Kroos L, Zhang B, Ichikawa H, Yu Y T
Department of Biochemistry, Michigan State University, East Lansing 48824, USA.
Mol Microbiol. 1999 Mar;31(5):1285-94. doi: 10.1046/j.1365-2958.1999.01214.x.
When starved, Bacillus subtilis undergoes asymmetric division to produce two cell types with different fates. The larger mother cell engulfs the smaller forespore, then nurtures it and, eventually, lyses to release a dormant, environmentally resistant spore. Driving these changes is a programme of transcriptional gene regulation. At the heart of the programme are sigma factors, which become active at different times, some only in one cell type or the other, and each directing RNA polymerase to transcribe a different set of genes. The activity of each sigma factor in the cascade is carefully regulated by multiple mechanisms. In some cases, novel proteins control both sigma factor activity and morphogenesis, co-ordinating the programme of gene expression with morphological change. These bifunctional proteins, as well as other proteins involved in sigma factor activation, and even precursors of sigma factors themselves, are targeted to critical locations, allowing the mother cell and forespore to communicate with each other and to co-ordinate their programmes of gene expression. This signalling can result in proteolytic sigma factor activation. Other mechanisms, such as an anti-sigma factor and, perhaps, proteolytic degradation, prevent sigma factors from becoming active in the wrong cell type. Accessory transcription factors modulate RNA polymerase activity at specific promoters. Negative feedback loops limit sigma factor production and facilitate the transition from one sigma factor to the next. Together, the mechanisms controlling sigma factor activity ensure that genes are expressed at the proper time and level in each cell type.
饥饿时，枯草芽孢杆菌会进行不对称分裂，产生两种命运不同的细胞类型。较大的母细胞会吞噬较小的前芽孢，然后对其进行滋养，最终裂解以释放出一种休眠的、具有环境抗性的孢子。驱动这些变化的是一个转录基因调控程序。该程序的核心是σ因子，它们在不同时间变得活跃，有些仅在一种细胞类型中活跃，每种σ因子都指导RNA聚合酶转录不同的一组基因。级联反应中每个σ因子的活性都受到多种机制的严格调控。在某些情况下，新型蛋白质既控制σ因子的活性又控制形态发生，将基因表达程序与形态变化协调起来。这些双功能蛋白质，以及其他参与σ因子激活的蛋白质，甚至σ因子本身的前体，都被靶向到关键位置，使母细胞和前芽孢能够相互通信并协调它们的基因表达程序。这种信号传导可导致σ因子的蛋白水解激活。其他机制，如抗σ因子以及可能的蛋白水解降解，可防止σ因子在错误的细胞类型中变得活跃。辅助转录因子在特定启动子处调节RNA聚合酶的活性。负反馈回路限制σ因子的产生，并促进从一种σ因子向下一种σ因子的转变。总之，控制σ因子活性的机制确保基因在每种细胞类型的适当时间和水平表达。
