Higgins D E, Nazareno E, DiRita V J
Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor 48109.
J Bacteriol. 1992 Nov;174(21):6974-80. doi: 10.1128/jb.174.21.6974-6980.1992.
Virulence gene expression in Vibrio cholerae is postulated to involve ToxR-dependent activation of the toxT gene followed by ToxT activation of virulence genes, including several of those involved in biogenesis of the toxin-coregulated pilus. ToxR is a transmembrane, DNA-binding protein which is a member of the OmpR subclass of two-component activator systems in bacteria. Data presented in this report demonstrate that ToxT is similar to the AraC family of transcriptional activators identified in a variety of gram-negative bacteria. The toxT open reading frame begins approximately 200 nucleotides from the end of the tcpF gene, which is part of a cluster of genes responsible for production of the toxin-coregulated pilus. Accumulation of toxT specific mRNA is ToxR dependent and is modulated by environmental conditions that modulate expression of the regulon. Within the intergenic region between tcpF and toxT is a potential stem-loop structure of an unusual nature which may play a role in regulating expression of toxT mRNA. Experiments with tcpF and toxT cloned behind a strong, constitutive promoter suggest that the two genes can be cotranscribed, but Northern (RNA) blot analysis of V. cholerae suggests that if they are, steady-state levels of their messages may be controlled by a posttranscriptional mechanism. Possible mechanisms for ToxR-dependent expression of toxT are discussed.
霍乱弧菌毒力基因的表达被推测涉及toxT基因的ToxR依赖性激活，随后是毒力基因的ToxT激活，包括参与毒素共调节菌毛生物合成的几个基因。ToxR是一种跨膜DNA结合蛋白，是细菌中双组分激活系统的OmpR亚类成员。本报告中的数据表明，ToxT与在多种革兰氏阴性细菌中鉴定出的AraC家族转录激活因子相似。toxT开放阅读框从tcpF基因末端约200个核苷酸处开始，tcpF基因是负责毒素共调节菌毛产生的基因簇的一部分。toxT特异性mRNA的积累依赖于ToxR，并受到调节调节子表达的环境条件的调控。在tcpF和toxT之间的基因间隔区内有一个性质异常的潜在茎环结构，它可能在调节toxT mRNA的表达中起作用。将tcpF和toxT克隆到一个强组成型启动子后面的实验表明这两个基因可以共转录，但霍乱弧菌的Northern（RNA）印迹分析表明，如果它们共转录，其mRNA的稳态水平可能受转录后机制控制。文中讨论了toxT的ToxR依赖性表达的可能机制。
