ULTRAVIOLET MUTAGENESIS AND ITS REPAIR IN AN ESCHERICHIA COLI STRAIN CONTAINING A NONSENSE CODON

Stanley Person ¹, John Allen McCloskey ¹, Wallace Snipes ¹, and Richard C. Bockrath ¹

¹ Biophysics Department, The Pennsylvania State University, University Park, Pennsylvania 16802 and Department of Microbiology, University of Indiana Medical School, Indianapolis, Indiana 46204

Ultraviolet mutagenesis and its repair were studied mainly in WU36-10-89, a uvr^- strain of Escherichia coli containing a UAG mutation in a gene for leucine biosynthesis. Following ultraviolet (UV) irradiation revertants appearing with or without direct photoreactivation (PR) were classified according to the presence and type of suppressor they contained. We find UV mutation production to be quite specific. An analysis of revertants produced by UV indicates they are formed mainly from GC AT and that the miscoding is due to a cytosine residue at the site of mutation in a cytosine-thymine (CT) dimer. We propose that the dimer serves as template during some aspects of repair replication and at the time of replication the C in the dimer directs the insertion of A in the complementary strand. We also note that C A and T G changes caused by a CT dimer occur much less frequently.

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