Once a permeability barrier is overcome, proflavin is highly mutagenic for Salmonella typhimurium and Escherichia coli. Mutagenesis of the his operon (in Salmonella) and lac operon (in E. coli) depends on derepression of the respective operons. No proflavin mutagenesis was detected in rec- strains. Over 100 proflavin-induced his mutants have been classified: 50% are base substitution types, 30% are stable (10% demonstrably multisite), and only 20% are probably frameshift mutations. None of the proflavin-induced frameshift mutations is of the type previously shown to be suppressed by frameshift suppressor mutations.
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