Bacterial Replication Termination: Difference between revisions
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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. | 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. | ||
[[Image:Bidirectionalrep2.jpg | thumb | Bacterial replication fork]] | [[Image:Bidirectionalrep2.jpg | thumb | right | 500px | Bacterial replication fork []]] | ||
==Termination (''ter'') Sites== | ==Termination (''ter'') Sites== | ||
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==Biological Significance== | ==Biological Significance== | ||
The role of the replication fork arrest was | The role of the replication fork arrest was believed to be of great importance for the faithful termination of replication, segregation of chromosomes, and inheritance of a stable genome. However recent studies where the ''rtp'' and ''tus'' genes of ''B.subtilis'' and ''E.coli'', respectively, were knocked out, suggested that this role is dispensable [4]. Indeed, bacterial systems that have mutations within these genes can survive in the environment and appear identical in both growth rate and cell morphology compared to wildtype bacteria, suggesting that replication termination is not a requirement for cellular replication [4]. However as it has analogous presence between different bacteria this suggests that this mechanism is beneficial for these prokaryotes. It has recently been suggested that this form of termination may have roles including; aiding the co-ordination and optimization of recombination events preceding replication; preventing over-replication and preventing the harmful affects of clashes that can occur between replication-transcription due to the bacterial bias in gene orientation. | ||
As most genes are orientated towards the terminus, from the origin, if replication is not arrested, it progresses into regions being actively transcribed and collides into the transcription RNA polymerase [4]. It is also suggested that termination may occur by specific dif sites | As most genes are orientated towards the terminus, from the origin, if replication is not arrested, it progresses into regions being actively transcribed and collides into the transcription RNA polymerase [4]. It is also suggested that termination may occur by specific "''dif''" sites; conserved sites that are located near the terminus region that are involved in homologous recombination []. In fact the ''dif''-terminus hypothesis proposes that termination occurs at or near these sites, where after termination of the replication forks, the two recombinases, XerC and XerD (proteins originating from ''E.coli''), cause site-specific recombination at these ''dif''-sites, and that this would resolve the concatenated chromosomes and complete replication [7]. This mechanism implies that this replication termination by RTP and Tus proteins is merely advantageous to the bacteria and not necessary. | ||
==References== | ==References== | ||