The bond in the bacteriophage phi X174 gene A protein--DNA complex is a tyrosyl-5'-phosphate ester.

The bacteriophage phi X174 gene A protein cleaves the viral strand of the double-stranded replicative form (RF) DNA of the phage at a specific site, the origin. It leaves a free 3'-OH at nucleotide 4305 (G) of the phi X DNA sequence and binds covalently to the DNA. The nature and position of the covalent bond have been determined using the octadecadesoxyribonucleotide CAACTTG[32P]ATATTAATAAC. This octadecamer, which corresponds to nucleotides 4299-4316 of phi X viral DNA, is cleaved by gene A protein. Gene A protein is bound to the labelled phosphate via a tyrosyl residue, indicating that binding occurs to the nucleotide corresponding to 4306 (A) of the phi X viral DNA strand.