Endospore formation in Bacillus subtilis is primarily dependent on the phosphorylation of the key transcription factor Spo0A by two major kinases, KinA and KinB, thought to be activated by distinct signals. Using a strategy designed to detect mutations that specifically affect the signalling pathway to KinB, we have isolated a Tn10 insertion mutant in one of two adjacent lrp-like genes coding for homologues of the Escherichia coli leucine-responsive regulatory protein (Lrp) and another mutant in the glyA gene encoding the serine hydroxymethyl transferase (SHMT). SHMT catalyses interconversion of serine and glycine while transferring the resulting one-carbon unit into the C1 pool through methylene tetrahydrofolate. Sporulation experiments performed in a series of supplemented media indicated that the role of SHMT in the KinB pathway is to feed the pool of C1 units recruited for the biosynthesis of key metabolites, which include the methyl donor S-adenosyl-methionine (SAM). The results of experiments using L-ethionine suggest that SAM is involved in post-synthetic methylation reactions or biosynthesis of metabolites that serve to activate KinB. Truncated LrpA and LrpB peptides that have retained the DNA-binding domain but have lost the C-terminal half of the protein appear to act as repressors of glyA transcription and KinB-dependent sporulation. However, deletions of lrpA, lrpB or lrpAB have little effect on glyA transcription or sporulation through KinB, suggesting that other effectors, such as additional Lrp homologues, may act in conjunction with LrpA and LrpB. Our results indicate that lrpA-lrpB together with the biosynthetic glyA gene lie on a common signalling pathway meant to activate the KinB sensor kinase.