The springboard for our work is a developmental checkpoint that regulates sporulation by cells of the bacterium Bacillus subtilis in response to replication status and DNA damage. When starved, cells of Bacillus subtilis can develop into spores capable of withstanding prolonged exposure to extreme environmental conditions. Cells initiate spore development by undergoing an asymmetric cell division to produce two specialized daughter cells, both of which are required for the production of a mature spore. The initiation of spore development is inhibited if cells are experiencing problems with DNA replication or have suffered DNA damage. Inhibiting the initiation of sporulation prevents the formation of daughter cells with incomplete or damaged copies of the genome.

We have recently identified the components of the signaling pathway that inhibits sporulation in response to replication defects and DNA damage. Replication defects and DNA damage are sensed through two convergent signaling pathways that activate transcription of a gene encoding an inhibitor of sporulation, Sda. Sda regulates sporulation by specifically inhibiting two kinases that are required to activate sporulation-specific gene expression in response to starvation signals.

We are currently focusing on several questions regarding the role of the Sda signaling pathway and the mechanisms by which it works, asking if it helps coordinate spore development with the cell cycle in cells that are replicating their chromosomes normally, if it contributes to cell viability and genome stability following transient perturbations in replication, how the signals regulating the pathway are integrated, and how Sda specifically recognizes its target kinases. We are also combining genetic approaches with fluorescence microscopy and biochemistry to identify and characterize other cell cycle-dependent signaling pathways in Bacillus subtilis regulating growth and development.

Understanding how these pathways work on a mechanistic level should provide insights into the developmental regulation of other bacteria, notably during pathogenesis and symbiosis, and identify potential targets for new antibiotics. It may also yield insights into those aspects of cell cycle regulation shared with eukaryotes due to the conservation of machinery involved in DNA replication, repair, and other key cellular processes.