Cleavage and circularization of single-stranded DNA: a novel enzymatic activity of phi X174 A* protein

Purified phi X gene A* protein cleaves phi X single stranded DNA. The cleavage appears to be stoichiometric, whereby a gene A* protein molecule cleaves a phosphodiester bond and binds to the DNA fragment. The size of the cleavage product was inversely proportional to the ratio of A* protein to DNA in the reaction mixture. The cleavage of the DNA resulted in the formation of an A* protein - ssDNA complex identified on SDS-polyacrylamide gels and by banding in CsCl. An A* protein-ssDNA complex was isolated by gel filtration and shown to be active in a ligating reaction in which the two ends of the DNA fragment were joined to form a covalently closed circle. The joining reaction required Mg++ ions and was accompanied by the release of the protein from the DNA.