Mechanisms of LysR-type Protein-Mediated Regulation of Gene Expression

点击翻译按钮获取中文摘要

9723452 Hatfield Much attention is currently being focused toward understanding how the transcriptional efficiencies of promoters are influenced by the superhelical density of the DNA template. In bacteria it has been shown that the transcription process alters the local super helical density of the chromosome; positive DNA supercoils are generated in front of a transcribing RNA polymerase and negative DNA supercoils are generated in its wake. Moreover, the local negative DNA supercoiling generated behind a transcribing RNA polymerase can activate transcription initiation from a nearby divergent promoter site. Although divergent promoter arrangements are common in all organisms, the physiological significance of these divergent promoters is not fully understood. In bacteria, the most common positive regulators of gene expression are the LysR family of proteins. Members of the LysR family are characterized as sequence-related, co-inducer responsive, transcriptional activator proteins that activate transcription of a target gene(s) and autoregulate their own synthesis. The IlvY protein is a LysR family protein that negatively regulates the expression of its own structural gene, ilvY, and, in the presence of a small molecule co-inducer, positively regulates the expression of a divergently transcribed ilvC gene which encodes the second enzyme of the parallel L-isoleucine, L-valine biosynthetic pathway. It recently has been demonstrated that this activation of transcription is significantly influenced by the superhelical density of the DNA template. On the basis of this observation and other preliminary evidence, it has been hypothesized that as co-inducer levels increase more co-inducer binds to a performed IlvY-operator DNA complex and further activates ilvC transcription. This increased ilvC promoter activity generates negative supercoils in its wake and enhances the transcriptional activity of the nearby, divergent, ilvPY promoter. This increased ilvPY promoter activity results in hig her IlvY production and greater IlvY occupancy at the tandem operator sites. Under this condition the ilvC gene can be induced to a higher maximal level of expression because of higher IlvY occupancy at the tandem operator sites. Thus, this transcriptional-coupling between the divergent ilvPC and ilvPY promoters provides a sensitive and energetically economical mechanism to adjust IlvY and isomeroreductase (ilvC gene product) production with substrate-inducer levels which reflect the demands of the cell for isomeroreductase activity for branched chain amino acid biosynthesis. This research will test the predictions of this transcriptional-coupling hypothesis and to elucidate the protein-protein and protein DNA interactions important for IlvY protein-mediated activation of transcription from the ilvC gene. It is anticipated that the results of this research will provide fundamental information for understanding the roles of DNA supercoiling-dependent structures in gene expression, will enhance the current level of understanding of the mechanism of action for other LysR-type gene activators, and will be of general interest for investigations of divergent promoter systems in lower and higher eukaryotic systems. %%% Much attention is currently being focused toward an understanding of the roles of DNA supercoiling-dependent (chromatin) structures on gene expression. In bacteria, the most common positive regulators of gene expression are the LysR family of proteins which activate transcription of a target gene(s) and autoregulate their own synthesis. This research will provide fundamental information for understanding the roles of DNA supercoiling-dependent structures in gene expression, will enhance the current level of understanding of the mechanism of action for other LysR-type gene activators, and will be of general interest for investigations of divergent promoter systems in lower and higher eukaryotic system. ***