Sandbox20: Difference between revisions

The replication of chromosomal DNA in most bacterial species occurs through a bidirectional mechanism, in which two replication forks progress in opposite directions from the unique origin of replication <ref>PMID: 4562743</ref>. A [[1b77| replisome]] polymerizes new DNA at each of the replication forks, and this creates a characteristic theta structure in the chromosome as an identical DNA loop is created. Although the use of two active polymerase complexes accelerates replication, it means the phase of termination must be carefully coordinated. The rate at which each replisome progresses may be altered by a number of factors, such as the presence of DNA-associated proteins. Although the location of their union is consequently not necessarily directly opposite the origin, it is not random either. Specific DNA termination sequences called ''Ter'' sites halt the advance of a replication fork from one direction only by recruiting specific protein factors. The position and orientation of these within the chromosome define a ''replication fork trap'', which allows either replisome to traverse only slightly more than half of the total DNA before being halted. Together, they define the restricted ''terminus region'' where replication is terminated.

The significance of the fork trap is a matter of debate as its inactivation has no clear consequences <ref>PMID: 6442251</ref>, and fork fusion itself is in fact not essential, as the genome can be stably linearized <ref>PMID:17218953</ref>. It may be that prevention of one replication body from synthesizing the entire chromosome is necessary to prevent conflicts with transcription processes. The majority of genes within the B. subtilis genome are oriented such that their promoters are proximal to the origin of replication, suggesting a possible evolutionary pressure towards co-directionality of DNA replication and transcription  <ref>PMID: 2118869</ref>. The importance, however, is underlined by the apparent independent evolution of terminator proteins in E. coli and B. subtilis.