Sandbox20: Difference between revisions

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===Replication Termination Activity===
===Replication Termination Activity===


Tus binds to a conserved cytosine residue which is not base paired <scene name='Sandbox20/Tus/13'>as shown by clicking here</scene>. The interactions between residues of the Tus protein and this unpaired cytosine nucleotide are shown in more detail <scene name='Sandbox20/Tus/9'>by clicking here</scene>.
The displacement of a <scene name='Sandbox20/Tus/13'>single conserved cytosine</scene> nucleotide from the double helix determines the polarity of fork arrest. (Ref!) This is located at the edge of the non-permissive face, and defines the point at which helicase activity is halted. Upon interaction with Tus the cyotsine is no longer base-paired, and is instead associated with residues within a well-defined recognition pocket. The specific interactions which stabilise this are shown in this <scene name='Sandbox20/Tus/9'>model</scene>.  


Its ability to do this depends on the conserved glutamate residue E49.
Before the structure of the Tus-Ter complex was determined, mutation of Glu49 was shown to eliminate anti-helicase activity without affecting DNA binding. This could not be explained by the [[1ecr|original crystal structure]] as it is not located close enough to make direct contact with the conserved cytosine. However, the [[2ewj|more recent structure]] revealed that this is due the water-mediated hydrogen bond formed between it and the adenine residue adjacent to cytosine. It is therefore likely, that the interaction is necessary to compensate for the disrupted H-bonding in the nucleotide adjacent to the displaced cytosine, as shown in this <scene name='Sandbox20/Tus/18'>model</scene>.
"The crystal structure of the Tus-Ter lock shows that Glu49 of Tus makes a water-mediated hydrogen bond with the 50- phosphate of the displaced A(7) nucleotide residue, and it would thus be expected to be partially defective in formation of the locked species."
This is shown <scene name='Sandbox20/Tus/18'>by clicking here</scene>
 
E49K does not affect DNA binding but does affect anti-helicase activity in the ‘trapped’ complex E49 makes an indirect hydrogen bond to the phosphate of the ‘displaced’ nucleotide.
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