spo0 Genes, the Phosphorelay, and the Initiation of Sporulation

The cell responds to signals which it continually monitors in the environment, and when these environmental signals reach a point that indicates it is time to shut down division and begin sporulation, the initiation process begins. The signal transduction system, the phosphorelay, for the initiation of sporulation in Bacillus subtilis revolves around the transcription factor Spo0A. This chapter discusses the genetic and chemical structure of the spo0 loci and their products, along with other components of the signal transduction system, and try to provide an historic and mechanistic perspective of the phosphorelay and the function of the Spo0A transcription factor. The role of the phosphorelay is to produce Spo0A-P, the activated form of the transcription factor responsible for transcription of genes involved in the initial stages of sporulation. The multiple points of control of the flow of phosphate through the phosphorelay begin with the kinases, KinA and KinB. The possibility exists that KinA might not be subject to activation by an effector ligand acting as a signal from the environment, but rather KinA might be functionally inactive in autophosphorylation in the absence of Spo0F. This would place responsibility for the initiation of phosphate flow in the phosphorelay on transcriptional control of the spo0F gene. It seems possible that one role of increased Spo0A~-P concentration during the initial stages of sporulation might be to prevent reinitiation of chromosome replication, which could be accomplished by preventing dnaA transcription and/or binding at the origin to inhibit the reinitiation of DNA replication.