The nonessential T4 genes uvsX and uvsY are involved in DNA repair and general recombination. Using newly isolated amber mutants of these genes, we have identified the gene products (gp) by sodium dodecyl sulfate (SDS)/polyacrylamide gel electrophoresis. Their relative molecular masses are 39 000 and 16000, respectively. In the normal wild-type infection process they are produced early but not late in infection. Their synthesis continues for a longer period when DNA synthesis is blocked. We have developed procedures to isolate these gene products at a purity of more than 95% for gpuvsX and at 70% for gpuvsY, as judged by SDS/polyacrylamide gel electrophoresis and staining with Coomassie brilliant blue dye. The purification procedures suggest that these products may be membrane proteins. Using both an agarose gel assay and electron microscopy, we find that the product of the gene uvsX catalyzes the assimilation of a linear single-stranded fd DNA fragment into superhelical double-stranded fd DNA (RFI). The reaction requires ATP and Mg2+ besides substrate DNAs and uvsX protein.The T4 uvsX protein therefore is similar to the Escherichia coli r e d protein in molecular size and function, but differs in antigenic property.During the infection cycle of bacteriophage T4, general recombination is an essential process for continuous DNA replication and for the repair of damaged DNA [l -41. Extensive studies on recombination of T4 have implicated nearly 20 genes in this process [5-121. Most of these genes are indispensable for phage reproduction, since they also participate in DNA replication [l, 4, 131 and maturation [14, 1.51. The other genes, including uvs W, uvsX and uvs Y , are dispensable for phage reproduction in laboratory host strains, although plating efficiencies, burst sizes and the formation of concatemeric DNA are reduced. Also, phage DNA replication in uvsX--or uvsY--infected cells initiated at the normal period with a normal rate, but the replication is arrested abruptly [ l l , 161.The process of general recombination comprises several steps [l, 171. Among these, the pairing of complementary strands is the most crucial. Since the product of the indispensable gene 32 has been shown to destabilize duplex DNA and to renature complementary strands [18], it could be supposed that gp32 catalyzes the crucial step of genetic recombination. However, the renaturation process [18] and the in vitro formation of recombinant DNA catalyzed by gp32 [19] required higher than physiological Mg2 + concentration. Strand pairing may induce de novo initiation of DNA replication by providing a 3'-strand terminus, and thus it would not be essential for primary replication but for secondary replication [2, 31. Genes that control such a function therefore may be dispensable when concatemeric DNA is available by other methods. We started to identify and purify gpuvsXand gpuvsY to study their function and found that gpuvsX catalyzed the assimilaCorrespondence to : T. Minagawa, Department of Botany, Faculty of Science, Kyoto University...