细菌转录起始调控机制*

原核生物同一种群的每个细胞都是和外界环境直接接触的,它们主要通过开启或关闭某些基因的表达来适应环境条件。所以,环境因子往往是调控的效应因子,必须严格调控转录来确保细胞对环境改变做出有效且充分的反应。原核生物基因的表达受多种因素的调控,而对于大多数细菌来说,调控基因表达的关键步骤是启动子识别和RNA聚合酶启动转录。在细菌的细胞中,可以通过调节RNA聚合酶的活性以及改变RNA聚合酶对启动子的结合来优化基因的转录过程以适应不同环境变化。总结了目前已发现的参与细菌细胞转录调节的各类因子,从这些因子对启动子的作用、RNA聚合酶的作用以及两者的相互作用等方面阐述它们调控基因表达的分子机制。总结多种基因调控的作用,加深对转录起始过程的认识,希望能对未来调控转录起始过程来实现目标基因的高效表达和不利基因的抑制表达提供思路,为以后的工业菌株改造提供依据。     

A complex regulatory DNA element associated with a major histocompatibility complex class I gene consists of both a silencer and an enhancer.

A novel regulatory element which contributes to the regulation of quantitative, tissue-specific differences in gene expression has been found between -771 and -676 bp upstream of the major histocompatibility complex (MHC) class I gene, PD1. Molecular dissection of this element reveals the presence of two overlapping functional activities: an enhancer and a silencer. Distinct nuclear factors bind to the overlapping enhancer and silencer DNA sequence elements within the regulatory domain. The levels of factors binding the silencer DNA sequence in different cell types are inversely related to levels of class I expression; in contrast, factors binding the enhancer DNA sequence can be detected in all cells. In cultured cell lines, inhibition of protein synthesis leads to the rapid loss of silencer complexes, with a concomitant increase in both enhancer complexes and MHC class I RNA. From these data, we conclude that a labile silencer factor competes with a constitutively expressed, stable enhancer factor for overlapping DNA-binding sites; the relative abundance of the silencer factor contributes to establishing steady-state levels of MHC class I gene expression.     

Sequence variations within PrfA DNA binding sites and effects on Listeria monocytogenes virulence gene expression

Reporter gene fusions were used to investigate the contributions of PrfA DNA binding sites to Listeria monocytogenes virulence gene expression. Our results suggest that the DNA sequence of PrfA binding sites determines the levels of expression of certain virulence genes, such as hly and mpl. Other virulence genes, such as actA and plcB, may depend upon additional factors for full regulation of gene expression.     

Gene regulation in response to DNA damage

To deal with different kinds of DNA damages, there are a number of repair pathways that must be carefully orchestrated to guarantee genomic stability. Many proteins that play a role in DNA repair are involved in multiple pathways and need to be tightly regulated to conduct the functions required for efficient repair of different DNA damage types, such as double strand breaks or DNA crosslinks caused by radiation or genotoxins. While most of the factors involved in DNA repair are conserved throughout the different kingdoms, recent results have shown that the regulation of their expression is variable between different organisms. In the following paper, we give an overview of what is currently known about regulating factors and gene expression in response to DNA damage and put this knowledge in context with the different DNA repair pathways in plants. This article is part of a Special Issue entitled: Plant gene regulation in response to abiotic stress.     

DNA sequence adjacent to flagellar genes and evolution of flagellar-phase variation

A variety of factors, including phase variation, are involved in the regulation of flagellin gene expression in Salmonella sp. Flagellar-phase variation refers to the alternate expression of two different flagellin genes, H1 and H2. Site-specific inversion of a DNA segment adjacent to the H2 gene is responsible for switching expression. The segment includes the H2 promoter as well as the hin gene, which is required to mediate the inversion. Sequences in this region have homology with the corresponding sequences adjacent to the H1 flagellin gene in Salmonella sp. and the hag flagellin gene in Escherichia coli. The hin gene has also been shown to be homologous to the gin gene, which is found on bacteriophage Mu. To understand gene expression and the origin of these relationships, we have compared the DNA sequence adjacent to all three flagellin genes. The sequence data suggest a mechanism for the evolution of the hin-H2 locus.     

Control of the adipsin gene in adipocyte differentiation. Identification of distinct nuclear factors binding to single- and double-stranded DNA

The mouse adipsin gene encodes a serine protease with complement factor D activity that is expressed during adipocyte differentiation and is deficient in several animal models of obesity. We have investigated the regulation of adipsin expression by transfecting preadipocytes and adipocytes with plasmids containing the 5'-flanking region of the adipsin gene linked to a reporter gene. Constructions containing a -950 to +35 segment of the adipsin promoter were preferentially expressed in adipose cells. Deletion experiments identified a region from -114 to -38 which contains a large inverted repeat sequence and negatively regulated gene expression in preadipocytes and positively regulated expression in fat cells. Exonuclease III protection and gel retardation assays indicated that this region of duplex DNA had multiple binding sites for nuclear factors, several of which were preadipose specific. In addition, we also identified two distinct factors that bound symmetrically and sequence specifically to the inverted repeat sequences only when they were in single-stranded form; one of these factors was induced during adipocyte differentiation. These results suggest that the control of the adipsin promoter in differentiation may involve an interplay of multiple regulated DNA-binding proteins, including two that have preferential affinity for single-stranded DNA.     

基因表达调控中的核因子作用

利用病毒和动物系统对基因表达调控进行了广泛和深入的研究,发现了顺式作用调节序列,鉴定了序列专一的DNA结合蛋白,DNA与蛋白质相互识别、结合及蛋白质与蛋白质相互作用中起作用的蛋白质结构域,并且对调节蛋白基因的克隆和序列进行了分析．基因表达调控领域又由于植物基因调控机制取得的发展而得到了补充,文章着重介绍植物基因中的DNA与蛋白质间的作用;植物调节蛋白基因的分离;这一领域的今后研究方向及展望．