Increase in UV Mutagenesis by Heat Stress on UV-Irradiated E. coli Cells

When (-) auxotrophs of , after UV irradiation, were grown at temperatures between 30 and 47A degrees C, the frequency of UV-induced mutation from (-) to (+) revertant increased as the UV dose and the temperature increased. For cells exposed to a UV dose of 45 J/m(2), the mutation frequency at 47A degrees C was 1.9 times that at 30A degrees C; for a dose of 90 J/m(2), it was 3.25 times; and for 135 J/m(2), it was 4.8 times. Similar enhancement of reversion frequency was observed when the irradiated cells were grown at 30A degrees C in the presence of a heat shock inducer, ethanol (8% v/v). Heat shock-mediated enhancement of UV mutagenesis did not occur in an mutant sigma 32 (heat shock regulator protein), but sigma 32 overexpression in the mutant strain (transformed with a -bearing plasmid) increased the UV-induced mutation frequency. These results suggest that heat stress alone has no mutagenic property, but when applied to UV-damaged cells, it enhances the UV-induced frequency of cell mutation.