ALTERATIONS IN THE FLOW OF ONE-CARBON UNITS AFFECT KINB-DEPENDENT SPORULATION IN BACILLUS-SUBTILIS

Endospore formation in Bacillus subtilis is primarily dependent on the phosphorylation of the key transcription factor Spo0A by two major ki nases, KinA and KinB, thought to be activated by distinct signals. Usi ng a strategy designed to detect mutations that specifically affect th e signalling pathway to KinB, we have isolated a Tn10 insertion mutant in one of two adjacent lrp-like genes coding for homologues of the Es cherichia 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 white tr ansferring the resulting one-carbon unit into the C1 pool through meth ylene tetrahydrofolate. Sporulation experiments performed in a series of supplemented media indicated that the role of SHMT in the KinB path way 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 i s involved in post-synthetic methylation reactions or biosynthesis of metabolites that serve to activate KinB. Truncated LrpA and LrpB pepti des that have retained the DNA-binding domain but have lost the C-term inal half of the protein appear to act as repressors of glyA transcrip tion and KinB-dependent sporulation. However, deletions of IrpA, IrpB or IrpAB have little effect on glyA transcription or sporulation throu gh KinB, suggesting that other effecters, such as additional trp homol ogues, may act in conjunction with LrpA and LrpB. Our results indicate that lrpA-lrpB together with the biosynthetic glyA gene lie on a comm on signalling pathway meant to activate the KinB sensor kinase.