A DNA fragment from the origin of single-strand to double-strand DNA replication of bacteriophage fd.

A specific complex is formed between fd DNA, Escherichia coli DNA unwinding protein, and RNA polymerase (EC 2.7.7.6; nucleosidetriphosphate:RNA nucleotidyltransferase) during the first steps in the conversion of the single-stranded viral DNA to the double-stranded replicative form. In this complex a unique DNA fragment of about 120 nucleotides is protected against nuclease digestion. Both the requirements for its isolation and its position on the map of the phage genome indicate that the fragment contains the origin of single-strand to double-strand DNA replication. The isolated DNA fragment possesses double-strand-like characteristics, which protect it from being covered by the DNA unwinding protein and thus indirectly positions the RNA polymerase to the origin of replication.