Isolation of mutants of bacteriophage T4 unable to induce thymidine kinase activity

New mutants of T4 have been isolated by using a strain of Escherichia coli lacking thymidine kinase activity. These T4 mutants, designated tk, are able to grow on this E. coli strain under light on plates containing 5-bromodeoxyuridine and were all found to be unable to induce thymidine kinase (ATP: thymidine 5'-phosphotransferase, EC 2.7.1.21). All of these tk mutants fall into one complementation group which maps just to the right of rI on the standard T4 genetic map, far from most other genes coding for enzymes involved in pyrimidine metabolism. The tk mutants grow as well as wild-type T4, indicating that thymidine kinase is a non-essential enzyme.