In Escherichia coli the SOS response, induced by DNA-damaging treatments, includes two systems of cell division inhibition, SfiA and SfiC, which are thought to prevent cell division by interacting with the division protein FtsZ. It is shown here that SfiA-mediated division inhibition is readily reversible, even in the absence of de novo protein synthesis, suggesting that functional FtsZ molecules can be recovered from SfiA-FtsZ complexes. The action of SfiC, on the other hand, is essentially irreversible; induction by expression of the recA (Tif) mutation for 60 min results in division inhibition that continues for at least 180 min after the end of the induction period. An excess of the presumed target molecule FtsZ, furnished by a multicopy plasmid, suppresses the action of SfiA but not SfiC. Simultaneous induction of SfiA and SfiC results in irreversible division inhibition, showing that SfiC is epistatic to SfiA. The irreversibility of SfiC action is most readily accounted for by assuming that the SfiC product, unlike SfiA, is stable. The reversibility of SfiA action is slower in a lon mutant, in which the SfiA protein is partially stabilized. From the kinetics of division resumption in the absence of protein synthesis, we estimated the in vivo half-life of the SfiA protein to be 10 min in a lon+ strain and 170 min in a lon mutant
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