A method has been devised for the rapid identification of nonsense mutations (UAG, UAA, UGA codons) in Salmonella . The mutations to be tested are reverted, and the revertants are replica-printed onto lactose plates spread with lawns of tester strains. These tester strains contain F′ lac episomes with nonsense mutations in the episomal Z gene. The revertants are infected with the episome from the tester strain lawn. Because S. typhimurium is unable to ferment lactose, only those revertants which have nonsense suppressors are able to grow on lactose. If colonies appear on the lactose plate, it may be concluded that the original strain carries a nonsense mutation, since nonsense suppressors suppress the mutant phenotype.

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