Xu H, Dingwall A, Shapiro L
Department of Molecular Biology, Albert Einstein College of Medicine, Bronx, NY 10461.
Proc Natl Acad Sci U S A. 1989 Sep;86(17):6656-60. doi: 10.1073/pnas.86.17.6656.
The Caulobacter crescentus flagellum is formed at a specific time in the cell cycle and its assembly requires the ordered expression of a large number of genes. These genes are controlled in a positive trans-acting hierarchy that reflects the order of assembly of the flagellum. Using plasmids carrying transcriptional fusions of either a neo or a lux reporter gene to the promoters of three flagellar genes representing different ranks in the hierarchy (the hook operon, a basal body gene flbN, and the flaO gene), we have measured the level of chimeric gene expression in 13 flagellar mutant backgrounds. Mutants in the hook operon or in basal body genes caused overproduction of both hook operon and basal body gene chimeric mRNAs, suggesting that negative regulation is superimposed on the positive trans-acting control for these early events in the flagellar hierarchy. Mutants in the structural genes and in genes involved in flagellar assembly had no effect on flaO expression, placing the flaO gene near the top of the hierarchy. However, flaO expression appears to be under negative control by two regulatory genes flaS and flaW. Negative control, as a response to the completion of specific steps in the assembly process, may be an important mechanism used by the cell to turn off flagellar gene expression once the gene product is no longer needed.
新月柄杆菌鞭毛在细胞周期的特定时间形成，其组装需要大量基因的有序表达。这些基因受正向反式作用层级调控，该层级反映了鞭毛组装的顺序。利用携带新霉素或荧光素酶报告基因与代表层级中不同等级的三个鞭毛基因（钩形操纵子、基体基因flbN和flaO基因）启动子转录融合的质粒，我们测量了13种鞭毛突变背景下嵌合基因的表达水平。钩形操纵子或基体基因的突变导致钩形操纵子和基体基因嵌合mRNA均过量产生，这表明在鞭毛层级中这些早期事件的正向反式作用控制之上叠加了负调控。结构基因和参与鞭毛组装的基因的突变对flaO表达没有影响，这表明flaO基因位于层级的顶部附近。然而，flaO表达似乎受两个调控基因flaS和flaW的负调控。负调控作为对组装过程中特定步骤完成的一种反应，可能是细胞在基因产物不再需要时关闭鞭毛基因表达的一种重要机制。
