The master regulator for entry into sporulation in Bacillus subtilis becomes a cell-specific transcription factor after asymmetric division.

Gene transcription at the onset of sporulation in Bacillus subtilis is governed by Spo0A, a member of the response regulator family of transcription factors. Spo0A is traditionally viewed as the master regulator for entry into development. We now report that Spo0A continues to function after the initiation phase of sporulation and that it becomes a cell-specific transcription factor when the sporangium is divided into a mother cell and forespore. We observed that (1) Spo0A and Spo0A-directed gene transcription reached high levels in the mother cell; (2) an activated form of Spo0A impaired sporulation when produced in the forespore but not when produced in the mother cell; and (3) an inhibitor of Spo0A called Spo0A-N impaired sporulation and Spo0A-directed transcription when produced in the mother cell but not when produced in the forespore. Spo0A-N, which corresponds to the NH(2)-terminal domain of Spo0A, was shown to compete with the full-length response regulator for phosphorylation by the phosphorelay protein Spo0B. We propose that Spo0A is the earliest-acting transcription factor in the mother-cell line of gene expression and that in terms of abundance and transcriptional activity Spo0A may function predominantly as a cell-specific regulatory protein.