SUMMARY: A mutant of was isolated which showed temperature-sensitive synthesis of DNA. The mutant (11 ) increased in cell mass at 38° without concurrent DNA synthesis, resulting in loss of viability and the production of long filaments. DNA synthesis at 38° continues for approximately 40 min. at a gradually decreasing rate giving an increase of DNA in Casamino acid minimal medium of about 55% over that present at the time of the shift. This residual DNA synthesis is not reduced by increasing the temperature to 42°, but can be increased by enriching of the medium in which the organisms are grown before the shift. The effect of high temperature on DNA synthesis in 11 mimics the effect of chloramphenicol, which is known to allow the completion of rounds of replication of DNA without allowing new initiations. The lesion is essentially irreversible and is not corrected by increasing the osmotic pressure of the growth medium. Phage P22 can develop normally in 11 if added at the time of the shift or 30 min. later. Even if the phage is added 3½ h. after the shift substantial multiplication occurs. Cell division continues for several hours after the temperature shift, resulting in the production of DNA-less cells as reported in a separate publication.


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