No abstract

A general mechanism by which proteins locate their target sites within large domains of DNA is a one-dimensional facilitated diffusion process in which the protein scans DNA in a nonspecifically bound state. An electrostatic contribution to this type of mechanism has been previously established. This study was designed to question whether other characteristics of a protein's structure might contribute to the scanning mechanism of target site location. In this regard, T4 endonuclease V was shown to establish an ionic strength dependent monomer-dimer equilibrium in solution. A protein dimer interaction site was postulated to exist along a putative alpha-helix containing amino acid residues 54-62. The conservative substitutions of Phe-60----Leu-60 and Phe-59, Phe-60----Leu-59, Leu-60 resulted in mutant enzymes which remained in the monomeric state independent of the ionic strength of the solution. The target site location mechanism of these mutants has also been altered. Under conditions where wild-type endonuclease V processively scans nontarget DNA, the target location mechanism of the monomeric mutant proteins was shifted toward a less processive search. This decrease in the processivity of the mutants was especially surprising because the nontarget DNA binding affinity was found to be significantly increased. Thus, an additional component of the endonuclease V DNA scanning mechanism appears to be the formation of a stable endonuclease V dimer complex.

Facilitated one-dimensional diffusion is a general mechanism utilized by several DNA-interactive proteins as they search for their target sites within large domains of nontarget DNA. T4 endonuclease V is a protein which scans DNA in a nonspecifically bound state and processively incises DNA at ultraviolet (UV)-induced pyrimidine dimer sites. An electrostatic contribution to this mechanism of target location has been established. Previous studies indicate that a decrease in the affinity of endonuclease V for nontarget DNA results in a decreased ability to scan DNA and a concomitant decrease in the ability to enhance UV survival in repair-deficient Escherichia coli. This study was designed to question the contrasting effect of an increase in the affinity of endonuclease V for nontarget DNA. With this as a goal, a gradient of increasingly basic amino acid content was created along a proposed endonuclease V-nontarget DNA interface. This incremental increase in positive charge correlated with the stepwise enhancement of nontarget DNA binding, yet inversely correlated with enhanced UV survival in repair-deficient E. coli. Further analysis suggests that the observed reduction in UV survival is consistent with the hypothesis that enhanced nontarget DNA affinity results in reduced pyrimidine dimer-specific recognition and/or binding. The net effect is a reduction in the efficiency of pyrimidine dimer incision.