A region near the C-terminal end of Escherichia coli DNA helicase II is required for single-stranded DNA binding

The role of the C terminus of Escherichia coli DNA helicase II (UvrD), a re gion outside the conserved helicase motifs, was investigated by using three mutants: UvrD Delta 107C (deletion of the last 107 C-terminal amino acids) , UvrD Delta 102C, and UvrD Delta 40C. This region, which lacks sequence si milarity with other helicases, may function to tailor UvrD for its specific in vivo roles. Genetic complementation assays demonstrated that mutant pro teins UvrD Delta 107C and UvrD Delta 102C failed to substitute for the wild -type protein in methyl-directed mismatch repair and nucleotide excision re pair. UvrD Delta 40C protein fully complemented the loss of helicase II in both repair pathways. UvrD Delta 102C and UvrD Delta 40C were purified to a pparent homogeneity and characterized biochemically. UvrD Delta 102C was un able to bind single-stranded DNA and exhibited a greatly reduced single-str anded DNA-stimulated ATPase activity in comparison to the wild-type protein (k(cat) 0.01% of the wild-type level). UvrD Delta 40C was slightly defecti ve for DNA binding and was essentially indistinguishable from wild-type Uvr D when single-stranded DNA-stimulated ATP hydrolysis and helicase activitie s were measured. These results suggest a role for a region near the C termi nus of helicase II in binding to single-stranded DNA.