Spontaneous and ICR191-A-induced Frameshift Mutations in the A Gene of Escherichia coli Tryptophan Synthetase

Frameshift mutant trpA21 was isolated after ultraviolet treatment and frameshift mutant trpA540 after ICR191-A (an acridine derivative) treatment of wild-type Escherichia coli K-12. The A proteins of spontaneous and ICR191-A-induced partial revertants of these mutants contained altered amino acid sequences one residue shorter than the comparable sequence in the A protein of wild-type bacteria. The data support the conclusion that ICR191-A causes frameshift mutations. The findings further indicate that both base additions and deletions are elicited by ICR191-A treatment and that mutagenesis by this compound sometimes affects more than one base pair. ICR191-A also weakly reverts some missense mutants. Analyses of the relevant peptides of the purified A protein show single amino acid replacements compatible with single base-pair changes. In addition, we found that some spontaneously revertible ICR191-A- and ultraviolet light-induced frameshift mutants are not further stimulated to revert by exposure to ultraviolet light.