セミナー情報

In response to starvation, a Gram-positive soil bacterium Bacillus subtilis undergoes developmental changes that result in the transition from vegetative state to spore formation. Gene expression programs during vegetative growth and sporulation are distinct and thus serve as a simple model system for studying the cell differentiation process. It is widely believed that a starvation-induced sporulation signal is the key that activates the phosphate signal transduction pathway (phosphorelay) and subsequently turns on the gene network responsible for sporulation. Although the cellular processes that lead to spore formation have been studied intensively, how the cells decide to switch from the vegetative cycle to the sporulation cycle upon starvation remains unknown. Moreover, all previous efforts to identify the sporulation signal(s) have been unsuccessful. In this study, we attempted to address these long-standing problems by studying the sensor histidine kinase KinA, the first component of the phosphorelay. Using in vivo analysis, we found that the most amino-terminal subdomain of KinA, which is believed to be essential for signal sensing, is disposable. We then provided evidence that the amino-terminal domain is essential for forming a stable tetramer as a functional kinase, but possibly not for sensing an as-yet unknown signal. Furthermore, we found that a threshold level of KinA is necessary and sufficient to activate the sporulation gene network via the phosphorelay. Based on these results, we propose a new model in which a threshold level of the kinase acts as a developmental switch for sporulation initiation.