Artificial activation of both σH and Spo0A in Bacillus subtilis enforced initiation of spore development at the vegetatively growing phase.

Abstract

When Bacillus subtilis cells face environmental deterioration, such as exhaustion of nutrients and an increase in cell density, they form spores. It is known that phosphorylation of Spo0A and activation of σ^H are key events at the initiation of sporulation. However, the initiation of sporulation is an extremely complicated process, and the relationship between these two events remains to be elucidated. To determine the minimum requirements for triggering sporulation initiation, we attempted to induce cell sporulation at the log phase, regardless of nutrients and cell density. In rich media such as Luria-Bertani (LB) medium, the cells of B. subtilis do not sporulate efficiently, possibly because of excess nutrition. When the amount of xylose in the LB medium was limited, σ^H -dependent transcription of the strain, in which sigA was under the control of the xylose-inducible promoter, was induced, and the frequency of sporulation was elevated according to the decreased level of σ^A. We also employed a fusion of sad67, which codes for an active form of Spo0A, and the IPTG-inducible promoter. The combination of lowered σ^A expression and activated Spo0A allowed the cells in the log phase to stop growing and rush into spore development. This observation of enforced initiation of sporulation in the mutant strain was detected even in the presence of the wild-type strain, suggesting that only intracellular events initiate and fulfill spore development regardless of extracellular conditions. Under natural sporulation conditions, the amount of σ^A did not change drastically throughout growth. Mechanisms that sequester σ^A from the core RNA polymerase and help σ^H to become active exist, but this has not yet been elucidated.