Sandbox20: Difference between revisions
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[[Image:Theta Model of Replication.jpg | thumb | upright=1.7| right| Schematic representation of the replication termination fork and fork traps of the E. coli and B. subtilis chromosomes.]] | [[Image:Theta Model of Replication.jpg | thumb | upright=1.7| right| Schematic representation of the replication termination fork and fork traps of the E. coli and B. subtilis chromosomes.]] | ||
The replication of chromosomal DNA in most bacterial species occurs through a bidirectional mechanism, whereby two replication forks derived from the same origin of replication travel in opposite directions.<ref>PMID: 4562743</ref> This gives rise to the characteristic theta-shaped structure while the nascent DNA loop connecting the replication forks is generated. While the use of two [[1b77| replisomes]], one at each fork, accelerates replication, the phase of termination must be carefully coordinated. Specific sequences known as ''Ter sites'' lie just beyond the halfway point for each replisome. These halt the advance of replication forks in a direction-specific manner, and thereby form a ''replication fork trap''. As a result, each replisome can traverse only slightly more than half of the DNA before it is arrested. Together, the two sets of Ter sites define the terminus region where replication is terminated and the two forks fuse. | The replication of chromosomal DNA in most bacterial species occurs through a bidirectional mechanism, whereby two replication forks derived from the same origin of replication travel in opposite directions.<ref>PMID: 4562743</ref> This gives rise to the characteristic theta-shaped structure while the nascent DNA loop connecting the replication forks is generated. While the use of two [[1b77| replisomes]], one at each fork, accelerates replication, the phase of termination must be carefully coordinated. Specific sequences known as ''Ter sites'' lie just beyond the halfway point for each replisome. <ref>PMID: 8665860</ref> These halt the advance of replication forks in a direction-specific manner, and thereby form a ''replication fork trap''. As a result, each replisome can traverse only slightly more than half of the DNA before it is arrested. Together, the two sets of Ter sites define the terminus region where replication is terminated and the two forks fuse. <ref>PMID: 19233209 </ref> | ||
The function of the replication fork trap is enacted by the binding of termination proteins to Ter sites. In ''E. coli'', this is done by termination utilisation substance (''Tus''), and the functionally corresponding protein in ''B. subtilis'' is replication termination protein (''RTP''). The importance of these replication fork traps is still a matter of debate, as their inactivation does not interfere greatly with cell cycle progression<ref>PMID: 6442251</ref>, nor does it affect the well-being of daughter cells following cell division. However, the fact that the structurally and sequentially dissimilar Tus and RTP proteins both bind to ill-conserved Ter sites to elicit the same biological function suggests an evolutionary pressure towards a termination mechanism that involves replication fork traps. It has been noted that the majority of genes in the ''B. subtilis'' genome have their promoters proximal to the origin of replication. Terminating replication before the replisome reaches the promoters may help the smooth running of transcriptional processes <ref>PMID:19019156 </ref> | The function of the replication fork trap is enacted by the binding of termination proteins to Ter sites. In ''E. coli'', this is done by termination utilisation substance (''Tus''), and the functionally corresponding protein in ''B. subtilis'' is replication termination protein (''RTP''). <ref>PMID: 7867058</ref> The importance of these replication fork traps is still a matter of debate, as their inactivation does not interfere greatly with cell cycle progression<ref>PMID: 6442251</ref>, nor does it affect the well-being of daughter cells following cell division. However, the fact that the structurally and sequentially dissimilar Tus and RTP proteins both bind to ill-conserved Ter sites to elicit the same biological function suggests an evolutionary pressure towards a termination mechanism that involves replication fork traps. It has been noted that the majority of genes in the ''B. subtilis'' genome have their promoters proximal to the origin of replication. <ref>PMID: 2118869</ref> Terminating replication before the replisome reaches the promoters may help the smooth running of transcriptional processes. <ref>PMID: 19019156</ref> | ||
During fork fusion, it is proposed that the fork arriving at the permissive face of the replication fork trap displaces the terminator protein and causes the disassembly of the replisome. A combination of helicase, toposiomerase, polymerase and ligase then fill the remaining gaps to make a chromosome dimer with the two full-length chromosomes joined. RecQ, topoisomerase III, single-strand binding protein (SSBP) and DNA polymerase I are said to play a role in this process <REF>PMID:18570879 </REF>. Finally, site-specific recombination takes place at the dif site to produce two identical monomeric chromosomes <ref>PMID:15554958 </ref>. | During fork fusion, it is proposed that the fork arriving at the permissive face of the replication fork trap displaces the terminator protein and causes the disassembly of the replisome. A combination of helicase, toposiomerase, polymerase and ligase then fill the remaining gaps to make a chromosome dimer with the two full-length chromosomes joined. RecQ, topoisomerase III, single-strand binding protein (SSBP) and DNA polymerase I are said to play a role in this process <REF>PMID:18570879 </REF>. Finally, site-specific recombination takes place at the dif site to produce two identical monomeric chromosomes <ref>PMID:15554958 </ref>. | ||