A method for isolating nonsense suppressors in enterobacteriaceae using an amber mutant of the drug resistance factor R1

We describe here the isolation of a mutant derivative of the drug resistance factor R1 (Meynell and Datta, 1966) that carries a nonsense mutation in a gene determining resistance to penicillins. We have used this mutant R1 to isolate derivatives of Escherichia coli and Klebsiella pneumoniae that contain nonsense suppressors (Sup^- strains) by screening penicillin-resistant revertants of strains containing the mutant R factor for the presence of such suppressors. This obviates the need to have known nonsense mutations in chromosomal genes. Theoretically, suppressor-containing derivatives of any bacterial species that can maintain and express R1 can be constructed.