In Drosophila the Abdominal‐B (Abd‐B) domain of the bithorax complex (BX‐C) spans over 100 kb and is responsible for specifying the identities of adult abdominal segments five (A5) to nine (A9), inclusive, and correspondingly, neuromeres 10–14 of the embryonic central nervous system. The domain consists of a region coding for two proteins, ABD‐BI (54 kd) and ABD‐BII (36 kd) and cis‐regulatory regions extending from infra‐abdominal‐5 (iab‐5) to iab‐9, inclusive. We have used a monoclonal anti‐ABD‐B antibody to infer that mutants in iab‐8 eliminate the expression of ABD‐BI in neuromeres 10–13, inclusive, and that mutants in iab‐9 eliminate expression of ABD‐BII in neuromere 14. ABD‐B expression is also analyzed in homozygotes for (i) loss‐of‐function mutants involving the iab‐5, iab‐6 and iab‐7 regions, (ii) gain‐of‐function mutants Miscadastral pigmentation (Mcp) and Superabdominal (Sab), and (iii) a trans‐regulator, Polycomb (Pc). ABD‐B expression along the antero‐posterior axis is colinear with the chromosomal order of the cis‐regulatory regions. The behavior of rearrangement‐associated iab‐6 and iab‐7 mutants suggests that the enhancer‐like region, iab‐5, and possibly also iab‐6, may be shared between the abd‐A and Abd‐B domains. Such sharing is proposed as a factor that tends to keep gene complexes intact during evolution.
A number of homeodomain proteins have been shown to regulate cellular development by stimulating the transcription of specific target genes. In contrast to their distinct activities in vivo, however, most homeodomain proteins bind indiscriminately to potential target sites in vitro, suggesting the involvement of cofactors which specify target site selection. One such cofactor, termed extradenticle, has been shown to influence segmental morphogenesis in Drosophila melanogaster by binding cooperatively with certain homeodomain proteins to target regulatory elements. Here we demonstrate that STF-1, an orphan homeodomain protein required for pancreatic development in mammals, binds cooperatively to DNA with Pbx, the mammalian homolog of extradenticle. Cooperative binding with Pbx requires a pentapeptide motif (FPWMK) which is well conserved among a large subset of homeodomain proteins. The FPMWK motif is not sufficient to confer Pbx cooperativity on other homeodomain proteins, however; the N-terminal arm of the STF-1 homeodomain is also essential. As cooperative binding with Pbx occurs on only a subset of potential STF-1 target sites, our results suggest that Pbx may specify target gene selection in the developing pancreas by forming heterodimeric complexes with STF-1.
The commitment of cells to specific lineages during development is determined in large part by the relative expression of various homeodomain (HOX) selector proteins, which mediate the activation of distinct genetic programs. But the mechanisms by which individual HOX genes are themselves targeted for expression in different cell types remain largely uncharacterized. Here, we demonstrate that STF-1, a homeodomain protein that functions in pancreatic morphogenesis and in glucose homeostasis is encoded by an "orphan" homeobox gene on mouse chromosome 5. When fused to a -galactosidase reporter gene, a 6.5-kilobase genomic fragment of 5-flanking sequence from the STF-1 gene shows pancreatic islet specific activity in transgenic mice. Two distinct elements within the STF-1 promoter are required for islet-restricted expression: a distal enhancer sequence located between ؊3 and ؊6.5 kilobases and a proximal E-box sequence located at ؊104, which is recognized primarily by the helix loop helix/leucine zipper nuclear factor USF. As point mutations within the ؊104 E-box that disrupt USF binding correspondingly impair STF-1 promoter activity, our results demonstrate that USF is an important component of the regulatory apparatus which directs STF-1 expression to pancreatic islet cells.
The development of endocrine cell types within the pancreas is thought to involve the progressive restriction of pluripotential stem cells, which gives rise to the four major cell types: insulin-, glucagon-, somatostatin-, and pancreatic polypeptide-expressing cells. The mechanism by which these peptide hormone genes are induced and then either maintained or repressed during development is unknown, but their coexpression in early precursor cells suggests the involvement of common regulatory factors. Here we show that the somatostatin transcription factor STF-1 is also a principal regulator of insulin expression in beta-cells of the pancreas. STF-1 stimulates the insulin gene by recognizing two well defined islet-specifying elements on the insulin promoter and by subsequently synergizing in trans with the juxtaposed helix-loop-helix protein E47. Within the STF-1 protein, an N-terminal trans-activation domain functions cooperatively with E47 to stimulate insulin transcription. As truncated STF-1 polypeptides lacking the N-terminal activation domain strongly inhibit insulin promoter activity in beta-islet cells, our results suggest that the specification of islet cell types during development may be in part determined by the expression of STF-1 relative to other islet cell factors.
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