Suppression of DNA-arrested synthesis in mutants defective in gene 59 of bacteriophage T4

Amber nonsense mutants of gene 59 (amC5, amHL628) in bacteriophage T4 will cause an arrest of DNA synthesis in a nonpermissive host, Escherichia coli B. The arrested synthesis of DNA is rapidly suppressed by the addition of chloramphenicol at 5-12 min after infection. To be effective, chloramphenicol must be added at a specific time after infection, and suppression requires continuous presence of chloramphenicol for at least 30 min after infection. Extragenic mutations in either gene 55 or gene 33 also restored DNA synthesis of the gene 59 mutants to the normal level. The physicochemical characteristics of intracellular DNA obtained by alkaline sucrose gradient centrifugation of a double mutant (amBL292-amC5) is strikingly similar to that obtained in amC5-infected cells treated with chloramphenicol. Since both gene 55 and 33 control the expression of late genomes, it is likely that at least one of the late proteins is involved in the phenotypic expression of arrested DNA synthesis, a conclusion consistent with the chloramphenicol results. We have compared the suppression of amC5 (gene 59) with the suppression of amHX39 which affects polynucleotide ligase (gene 30) and which in its mutant form causes DNA arrested synthesis. Unlike its effect on the polynucleotide ligase mutant, an extragenic rII mutation introduced into amC5 did not affect the arrest of DNA synthesis of amC5. However, the plating efficiency in E. coli B was greatly increased.