Bacteriophage-coded thymidylate synthetase. Evidence that the T4 enzyme is a capsid protein

It has previously been shown that T4 bacteriophage-coded dihydrofolate reductase is a capsid protein, specifically an element of the tail plate. This paper presents evidence that thymidylate synthetase is also a structural protein. Antiserum prepared against purified T4 thymidylate synthetase neutralizes T4 infectivity. Evidence is presented that structural thymidylate synthetase is the target of the antiphage component of the serum.The td gene in T4 codes for thymidylate synthetase. We have crossed the td gene from phage T6 into T4 and eliminated other T6 genetic material from the hybrid phage by extensive backcrossing. The hybrid phage, T4td^T6, is inactivated at 60 °C significantly more rapidly than the parent phage, T4D. Thus, the td gene is a determinant of a physical property of the virion, providing direct confirmation that thymidylate synthetase is a capsid protein. At present the role of the virion-bound enzyme is unknown.