MUTATIONAL SPECIFICITY OF ULTRAVIOLET LIGHT IN ESCHERICHIA COLI WITH AND WITHOUT THE R PLASMID pKM101

R. G. Fowler ¹, L. McGinty ¹, and K. E. Mortelmans ¹

¹ Department of Biological Sciences, San Jose State University, San Jose, California 95192 Microbial Genetics Department, SRI International, Menlo Park, California 94025

Plasmid pKM101 provides UV protection and increases the frequency of spontaneous and UV-induced mutations in Escherichia coli. By analyzing reversion patterns of defined trpA alleles, we showed that pKM101 altered the mutational specificity of UV-induced mutations. Certain UV-induced base-pair substitutions were strongly enhanced, while others were decreased in frequency in the presence of pKM101. This result suggests an interaction between cellular misrepair and an error-prone repair function(s) provided by pKM101. We have also examined UV mutational specificity in the absence of pKM101 and found the following: (1) UV preferentially enhances missense, as well as nonsense, intergenic suppressor mutations; (2) UV causes all possible base-pair substitutions as well as frameshift mutations; (3) G·C base pairs are more susceptible to UV mutagenesis than A·T base pairs at the same nucleotide positions; and (4) UV-induced mutations can occur at nucleotide positions that are not part of pyrimidine-pyrimidine sequences.