SUMMARY: A mutant of Salmonella typhimurium was isolated which showed temperature-sensitive synthesis of DNA. The mutant (11 G) 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 G 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 G 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.
SUMMARY: DNA breakdown was detected 3 to 4 min. after addition of colicin E2 to sensitive cells; inhibition of cell division followed 5 to 10 min. later, but inhibition of DNA synthesis was observed only after several more minutes. Adsorption of E2, which takes place even at 4°, led to the formation of a specific surface complex (I). Complex I did not promote DNA breakdown. We suggest that the transition from this complex to a surface complex (II) which promoted DNA breakdown depended upon several factors which include temperature, concentration of E2, specific membrane proteins and, under certain conditions, high concentrations of extracellular KH2PO4. The formation of complex II did not depend on concomitant DNA or protein synthesis. The continued promotion of DNA breakdown by complex II and its associated nuclease was blocked by inhibition of energy metabolism. In addition, the removal of E2 from the cell surface by trypsin treatment during the early stages of the process greatly decreased the rate of DNA breakdown. E2-induced DNA breakdown, which appears to commence from a limited number of chromosomal sites, proceeded normally in UVr--, RecB--, RecC--, Hsr--, Hss--, PolA-- and in several tsDNA replication mutants.