assisted by Sandra Duperrier
We are studying the sporulation process in Bacillus subtilis. Sporulation is a simple model of morphological development, involving sequential expression of hundreds of genes in response to nutrient starvation. Two identical chromosomes are segregated into two unequal compartments generated by a polar division. The smaller compartment is engulfed by the larger one, the mother cell, within which it becomes an organelle, the forespore. Specific genes are expressed by the two genomes in coordinate fashion. Consecutive morphological steps lead to the acquisition by the forespore of various resistance properties until a free mature spore is released in the external medium.
The successive steps of sporulation are controlled by a cascade of transcriptional factors that allow RNA polymerase to recognize new sets of promoters. Four sigma factors have been identified that are synthesized only during sporulation. In each case these sigma factors appear in inactive form and complex regulatory mechanisms allow their activation at a specific developmental stage in one of the two compartments. The whole sporulation process after the unequal division step appears as a dialogue between the two cells present in the sporulating bacterium, each cell sending to the other a signal inducing the switch to the next developmental stage at a specic time. This crisscross regulation ensures coordination of gene expression between the two cells. Our current work focuses on the apparition of the late sigma factor sigmaG in the forespore. sigmaG is encoded by a gene controlled by the early forespore sigma factor, sigmaF. However, sigmaG synthesis is delayed as compared to other genes induced by sigmaF. The molecular mechanisms leading to this delay are still mysterious and the trans-acting macromolecules involved have not been identified. We have discovered that one of the genes induced by sigmaF encodes a protein which is a potent inhibitor of sigmaG-mediated transcription. This small protein, Gin, which is present in many endospore formers, acts as an oligomer containing a zinc ion presumably coordinated by cysteine pairs conserved in all Gin orthologues. We have shown that Gin acts directly on sigmaG, without any intermediary partner, and that sigmaG inhibition requires an additional step beyond the initial interaction between the two proteins. sigmaG appears at the end of engulfment, indicating that the inhibitory controls acting on synthesis and activity of sigmaG are both lifted at that stage. Several genes are known to be involved in that activatory step, most of them being under the control of sigmaE, the early sigma factor in the mother cell. Our goal is to understand the function of their products and to unravel the molecular mechanisms of the dialogue between the mother cell and the forespore at that developmental stage.
Recent articles :
Karmazyn-Campelli, C., Rhayat, L., Carballido-López, R., Duperrier, S., Frandsen, N., and Stragier, P. 2008. How the early sporulation sigma factor sigmaF delays the switch to late development in Bacillus subtilis. Mol. Microbiol. 67: 1169-1180.
Rhayat, L., Duperrier, S., Carballido-López, R., Pellegrini, O., and Stragier, P. 2009. Genetic dissection of an inhibitor of the sporulation sigma factor sigmaG. J. Mol. Biol. 390: 835-844.