Protease‐deficient SOS constitutive cells have RecN‐dependent cell division phenotypes

Summary In Escherichia coli, after DNA damage, the SOS response increases the transcription (and protein levels) of approximately 50 genes. As DNA repair ensues, the level of transcription returns to homeostatic levels. ClpXP and other proteases return the high levels of several SOS proteins to homeostasis. When all SOS genes are constitutively expressed and many SOS proteins are stabilized by the removal of ClpXP, microscopic analysis shows that cells filament, produce mini‐cells and have branching protrusions along their length. The only SOS gene required (of 19 tested) for the cell length phenotype is recN. RecN is a member of the Structural Maintenance of Chromosome (SMC) class of proteins. It can hold pieces of DNA together and is important for double‐strand break repair (DSBR). RecN is degraded by ClpXP. Overexpression of recN+ in the absence of ClpXP or recN4174 (A552S, A553V), a mutant not recognized by ClpXP, produce filamentous cells with nucleoid partitioning defects. It is hypothesized that when produced at high levels during the SOS response, RecN interferes with nucleoid partitioning and Z‐Ring function by holding together sections of the nucleoid, or sister nucleoids, providing another way to inhibit cell division. Expression of high levels of SOS proteins in the absence of ClpXP protease causes problems in Z‐ring placement, assembly and or function such that long cells, mini‐cells and branched cells are produced. Critical to this phenotype is the inability to correctly segregate chromosomes. Overproduction of RecN (an SOS protein that is rapidly degraded by ClpXP) is able to reconstitute the majority of these phenotypes.