the genes in the late prepupal phase of the molting regulatory hierarchy. Of interest is the observation that FTZ-F1 binds to the 75CD puff itself, raising the possibility of an autoregulatory interaction. Among the 25 puffs that show decreased activity in late prepupae, 7 are bound by FTZ-F1. It is therefore possible that FTZ-F1 may also participate in the repression of these puff loci (Lavorgna, 1993).

Where does FTZ-F1 fit into the hierarchy of regulatory genes expressed during metamorphosis? It has been suggested that its action may serve as a bridge between early and late gene expression during the process of metamorphosis. Early genes include Ecdysone receptor, the master regulator whose dimerization partner is Ultraspiracle. A third early protein, coded for by E75A, is another nuclear receptor superfamily member. These proteins are induced during the third instar larval period beginning during the fourth day of fly development. These early genes both repress their own expression and induce a large set of late genes. The induction of two late genes, E78A and DHR3, is delayed, relative to that of the early genes, apparently owing to an additional requirement for early ecdysone-induced protein synthesis. FTZ-F1, expressed during the fifth day of fly development, is repressed by both itself and ecdysone, thus restricting its expression to the brief interval of low ecdysone titer in midprepupae. FTZ-F1 appears to provide the competence for the "early genes" E54A, E75A, Broad Complex and E93 (an early gene expressed later in prepupal development at 5.5 to 6 days). These latter genes regulate the expression of late genes, expressed in late prepupal development and the pupal phase, beginning 5.5 days after fertilization (Woodard, 1994 and Thummel, 1995). Thus FTZ-F1 acts as a bridge between expression of the earliest genes involved in metamorphosis (Ecdysone receptor and E75) and the late genes.

GENE STRUCTURE

PROTEIN STRUCTURE

Amino Acids - 1043 (early) 816 (late)

Structural Domains

FTZ-F1 is a member of the nuclear hormone receptor superfamily. The conserved regions include the DNA binding domain that bears two potential Cys2-Cys2 zinc finger motifs, and the ligand binding domain of the nuclear receptor superfamily. The putative DNA binding domain is well conserved across the nuclear receptor superfamily, showing identity with all 20 invariant amino acids. Nonetheless, FTZ-R1 is somewhat distinct from the two major classes of nuclear receptors represented by those that bind either the glucocorticoid or the estrogen-thyroid hormone response elements (Lavorgna, 1991).

The C-terminal half of the FTZ-F1 protein shows sequence similarity to the ligand binding domain of hormone receptors. Sequence alignment identifies two core regions (region II and III) that are conserved across the nuclear receptor superfamily.. The FTZ-F1 sequence in these regions is most similar to the human COUP transcription factor 9 (also called erb-A related 3 or EAR3), and also to the Drosophila neural determination protein Sevenup. The sequence conservation in the ligand binding domain suggests that FTZ-F1 is a receptor for a hormonal ligand in Drosophila, but the nature of this ligand has yet to be determined (Lavorgna, 1991).

The late FTZ-F1 isoform (FTZ-F1ß) differs from the early isoform (FTZ-F1alpha) by replacement of the N-terminal 401 amino acids with a 173 amino acid segment. Both 5.6 and 4.8 kb mRNAs code for an identical late protein form. The late protein corresponds to the protein generated during the midprepupal stage that plays a role in ecdysone-induced gene expression (Lavorgna, 1993).

date revised:  19 Dec 96

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