A unique sigma/anti-sigma system involved in dormancy and revival of bacterial sporangiospores

Abstract Bacterial spores convert from dormant to active cells upon exposure to environmental stimuli; however, their molecular mechanisms remain unclear. Here, we report a unique sigma/anti-sigma system involved in the dormancy and revival of sporangiospores in the filamentous bacterium Actinoplanes missouriensis . We propose that the sigma factor σ SsdA activates transcription of a subset of genes for the maintenance of dormancy, including the oxidative stress response, directly or indirectly in sporangiospores. The cognate anti-sigma factor SipA does not seem to be a switch for spore revival; rather, it seems to modulate σ SsdA function to achieve appropriate transcription levels of σ SsdA -dependent genes during and/or after sporangium formation. Both SipA and σ SsdA possess an anti-sigma factor antagonist domain, thereby enabling interactions between the anti-sigma factor domain of SipA and the anti-sigma factor antagonist domain of σ SsdA . We also identified a two-component regulatory system (RsdK-RsdR) indirectly activated by σ SsdA as a key factor that represses revival of spores, more specifically, initiation of sporangium dehiscence. Thus, RsdK-RsdR plays a pivotal role in a molecular mechanism that delays the initiation of awakening of sporangiospores. This study reveals a previously unknown aspect of the general biological phenomena of cell dormancy and awakening.