Bacterial Replication Termination: Difference between revisions
From Proteopedia
Jump to navigationJump to search
No edit summary |
No edit summary |
||
| Line 1: | Line 1: | ||
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 | 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 approximately 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 [2]. 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 [1]. 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 [1]. | ||
[[Image:Bidirectionalrep2.jpg | thumb | right | 500px | Bacterial replication fork [3]]] | [[Image:Bidirectionalrep2.jpg | thumb | right | 500px | Bacterial replication fork [3]]] | ||
| Line 16: | Line 16: | ||
Active RTP is a homodimer composed of 14.5 kDa subunits [8]. The structure of the protein has been determined to a 2.6 A resolution using X-ray crystallography [7]. The RTP protein contains three major structural domains for its specific functionality; DNA-binding, | Active RTP is a homodimer composed of 14.5 kDa subunits [8]. The structure of the protein has been determined to a 2.6 A resolution using X-ray crystallography [7]. The RTP protein contains three major structural domains for its specific functionality; DNA-binding, | ||
<scene name='User:Bianca_Varney/Bacterial_Replication_Termination/Dnab_interacting_domain/3'> | <scene name='User:Bianca_Varney/Bacterial_Replication_Termination/Dnab_interacting_domain/3'>DNA helicase</scene> and dimer-dimer interaction domains [7]. The RTP is organized into a dimer by the association of their | ||
<scene name='User:Bianca_Varney/Bacterial_Replication_Termination/Dimer_interaction_domain/1'>long α helices</scene> within the C-terminus [7]. The ‘<scene name='User:Bianca_Varney/Bacterial_Replication_Termination/Dna-binding/3'>winged helix</scene>’ is believed to be involved as the major DNA-binding domain; with the two helices slotting into the major grove and two β strands inserting into the minor grove [7]. This binding interaction is vastly different from the Tus-''ter'' interactions. | <scene name='User:Bianca_Varney/Bacterial_Replication_Termination/Dimer_interaction_domain/1'>long α helices</scene> within the C-terminus [7]. The ‘<scene name='User:Bianca_Varney/Bacterial_Replication_Termination/Dna-binding/3'>winged helix</scene>’ is believed to be involved as the major DNA-binding domain; with the two helices slotting into the major grove and two β strands inserting into the minor grove [7]. This binding interaction is vastly different from the Tus-''ter'' interactions. | ||
| Line 36: | Line 36: | ||
Two models were proposed to explain the mechanism of Tus activity generated by early experiments. The "clamp model" proposed that the Tus-''ter'' complex created a barrier that arrested the progression of the replication machinery from one direction but not the other, by DNA binding [12]. The "interaction model" suggested that a particular region of Tus specifically interacted with the progressing helicase, causing it to halt the fork, and this interaction would only be possible at one face of the protein [4]. The structure of the tus-DNA complex has recently been solved [12]. It suggests that the protein can block helicase approaching from one direction and not the other, without the necessity of specific Tus–helicase interactions. The Tus-''Ter'' complex could act as a physical barrier against the replication fork at the non-permissive face; the α helical regions protrude from the protein around the DNA and block the helicase from accessing the region tightly bound to the DNA [13]. On the other hand, when the helicase advances from the opposite direction it does not encounter the α helical barriers and can disrupt Tus-DNA binding by interrupting the interdomain β strands, causing Tus to be released [13]. This simple model is supported by studies where mutants were screened after exhibiting a reduction in their ability to arrest replication [13]. Most of the mutations occurred in the interdomain β-strands and none of these mutations occurred in the blocking surface that may contact the progressing helicase [13]. However it is important to note that a specific interaction between the blocking face and the helicase cannot be ruled out based on structural studies, and that it if present it may have a role to enhance the physical barrier’s effectiveness. | Two models were proposed to explain the mechanism of Tus activity generated by early experiments. The "clamp model" proposed that the Tus-''ter'' complex created a barrier that arrested the progression of the replication machinery from one direction but not the other, by DNA binding [12]. The "interaction model" suggested that a particular region of Tus specifically interacted with the progressing helicase, causing it to halt the fork, and this interaction would only be possible at one face of the protein [4]. The structure of the tus-DNA complex has recently been solved [12]. It suggests that the protein can block helicase approaching from one direction and not the other, without the necessity of specific Tus–helicase interactions. The Tus-''Ter'' complex could act as a physical barrier against the replication fork at the non-permissive face; the α helical regions protrude from the protein around the DNA and block the helicase from accessing the region tightly bound to the DNA [13]. On the other hand, when the helicase advances from the opposite direction it does not encounter the α helical barriers and can disrupt Tus-DNA binding by interrupting the interdomain β strands, causing Tus to be released [13]. This simple model is supported by studies where mutants were screened after exhibiting a reduction in their ability to arrest replication [13]. Most of the mutations occurred in the interdomain β-strands and none of these mutations occurred in the blocking surface that may contact the progressing helicase [13]. However it is important to note that a specific interaction between the blocking face and the helicase cannot be ruled out based on structural studies, and that it if present it may have a role to enhance the physical barrier’s effectiveness. | ||
Interestingly, RTP has been found to arrest replication in ''E.coli'' when bound to ''E. coli'' specific''ter'' sequences. This suggests that the Tus-''ter'' complex provides a physical barrier that is not specific to the replication fork [12]. However there is some evidence to suggest that RTP specifically recognizes the ''E.coli''DnaB helicase allowing it to functionally block replicative progression, and that Tus may act similarly [14]. Mutational analysis | Interestingly, RTP has been found to arrest replication in ''E.coli'' when bound to ''E. coli'' specific''ter'' sequences. This suggests that the Tus-''ter'' complex provides a physical barrier that is not specific to the replication fork [12]. However there is some evidence to suggest that RTP specifically recognizes the ''E.coli''DnaB helicase allowing it to functionally block replicative progression, and that Tus may act similarly [14]. Mutational analysis a contrahelicase region has shown that mutations within these regions abolish RTP's ability to arrest DnaB. This indicates that protein-protein interactions occur between these two proteins, and further structural analysis has identified that these amino acid region interacts with a hinge region on DnB helicase. These reports mean that specific surfaces of the termination proteins, RTP and Tus, could be recognizing the identical or variable surfaces of the helicases [13]. | ||
</StructureSection> | </StructureSection> | ||
