DNA binding and cleavage selectivity of the Escherichia coli DNA G:T-mismatch endonuclease (vsr protein)

The Escherichia coli vsr endonuclease recognises T:G base-pair mismatches in double-stranded DNA and initiates a repair pathway by hydrolysing the phosphate group 5 ' to the incorrectly paired T. The gene encoding the vsr endonuclease is next to the gene specifying the E. coli dent DNA-methyltransferase; an enzyme that adds CH3 groups to the first dC within its target sequence CC[A/T]GG, giving (CC)-C-5Me[A/T]GG. Deamination of the d(5Me)C results in CT[A/T]GG in which the first T is mis-paired with de and it is believed that the endonuclease preferentially recognises T:G mismatches within the dcm recognition site. Here, the preference of the vsr endonuclease for bases surrounding the T:G mismatch has been evaluated. Determination of specificity constant (k(st)/K-D; k(st) = rate constant for single turnover, K-D = equilibrium dissociation constant) confirms vsr's preference for a T:G mismatch within a dcm sequence i.e. C (T) under bar [A/T]GG (the underlined T being mis-paired with de) is the best substrate. However, the enzyme is capable of binding and hydrolysing sequences that differ from the dcm target site by a single base-pair (dcm star sites). Individual alteration of any of the four bases surrounding the mismatched T gives a substrate, albeit with reduced binding affinity and slowed turnover rates. The vsr endonuclease has a much lower selectivity for the dcm sequence than type II restriction endonucleases have for their target sites. The results are discussed in the light of the known crystal structure of the vsr protein and its possible physiological role. (C) 2001 Academic Press.