CELL-CYCLE CHECKPOINTS IN BACTERIA

When DNA replication is interrupted in bacteria. a specific inhibitor (SfiA), a component of the SOS system, is synthesised which transientl y blocks cell division. This is the prototype, dispensable, cell cycle checkpoint, essential for maximal survival under a particular stress. In contrast. no process specifically signalling the termination of ch romosomal replication to activate the subsequent division event. which might be termed an essential checkpoint. has yet been demonstrated. I n E coli. a specific mechanism is apparently required to reactivate re plication forks blocked by damage. but its molecular basis is unclear. induction of the stringent response. mediated by RclA via the level o f ppGpp. presumably to optimise macromolecular synthesis according to the availability of nutrients. activates a control system which inhibi ts DNA replication in both E coli and B subtilis. In E toll. this bloc ks new rounds of initiation Lit oriC. although the mechanism is not cl ear. Conversely, initiation is not blocked in B subtilis. but replicat ion is blocked apparently at a number of distinct sites 100-200 kb dow nstream and either side of oriC. This nutrient-dependent replicating c heckpoint specifically requires RTP, the chromosomal terminator protei n, and new evidence indicates that specific RTP binding sites ma be in volved in this post-initiation control mechanism. A similar post-initi ation control mechanism appears to block replication reversibly after premature initiation in B subtilis. indicating that this system may ha ve a dual function, limiting replication in starvation conditions anti as a mechanism to compensate for premature initiations.