Summary: The apparent sensitivity of K12 to mild heat was increased by (), and , but not by , or mutations. However, addition of catalase to the rich plating medium used to assess viability restored counts of heat-injured , and strains to wild-type levels. p3478 was sensitized by heat to a concentration of hydrogen peroxide similar to that measured in autoclaved recovery medium. The apparent heat sensitivity of DNA-repair mutants is thus due to heat-induced sensitivity to the low levels of peroxide present in rich recovery media. It is proposed that DNA damage in heated cells could occur indirectly by an oxidative mechanism. The increased peroxide sensitivity of heat-injured cells was not due to a decrease in total catalase activity but may be related specifically to inactivation of the inducible catalase/peroxidase (HPI).


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