Protein p3 is linked to the DNA of phage phi 29 through a phosphoester bond between serine and 5'-dAMP.

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Abstract
To investigate the role of protein p3 in bacteriophage .vphi. initiation of replication, the nature of the covalent linkage between protein p3 and .vphi.29 DNA was studied. The protein-DNA compound was digested with micrococcal nuclease and Pronase resulting in a nucleotidyl-peptide that was further digested by alkaline phosphatase and snake venom phosphodiesterase yielding 5''-dAMP. The DNA-protein linkage is sensitive to alkali. Treatment of the nucleotidyl-peptide with 0.1 M NaOH at 37.degree. C for 3 h after phosphatase digestion released 5''-dAMP. Hydrolysis of the nucleotidyl-peptide with 5.8 M HCl at 110.degree. C for 90 min yielded O-phosphoserine. These results, together with the sensitivity of the DNA-protein linkage to snake venom phosphodiesterase and its resistance to hydroxylamine, indicate that protein p3 is covalently linked at .vphi.29 DNA through a phosphoester bond between L-serine and 5''-dAMP, namely a O,5''-deoxyadenylyl-L-serine bond.