Escherichia coli DNA helicase II is active as a monomer

Helicases are thought to function as oligomers (generally dimers or hexamer s), Here me demonstrate that although Escherichia coil DNA helicase II (Uvr D) is capable of dimerization as evidenced by a positive interaction in the yeast two-hybrid system, gel filtration chromatography, and equilibrium se dimentation ultracentrifugation (K-d = 3.4 mu M), the protein is active in vivo and in vitro as a monomer, A mutant lacking the C-terminal 40 amino ac ids (UvrD Delta 40C) failed to dimerize and yet was as active as the wild-t ype protein in ATP hydrolysis and helicase assays. In addition, the uvrD De lta 40C allele fully complemented the loss of helicase II in both methyl-di rected mismatch repair and excision repair of pyrimidine dimers, Biochemica l inhibition experiments using wild-type UvrD and inactive UvrD point mutan ts provided further evidence for a functional monomer. This investigation p rovides the first direct demonstration of an active monomeric helicase, and a model for DNA unwinding by a monomer is presented.