Bacterial Replication Termination: Difference between revisions

In most bacterial DNA replication initiation occurs at an origin where, due to the circular nature of the chromosome, the replication forks move bidirectionally to end at approxiametly 180 degrees away, at a specific sequence termini region [1]. Bacterial replication termination systems have been well studied in ''Eschericia coli'' and ''Bascillus subtilis''. In both systems a ''trans''-acting replication termination protein binds to a specific ''cis''-acting DNA sequences, the replication termini (''ter''), and the DNA-protein complex arrests the progression of replication forks. The terminator sites are orientated so that protein binding is asymmetric, allowing the complexes to block the replication machinery from only one direction while letting them proceed unimpeded from the other direction [2]. In this way they are said to act in a polar manner. The proteins involved in this termination are non-homologous and differ structurally in ''E.coli'' and ''B.subtilis'', although each contains similar contrahelicase activity and performs similar functions in arresting replication.  

==Termination (''ter'') Sites==

Replication is terminated in bacterial systems such as ''E.coli'' and ''B.subtilis'' by a "replication fork trap", studded with termination sites which causes the bidirectional forks to pause, encounter and fuse within a region called the terminus region. In ''E.coli'' the termination regions are spread across nearly half the chromosome compared to ''B.subtilis'' where they cover only ~10%. Termination regions are made up of two groups, opposite to each other, containing inverted sequences for the polar arrest of the replication helicase. In ''E.coli'' the 5 ''ter'' sites, J, G, F, B and C are arranged opposed to ''ter'' sites H, I, E, D and A, and can arrest the fork progressing in the clockwise direction and can block the anticlockwise direction, respectively. The replication fork progressing in a clockwise direction will encounter the ''terC'' site first and pause. If the fork progressing from the anticlockwise direction meets the clockwise fork while paused, replication is terminated, however if it does not meet its anti-fork it will proceed until it reaches the next termination site, ''terB'', where it will pause again, etc [8]. Therefore multiple ''ter'' sites are important as infrequently utilized backups, to ensure that the fork does not leave the terminus region, and that termination is completed. Multiple regions to entrap the replication fork means that if an inactivating mutation arises within a ''ter'' site, then arrest can still occur at another ''ter'' sequence [6].