| Endospore formation in the Gram-positive bacterium Bacillus subtilis may be viewed as the ultimate example of adaptation of bacteria in response to less than optimal growth conditions. Several physiological and environmental signals are required for the initiation of endospore. Starvation for a carbon, nitrogen, or a phosphorous source is the primary signal needed to initiate sporulation. In addition, cells must be at high density and capable of undergoing DNA replication and cytokinesis.; Sporulation in B. subtilis can be triggered by catabolite limitation. Conversely, carbon and nitrogen source excess, DNA synthesis blockage, and defects in chromosomal partitioning and cell division effectively inhibit stage II sporulation gene expression and sporulation. A common feature of inhibition of sporulation under these conditions appears to be the in vivo regulation of Spo0A∼P production and/or accumulation. Spo0A is a pivotal transcription regulator that becomes activated by phosphorylation, when the environment is no longer conducive to active vegetative growth, through the phosphorelay, which is an expanded form of a two component regulatory system. Recent studies have implicated a pH-sensing mechanism, involving AbrB, the TCA cycle, Spo0K, and σH in the control of catabolite repression of sporulation. We have determined that in excess glucose, stage 0 gene expression including spo0A, spo0F, kinA, components of the phosphorelay, spo0H, gene encoding SigH, and the sinIR operon is inhibited. Mutations in abrB or the addition of a pH stabilizer, MOPS (pH 7.5), did not relive the catabolite repression of stage 0 gene expression or sporulation, suggesting that a pH-sensing mechanism may not be involved in the catabolite repression of sporulation. On the other hand, loss of function mutations in hpr ( scoC) restored sporulation gene expression and sporulation in the presence of excess glucose, indicating that catabolite repression is mediated by Hpr.; Ireton and Grossman have reported that blocking DNA synthesis leads to inhibition of stage II sporulation gene expression and sporulation and that two, a RecA-dependent and a RecA-independent, signal transduction pathways may couple DNA replication to the onset of sporulation. They have further shown that perturbations in chromosome partitioning and cell division also lead to inhibition of stage II gene expression and sporulation. They have postulated that a mechanism, involving the spo0J operon, may function as a checkpoint, coupling sporulation to chromosome partitioning and cytokinesis in B. subtilis. We have determined that the phosphorelay is also a primary target that couples sporulation to DNA replication. However, we have found only one pathway, independent of RecA, involving Hpr (ScoC) and the phosphorelay that also mediates the signal transduction between DNA replication and initiation of sporulation. Moreover, the signaling pathway that couples cell division and sporulation is also incorporated in the phosphorelay. Perturbations in cell division adversely affect the phosphate flow to Spo0A through the phosphorelay. However, loss of function mutations in hpr does not restore stage 0 gene expression or sporulation, suggesting that a different pathway may couple cell cycle events to sporulation. We have also determined that the role of the spo0J operon and the recA on stage II sporulation gene regulation, observed by Ireton and Grossman, is downstream from the point of signal transduction, the phosphorelay. |
