Amber mutants of bacteriophage T4 defective in deoxycytidine diphosphatase and deoxycytidine triphosphatase. On the role of 5-hydroxymethylcytosine in bacteriophage deoxyribonucleic acid.

Infection of Escherichia coli B with T4 amber mutants in gene 56 fails to cause the appearance of either deoxycytidine trkghosphatase or deoxycytidine diphosphatase, and little or no DNA synthesis results. Infection of E. coli B with a 7:3 mixture of a gene 56 amber mutant and wild type T4 results in the appearance of only 30% as much dCTPase activity as normal but gives a 100% yield of mixed phage. This indicates that dCTPase is present in at least a 3-fold excess in the wild type infection. Escherichia coli W4597, shown by others to support the synthesis by wild type ‘I4 of DNA in which the bydroxymethylcytosine is virtually unglucosylated, also fails to support the synthesis of DNA upon infection with a gene 56 amber mutant. This and other evidence indicate that in ‘I4 DNA a crucial function of hydroxymethylcytosine, whether glucosylated or not, is the protection of progeny phage DNA against nuclease activity that degrades the cytosine-containing DNA of the host.