recD: the gene for an essential third subunit of exonuclease V

Exonuclease V (EC 3.1.11.5) of Escherichia coli, an enzyme with multiple activities promoting genetic recombination, has previously been shown to contain two polypeptides, the products of the recB and recC genes. We report here that the enzyme contains in addition a third polypeptide (alpha) with a molecular mass of about 58 kDa. The alpha polypeptide is not synthesized by a class of mutants (previously designated recB) lacking the nuclease activity of exonuclease V but retaining recombination proficiency. The gene, recD, coding for the alpha polypeptide is located near recB in the order thyA-recC-ptr-recB-recD-argA on the E. coli chromosome. The recB and recD genes appear to be governed by a common promoter to the left of recB; a weaker promoter appears to govern recD alone. In the light of these results we discuss the relation between the structure and function of the three polypeptides of exonuclease V, hereby alternatively designated RecBCD enzyme.