Infection of a thymineless strain of with either the temperate phage φ3 or its clear plaque mutant φ3c enabled DNA to be formed in the absence of added thymine. Growth of phage φ3c under these conditions was associated with an enhanced rate of DNA synthesis compared with uninfected cultures growing in the presence of thymine; when phage φ3 became a prophage the lysogenized cells synthesized DNA at the same rate as the uninfected bacteria with thymine. The thymine independence of phage-infected bacteria was due to the acquisition of the enzyme thymidylate synthetase absent in the uninfected mutant bacteria. Phage φ3c growth caused a rapid rise in the specific activity of the enzyme after a short lag, and by the time the cells lysed there was approximately ten times more activity than in uninfected wild-type . In thymineless lysogenized with phage φ3 the amount of thymidylate synthetase was the same as in wild-type . Phage DNA was able to transform thymine-requiring to thymine independence, showing that it contained the gene for thymidylate synthetase. Plaque-forming particles could not be separated by density gradient centrifugation from those possessing the thymidylate synthetase gene. This result, together with the failure to get ‘thymineless’ mutants of the phage to regain their ability to promote thymine synthesis, suggested that phage φ3 was a converting phage. However, the results of transformation implied that the phage thymidylate synthetase gene was closely related to that of the recipient cells, and may have originated from the bacterium.


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