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 transiently 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 chromosomal replication to activate the subsequent division event, which might be termed an essential checkpoint, has yet been demonstrated. In E coli, a specific mechanism is apparently required to reactivate replication forks blocked by damage, but its molecular basis is unclear. Induction of the stringent response, mediated by RelA via the level of ppGpp, presumably to optimise macromolecular synthesis according to the availability of nutrients, activates a control system which inhibits DNA replication in both E coli and B subtilis. In E coli, this blocks new rounds of initiation at oriC, although the mechanism is not clear. Conversely, initiation is not blocked in B subtilis, but replication is blocked apparently at a number of distinct sites 100-200 kb downstream and either side of oriC. This nutrient-dependent replicating checkpoint specifically requires RTP, the chromosomal terminator protein, and new evidence indicates that specific RTP binding sites may be involved in this post-initiation control mechanism. A similar post-initiation control mechanism appears to block replication reversibly after premature initiation in B subtilis, indicating that this system may have a dual function, limiting replication in starvation conditions and as a mechanism to compensate for premature initiations.