E. coli dnaZ gene product is required for conversion of phiX174, fd, and ST-1 single-stranded phage DNAs to duplex DNAs in vitro. This protein has been purified about 5000-fold. It functions in the elongation of RNA- or DNA-primed single-stranded DNA that is catalyzed by DNA polymerase III(DNA nucleotidyltransferase; deoxynucleosidetriphosphate: DNA deoxynucleotidyltransferase; EC 2.7.7.7) in conjunctions with two other E. coli protein preparations referred to as DNA elongation factors I and III. It also functions in similar reactions catalyzed by DNA polymerase II in combination with E. coli DNA binding protein and DNA elongation factors I and III.