Ultraviolet mutagenesis and its repair in an Escherichia coli strain containing a nonsense codon

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. UV mutation production was found 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. It is proposed that the dimer serves as template during some aspects of repair replication and the time of replication the C in the dimer directs the insertion of A in the complementary strand. It was also noted that C → A and T → G changes caused by a CT dimer occur much less frequently.