REGULATION OF PYRIMIDINE GENE EXPRESSION IN BACTERIA

本提案的目的是了解 调节大肠杆菌中嘧啶基因的表达。 在这 细菌，从头合成UMP，所有嘧啶的前体 核苷酸是由六种酶催化的，这六种酶由六个不相连的基因编码。 和小操纵子：carAB、pyrBI、pyrC、pyrD、pyrE和pyrF。 的 这些基因和操纵子的表达受到以下因素的非协调调节： 尿苷或胞苷核苷酸的细胞内水平。 最近 研究表明，pyrBI和pyrE的表达主要受 通过一种衰减控制机制， 在紧接在前的Rho独立终止子（衰减子）处， pyr结构基因受UTP- 前导区内的敏感转录和偶联翻译 衰减器的上游。 目前，人们对这一问题知之甚少。 调节其他嘧啶基因表达的机制。 在这项研究中，实验旨在确定新的调节 元素，并测试可能的模型，为调节嘧啶 基因表达。 拟议的研究将集中在pyrBI操纵子 以及pyrC和pyrF基因。 计划进行实验以完成 关键调控元件的遗传和生化表征 参与pyrBI表达的衰减控制，包括 核苷酸（UTP和GTP）特异性转录暂停。 菌株 显示出改变的衰减控制将被隔离， 提供了多种感兴趣的突变，包括Rpo突变 它将被用来描述RNA的基本特性 聚合酶。 pyrBI表达的非衰减子依赖性调节， 其显著有助于嘧啶介导的对 这种操纵子也将被表征。 特别感兴趣的是 可能涉及UTP敏感的流产转录 启动独立于衰减器的调节。 最近的结果表明， pyrC的表达受到一种新的衰减控制因子的调控， 转录终止在Rho依赖的 pyrC结构基因内的寄生虫是由 结合转录和翻译。 实验计划测试 rho突变和改变pyrC表达速率的突变的影响 翻译起始对pyrC表达的调控。 在 此外，由purR编码的嘌呤阻遏物可能发挥的作用 基因，在调控pyrC，pyrD，和汽车AB的表达将是 考察 剩下的实验将研究这种机制 调节pyrF表达。 可能的调节作用的反式- 作用因子、败育起始和orF基因（第二基因 在pyrF-orfF操纵子中）将被探索。

The objective of the present proposal is to understand the mechanisms regulating pyrimidine gene expression in Escherichia coli. In this bacterium, the de novo synthesis of UMP, the precursor of all pyrimidine nucleotides is catalyzed by six enzymes encoded by six unlinked genes and small operons: carAB, pyrBI, pyrC, pyrD, pyrE, and pyrF. The expression of these genes and operons is noncoordinately regulated by the intracellular levels of uridine or cytidine nucleotides. Recent studies have shown that pyrBI and pyrE expression is regulated primarily by an attenuation control mechanism in which transcriptional termination at a Rho-independent terminator (attenuator) immediately preceding the pyr structural gene(s) is regulated by the relative rates of UTP- sensitive transcription and coupled translation within a leader region upstream of the attenuator. At present, little is known about the mechanisms regulating the expression of the other pyrimidine genes. In this study experiments are designed to identify new regulatory elements and to test possible models for the regulation of pyrimidine gene expression. The proposed research will focus on the pyrBI operon and the pyrC and pyrF genes. Experiments are planned to complete the genetic and biochemical characterization of key regulatory elements involved in attenuation control of pyrBI expression, including nucleotide (UTP and GTP)specific transcriptional pausing. Strains exhibiting altered attenuation control will be isolated and should provide a variety of interesting mutations, including rpo mutations which will be used to characterize fundamental properties of RNA polymerase. Attenuator-independent regulation of pyrBI expression, which significantly contributes to the pyrimidine-mediated control of this operon, also will be characterized. Of particular interest is the possible involvement of UTP-sensitive abortive transcriptional initiation in attenuator-independent regulation. Recent results suggest that pyrC expression is regulated by a novel attenuation control mechanism in which transcriptional termination at a Rho-dependent erminator within the pyrC structural gene is controlled by the extent of coupled transcription and translation. Experiments are planned to test the effect of rho mutations and mutations that alter the rate of pyrC translational initiation on the regulation of pyrC expression. In addition, the possible role of the purine repressor, encoded by the purR gene, in the regulation of pyrC, pyrD, and car AB expression will be examined. The remaining experiments outlined will examine the mechanism regulating pyrF expression. The possible regulatory roles of trans- acting factors, abortive initiation, and the orF gene (the second gene in the pyrF-orfF operon) will be explored.