Anti-mutagenic effect of ultraviolet light on spontaneous tyrosine tRNA ochre suppressor mutations in Escherichia coli.

The numbers of tyrosine tRNA ochre suppressor mutations arising spontaneously or after UV irradiation in different strains of Escherichia coli K12 are considered. The DNA sequence change requisite for this type of mutation would be a transversion at a cytosine between two purines, where pyrimidine-pyrimidine photoproducts could not form. We find that UV mutagenesis does not produce these tyrosine tRNA ochre suppressor mutations. With lexA51 recA441 defective cells, the spontaneous yield of these mutations is elevated and UV irradiation produces a significant decrease in the numbers of this particular mutation. As explanation we suggest that the spontaneous appearance of these mutations reflects mutation at apurinic sites, the efficiency of which is elevated in lexA51 recA441 cells (with derepressed SOS functions and an activated form of RecA protein). The addition of UV damage in the DNA of these cells cannot further stimulate the positive functions that are required for the production of these mutations and are typically associated with UV mutagenesis (induction of SOS functions, activation of RecA protein and introduction of a targeting photoproduct) but apparently can have a negative effect on mutagenesis, hitherto not realized.