Bacteriophage phi X 174 A protein binds in vitro to the phage phi X 174 DNA by a phosphodiester bond via a tyrosine residue.

Nuclease digestion of the phi X 174 A* protein covalent complex with /P32/ - phi X 174 phage DNA results in the transfer of 32p-label to the protein. Acid hydrolysis of the 32p-labelled A* protein yields (4)0-phosphotyrosine.