COLD SHOCK OF BACTERIA

IV. INVOLVEMENT OF DNA LIGASE REACTION IN RECOVERY OF ESCHERICHIA COLT FROM COLD SHOCK

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The lethal effect of cold shock to a thymine-requiring Escherichia coli was investigated with special reference to the nature of DNA after the cold shock. The cells were labeled with ³H-thymine during growth. Labeled cells were cold-shocked and lysozyme-lysates prepared. The depolymerization of DNA by endogenous nucleases was faster with the lysate prepared from cold-shocked cells than those from unshocked control and magnesium-recovered cells. The result indicates that cold-shocked cells contain DNA which is more susceptible to endogenous nucleases than DNA in unshocked or recovered cells. The lysates from cold-shocked, unshocked, and recovered cells were heat-treated and quenched rapidly at high salt concentrations. The resulting single-strand DNA was adsorbed by nitrocellulose. It was found that DNA of cold-shocked cells contain more single-strand breaks (nicks) than the unshocked or recovered cells. These results provide an evidence for the involvement of DNA-ligase reaction in the process of recovery from the cold shock. Possible mechanism of the lethal effect of cold shock is discussed.