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Effect of Tn10/Tn5 transposons on the survival and mutation frequency of halogen light-irradiated AB1157 Escherichia coli K-12.

We show here that the Tn10/Tn5 transposon when inserted into the chromosome of strain AB1157 makes the bacteria more sensitive to and less mutable by halogen light irradiation. These effects are most probably caused by depletion of UmuD and UmuC proteins since: (i) transformation of the transposon-bearing bacteria with plasmids harbouring umuD'C (or umuDC) leads to recovery of the original survival and mutation frequencies; (ii) insertion of Tn10/Tn5 into chromosomal DNA has no effect on the level of mutation induced by ethyl methane-sulphonate treatment, a mutagen whose activity is umuDC-independent; (iii) the decline in survival is in about the same range for Tn10-bearing bacteria as for bacteria with deleted umuDC. However, whereas transformation of bacteria deleted in umuDC with plasmids carrying umuD'C/umuDC leads to full recovery of halogen light-induced mutability, recovery of survival is poor. This suggests that the mechanisms leading to umuDC-dependent mutagenesis and umuDC-dependent protection of cell survival are different. None of these effects occurs in bacteria bearing the Tn9 transposon in their DNA.