Stage 0 sporulation (spo0) mutants of Bacillus subtilis are defective in the signal transduction system initiating sporulation. Two of the products of these genes, Spo0A and Spo0F, are related to response regulator components of two-component regulatory systems used to control environmental responses in bacteria. The Spo0F response regulator was found to be the primary substrate for phosphorylation by the sporulation-specific protein kinase, KinA. Phosphorylated Spo0F was the phosphodonor for a phosphotransferase, Spo0B, which transferred the phosphate group to the second response regulator, the transcription regulatory protein Spo0A. This phosphorelay provides a mechanism for signal gathering from several protein kinases using Spo0F as a secondary messenger. These divergent signals are integrated through Spo0B phosphotransferase to activate the Spo0A transcription factor. This system provides for many levels of control to prevent capricious induction of sporulation.