Isolation and preliminary characterization of amber mutants of bacteriophage phi W-14 defective in DNA synthesis.

Of 42 amber mutants of bacteriophage phi W-14, 6 were defective in DNA synthesis. Three of the mutants synthesized DNA in the nonpermissive host, but were defective in post-replicational modification of the DNA. The DNA synthesized by two of these mutants, am36 and am42, contained more thymine and less alpha-putrescinylthymine than did wild-type DNA; that synthesized by the third mutant, am37, contained the normal amount of thymine, no alpha-putrescinylthymine, and hydroxymethyluracil. The properties of these mutants suggested that the presence of the normal amount of alpha-putrescinylthymine in phi W-14 DNA was essential for the production of viable progeny. Three of the mutants, am6, am35, and am45, failed to synthesize DNA in the nonpermissive host. These mutants were analogous to the DNA off mutants of T4. Nonpermissive cells infected with DNA off mutants accumulated dATP, dGTP, dCTP, and hydroxymethyl dUTP, but not dTTP or alpha-putrescinyldeoxythymidine triphosphate, confirming that both thymine and alpha-putrescinylthymidine in phi W-14 DNA are formed from hydroxymethyluracil at the polynucleotide level. The synthesis of phi W-14 DNA is unusual because (i) thymine is formed from hydroxymethyluracil at the polynucleotide level, (ii) the hypermodification forming alpha-putrescinylthymine is essential, and (iii) thymine and alpha-putrescinylthymine must be made in the correct proportions. Complementation tests showed that the mutants defined three genes involved in DNA polymerization and two genes involved in post-replicational modification.