Regulatory pathways controlling cell division after DNA damage in

All cells must coordinate DNA replication with cell division in order to faithfully propagate whole chromosomes to daughter cells. During episodes of DNA damage, cells often delay division until the lesions have been repaired and replication has completed. The paradigm for the bacterial response to DNA damage is the transcriptional induction of "SOS" genes, and many organisms encode an SOS-induced cell division inhibitor. However, the mechanistic details of division inhibition are understood only in the y-proteobacterium E. coli, and it is unclear whether there are SOS-independent modes of division inhibition. I have studied the DNA damage response in the u-proteobacterium Caulobacter crescentus and identified two damage-induced cell division inhibitors. sidA is an SOS-induced division inhibitor whereas didA is induced in an SOS-independent fashion. Unlike most division inhibitors, SidA and DidA do not disrupt the localization of the cell division scaffold FtsZ or any other component of the cell division machinery or "divisome". Instead, SidA and DidA target the lateacting division proteins FtsW, FtsI, and FtsN to prevent divisome constriction, demonstrating that divisome components other than FtsZ can serve as regulatory targets. I have characterized mutations infts W andftsI which suppress the activities of both inhibitors, likely by causing cells to divide hyperactively. These results suggest that the FtsW/FtsI/FtsN subcomplex serves as an important regulatory node and may play an unexpected role in triggering divisome constriction in Caulobacter. I show that cells require at least one inhibitor to properly delay division following DNA damage, as cells lacking both inhibitors divide prematurely and suffer a viability defect in the presence of the DNA damaging agent mitomycin C (MMC). This finding suggests that some degree of redundancy exists within the Caulobacter response to MMC. Finally, I describe ongoing experiments which explore the origins of the SOS-independent induction of didA. Thesis supervisor: Michael T. Laub Title: Associate Professor of Biology