The Escherichia coli recQ gene, a member of the RecF recombination gene family, was set in an overexpression plasmid, and its product was purified to near-homogeneity.The purified RecQ protein exhibited a DNA-dependent ATPase and a helicase activity. Without DNA, no ATPase activity was detected. The capacity as ATPase cofactor varied with the type of DNA in the following order: circular single strand > linear single strand >» circular or linear duplex. As a helicase, RecQ protein displaced an annealed 71-base or 143-base singlestranded fragment from circular or linear phage M13 DNA, and the direction of unwinding seemed to be 3' --5' with respect to the DNA single strand to which the enzyme supposedly bound. Furthermore, the protein could unwind 143-base-pair bluntended duplex DNA at a higher enzyme concentration. It is concluded that RecQ protein is a previously unreported helicase, which might possibly serve to generate single-stranded tails for a strand transfer reaction in the process of recombination.The recQ gene of Escherichia coli (1) is a member of the so-called RecF gene family. The genes ofthis group, the other known members of which are recA, recF, recJ, recN, recO, recR, and ruv (for review, see refs. 2 and 3), share the common denominator that their mutations abolish conjugal recombination proficiency and UV resistance in a mutant lacking active exonuclease V (RecBCD enzyme) and exonuclease I (SbcB protein). In such a mutant, the conjugal recombination system operative in wild-type cells (the RecBCD pathway), which is also implicated in the postreplication recombinational repair of UV-damaged DNA (4), is no longer functional due to the loss of the key enzyme exonuclease V (see ref.2). To explain the conjugal recombination proficiency and UV resistance of the mutant, it is generally assumed that the absence of exonuclease I somehow activates a substitute system, the RecF pathway (5), which is dependent on the RecF family genes (2). In addition to the above assignments, recent work has revealed the roles for the RecF family genes in plasmid recombination (6)(7)(8).