A novel membrane stress-response that blocks chromosomal replication by targeting the DnaA initiator via the ClpP protease
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In Escherichia coli , membrane-stress due to interrupted lipoprotein (Lpp) maturation impairs DNA replication and arrests cell growth. How the two disparate processes of Lpp maturation and DNA replication are connected remains unclear. We demonstrate that upon membrane-stress the Rcs stress-response pathway is activated and the replication initiator DnaA is lost, which explains the replication block. However, the Lon protease, a key regulator of the Rcs pathway, is not required for the DnaA loss. We further ruled out the involvement of (p)ppGpp, one of the major mediators of stress-response in bacteria. However, upon deletion of the ClpP protease gene, DnaA was stable, replication initiated, and there was no cell-growth arrest. In wildtype cells, overexpression of DnaA was lethal even without the membrane-stress apparently from hyper-initiation. The hyper-initiation was restrained in Δ crp cells, and overexpression of DnaA was able to overcome the growth-arrest. Δ fis cells, which were earlier found resistant to the membrane-stress, showed DnaA stability and normal replication upon induction of the membrane-stress. We conclude that DnaA loss suffices to explain the growth-arrest upon the membrane-stress. The stress-response pathway described here appears novel because of its independence from Lon and (p)ppGpp, which have been implicated in other stress-responses that block DNA replication.
Significance
The seminal observation that DNA replication-stress can block cell division in E. coli (SOS response), introduced the concept of checkpoint control in the cell cycle. Here, we describe a novel checkpoint control that functions in the opposite direction: membrane-stress causing replication block. We show how two apparently unrelated outcomes of reducing a membrane phospholipid, accumulation of a precursor lipoprotein (pLpp) and block of replication initiation, could be linked. The pLpp accumulation stresses the membrane that causes a response culminating in activating the ClpP protease that blocks replication by targeting the initiator DnaA. DnaA being vital and highly conserved, the detail understanding of the response pathway is likely to open new avenues to treat bacterial infection.
