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Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located within the 5'-flanking region of the insulin gene. Transcription from the enhancer is controlled by both positive- and negative-acting cellular transcription factors. It was previously shown that both the 243- and 289-amino-acid adenovirus type 5 E1a proteins can repress insulin gene transcription in vivo. To localize the insulin DNA sequences involved in this response, we examined the effects of a number of mutations within the 5'-flanking region of the rat insulin II gene on E1a-mediated repression of insulin gene transcription. We have found that E1a proteins inhibit enhancer-stimulated transcription of the insulin gene. The enhancer appears to contain at least two genetically separable and independent E1a target sequence elements. Interestingly, these same regions of the insulin enhancer have been shown to be negatively regulated by cellular transcription factors. These results suggest that E1a-like cellular factors may function in the pancreatic beta-cell-specific expression of the insulin gene.
Monoclonal antibody 51F recognizes determinants in the helix 4 region of the native form of the N-terminal domain of lambda repressor. A cassette mutagenesis method was used to introduce changes within this region, and antibody-reactive candidates were isolated and sequenced. The resulting data allow the identification of repressor side chains that are critical determinants of antibody binding. Four of these side chains are on the surface of the N-terminal domain and probably contact the antibody directly. These contact positions were then mutagenized individually, and the antibody binding phenotypes of a large number of singly mutant repressors were determined. Taken together, the mutational data allow a functional map of the recognition surface to be constructed and the physical nature of some of the specific interactions that stabilize the antibody-antigen complex to be surmised.
Monoclonal antibodies reactive with distinct regions of the N-terminal domain of the lambda repressor protein have been isolated. By comparing the affinities of these antibodies for mutant repressors with increased and decreased thermal stabilities, each of the antibodies can be shown to bind to epitopes accessible in the native conformation of the N-terminal domain. Experiments probing antibody binding to protein fragments, mutant variants, and peptides have also been used to define likely regions of contact between the antibodies and the N-terminal domain.
Adenovirus E1A transforming function requires two distinct regions of the protein. Transforming activity is closely linked with the presence of a region designated conserved domain 2 and the ability of this region to bind the product of the cellular retinoblastoma tumor suppressor gene. We have investigated the biological properties of the second transforming region of E1A, which is located near the N terminus. Transformation-defective mutants containing deletions in the N terminus (deletion of residues between amino acids 2 and 36) were deficient in the ability to induce DNA synthesis and repress insulin enhancer-stimulated activity. The function of the N-terminal region correlated closely with binding of the 300-kilodalton E1A-associated protein and not with binding of the retinoblastoma protein. These results indicate that transformation by E1A is mediated by two functionally independent regions of the protein which interact with different specific cellular proteins and suggest that the 300-kilodalton E1A-associated protein plays a major role in E1A-mediated cell growth control mechanisms.
We report that two novel alternatively spliced products of the murine Oct-2 gene encode Mini-Oct (Oct-2d), a protein consisting of almost only the POU domain, and Oct-2c, a protein lacking the last 12 amino acids of Oct-2a. Ectopic expression in HeLa cells shows that Oct-2c is a transactivator, whereas Mini-Oct fails to transactivate if the octamer motif is in a promoter position next to TATA box. Mini-Oct can repress the transcriptional signal generated by endogenous octamer factors in F9 cells. It seems that Mini-Oct has the potential to serve as a transcriptional modulator for genes regulated by different octamer-binding factors. In situ hybridization reveals that Mini-Oct expression follows the general pattern of other known Oct-2 transcripts. However, it is absent from the Purkinje cell layer in the cerebellum of adult mice, and strong expression is observed in the developing nasal neuroepithelium and primary spermatids. Differential expression patterns of the Oct-2 transcripts with different transactivation/repression capacities of the encoded proteins may have a specific role in gene expression in the developing nervous system and in adult brain.
Long-chain fatty acids (FA) have been shown to regulate expression of the gene for the adipocyte FA-binding protein aP2. We examined whether this effect was exerted by FA themselves or by a FA metabolite. The alpha-bromo derivative of palmitate, an inhibitor of FA oxidation, was synthesized in the radioactive form, and its metabolism was investigated and correlated with its ability to induce aP2 in Ob1771 preadipocytes. alpha-Bromopalmitate was not utilized by preadipocytes. It was not cleared from the medium over a 24-hr period and was not incorporated into cellular lipids. Short incubations indicated that alpha-bromopalmitate exchanged across the preadipocyte membrane but remained in the free form inside the cell. In line with this, preadipocyte homogenates did not activate alpha-bromopalmitate to the acyl form. However, although it was not metabolized, bromopalmitate was much more potent than native FA in inducing aP2 gene expression. Induction exhibited the characteristics previously described for native FA, indicating that a similar if not identical mechanism was involved. The data indicated that induction of aP2 was exerted by unprocessed FA. Finally, in contrast to preadipocytes, adipocytes metabolized bromopalmitate. This reflected increased activity with cell differentiation of a palmitoyl-CoA synthase that could activate palmitate and bromopalmitate at about one-fifth the rate for palmitate. In preadipocytes, the predominant fatty-acyl-CoA synthase, arachidonyl-CoA synthase, had very low affinity for both FA. Increased activity of the palmitoyl-CoA synthase, which has a wider substrate range, is likely to be important for initiation of lipid deposition.