Mutations in a nonessential viral gene permit bacteriophage T4 to form plaques on Escherichia coli valS ts relA.

During viral development bacteriophage T4 modifies the valyl-transfer RNA synthetase of its host Escherichia coli, but the function of the modification has remained elusive. A strain of Escherichia coli has now been identified which is nonpermissive for wild-type bacteriophage T4, but permissive for bacteriophage mutants impaired in the modification reaction. A comparison with other bacteria suggests that nonpermissiveness is due to synthesis of a thermolabile valyl-transfer RNA synthetase and relaxed control of RNA accumulation.