CrebA
The promoter lacks a TATA box. A canonical CRE sequence, 5'TGACGTCA3', is found 61 bp upstream from the 5'-most start suggesting that, like a number of other transcription factors, CrebA may autoregulate in vivo (Rose, 1997).
Expression of CrebA in salivary gland depends on Sex combs reduced, since Scr mutants do not express CrebA in salivary glands and embryos expressing Scr in new places also express CrebA in new places. Activation is blocked by the trunk gene, teashirt and the posterior homeotic gene Abdominal-B. As with two other salivary gland genes, forkhead and trachealess, activation of CrebA in the salivary gland by Scr is blocked by dpp (Andrew, 1997).
Salivary gland formation in the Drosophila embryo is dependent on Scr. When Scr
function is missing, salivary glands do not form, and when Scr is expressed everywhere in the embryo, salivary glands form
in new places. Scr is normally expressed in all the cells that form the salivary gland. However, as the salivary gland
invaginates, SCR mRNA and protein disappear. Homeotic genes, such as Scr, specify tissue identity by regulating the
expression of downstream target genes. For many homeotic proteins, target gene specificity is achieved by cooperatively
binding DNA with cofactors. Therefore, it is likely that Scr also requires a cofactor(s) to specifically bind to DNA and
regulate salivary gland target gene expression. Two homeodomain-containing proteins encoded by the
extradenticle and homothorax genes are also required for salivary gland formation. exd and hth function at two
levels: (1) exd and hth are required to maintain the expression of Scr in the salivary gland primordia prior to invagination
and (2) exd and hth are required in parallel with Scr to regulate the expression of downstream salivary gland genes. Scr regulates the nuclear localization of Exd in the salivary gland primordia through repression of homothorax expression, linking the regulation of Scr activity to the disappearance of Scr expression in invaginating salivary
glands (Henderson, 2000).
To determine if Exd cooperates with Scr to control salivary gland gene expression, the accumulation
of two early salivary gland proteins, CrebA and
Trh, was examined in embryos lacking exd function. Zygotic loss of exd
function results in a reduction in the number of
salivary gland cells expressing CrebA and Trh, as well
as a decrease in the level of protein made in these cells. This reduced level of salivary gland
protein expression is not as severe as the one seen in Scr mutant
embryos. Unlike SCR, EXD mRNA is
supplied maternally and, thus,
the maternal contribution may partially compensate for the
loss of zygotic function. To test this possibility, the maternal
contribution of exd was removed using the FLP-FRT
system. In embryos lacking
maternal exd function, salivary gland expression of
CrebA and Trh is at wild-type levels. However, salivary gland expression of CrebA and
Trh is completely absent in embryos lacking both the
maternal and the zygotic contributions of exd. Thus, exd is required for embryonic salivary
gland gene expression. Moreover, zygotically provided exd
can rescue the loss of maternally provided exd and maternally
provided exd can partially compensate for zygotic loss
of exd (Henderson, 2000).
The sequences required for tissue-specific and temporal expression of the Adh genes of D. melanogaster (Dme) and D. mulleri (Dmu) have been characterized. The Dme Adh gene is expressed from two promoters: the proximal promoter primarily active in larvae, and the distal promoter primarily active in adults. In contrast the Dmu Adh locus contains two functional genes: one that is expressed in larvae and another that is expressed in adults. The Dme Adh adult fat body element (AAE) acts to simulate transcription from the distal promoter of adults. The Dmu adult enhancer is located about 2500 bp upstream of the Adh-2 promoter. The Dmu larval fat body enhancer, box B, is a 90 bp regulatory element located about 200 bp upstream of the Adh-1 gene. Box B drives expression of Adh-1 gene in larvae. Both box B and AAE act as fat body-specific enhancers when linked to a heterologous promoter. CrebA binds to the Dmu box B (the larval fat body enhancer of the Adh-1) and also binds to the AAE of Dme adult fat body element. A CrebA binding site is also located within the Dmu adult enhancer, which directs expression of the Adh-2 gene, although CrebA binds about threefold less well to this sequence than to box B and the AAE. CrebA also binds to a region of the Yolk protein gene fat body enhancer. dCREB-A acts as a transcriptional activator from box B, and binds to two mammalian liver-specific regulatory elements, the hepatocyte-specific enhancer of rat tyrosine aminotransferase and the mammalian Adh (Abel, 1992).
Although shown to bind fat-body and liver-specific regulatory elements, CrebA is not expressed in the fat body during any developmental stage (Andrew, 1997).
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CrebA:
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