Nucleic Acids Research Advance Access originally published online on September 29, 2006
Nucleic Acids Research 2006 34(18):5280-5290; doi:10.1093/nar/gkl519
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Nucleic Acids Research, 2006, Vol. 34, No. 18 5280-5290
© 2006 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Computational Biology |
Hypothesis: bacterial clamp loader ATPase activation through DNA-dependent repositioning of the catalytic base and of a trans-acting catalytic threonine
Cold Spring Harbor Laboratory, 1 Bungtown Road PO Box 100, Cold Spring Harbor, NY 11724, USA
Tel: +1 516 367 6802; Fax: +1 516 367 8461; Email: neuwald{at}cshl.org
Received June 2, 2006. Revised July 5, 2006. Accepted July 6, 2006.
The prokaryotic DNA polymerase III clamp loader complex loads the ß clamp onto DNA to link the replication complex to DNA during processive synthesis and unloads it again once synthesis is complete. This minimal complex consists of one 
, one ' and three 
subunits, all of which possess an AAA+ module—though only the subunit exhibits ATPase activity. Here clues to underlying clamp loader mechanisms are obtained through Bayesian inference of various categories of selective constraints imposed on the and ' subunits. It is proposed that a conserved histidine is ionized via electron transfer involving structurally adjacent residues within the sensor 1 region of 's AAA+ module. The resultant positive charge on this histidine inhibits ATPase activity by drawing the negatively charged catalytic base away from the active site. It is also proposed that this arrangement is disrupted upon interaction of DNA with basic residues in implicated previously in DNA binding, regarding which a lysine that is near the sensor 1 region and that is highly conserved both in bacterial and in eukaryotic clamp loader ATPases appears to play a critical role. ATPases also appear to utilize a trans-acting threonine that is donated by helix 6 of an adjacent or ' subunit and that assists in the activation of a water molecule for nucleophilic attack on the phosphorous atom of ATP. As eukaryotic and archaeal clamp loaders lack most of these key residues, it appears that eubacteria utilize a fundamentally different mechanism for clamp loader activation than do these other organisms.