Replication fork arrest and termination of chromosome replication in Bacillus subtilis

Sporulation in Bacillus subtilis provided the first evidence for the presence of sequence-specific replication fork arrest (Ter) sites in the terminus region of the bacterial chromosome. These sites, when complexed with the replication terminator protein (RTP), block movement of a replication fork in a polar manner. The Ter sites are organized into two opposed groups which force the approaching forks to meet and fuse within a restricted terminus region. While the precise advantage provided to the cell through the presence of the so-called replication fork trap is not patently obvious, the same situation appears to have evolved independently in Escherichia coli. The molecular mechanism by which the RTP-Ter complex of B. subtilis (or the analogous but apparently unrelated complex in E. coli) functions is currently unresolved and subject to intense investigation. Replication fork arrest in B. subtilis, requiring RTP, also occurs under conditions of the stringent response at so-called STer sites that lie close to and on both sides of oriC. These sites are yet to be identified and characterized. How they are induced to function under stringent conditions is of considerable interest, and could provide vital clues about the mechanism of fork arrest by RTP-terminator complexes in general.