AbstractExpression of mutant rRNAs from multicopy plasmids in Escherichia coli has proved a valuable means of studying how rRNA functions in protein synthesis (1–6). Plasmids with selectable antibiotic resistance markers in 16S and 23S rRNAs have been constructed (3), allowing recessive rRNA mutations to be followed. Genetic analysis of dominant lethal rRNA mutations is also possible by conditional expression from an inducible promoter (7,8). Biochemical analysis of mutant rRNAs proved more difficult, however, as expression from the seven rRNA (rrn) operons on the E. coli chromosome gives rise to an appreciable background of wild type rRNA molecules. This problem is overcome by allele-specific priming, which enables the biochemical characterization of mutant rRNAs in organisms that have multiple rrn operons. Allele-specific priming involves introduction of phenotypically silent mutations in the plasmid-coded rRNA, so that rRNAs can be selectively primed with complementary oligodeoxynucleotides (9). In this manner, the molecular genetic technology using multicopy plasmids can be elegantly combined with biochemical probing of the rRNAs and their analysis by the primer extension technique with reverse transcriptase (10,11).KeywordsMulticopy PlasmidValuable MeansMutant rRNAsMolecular Genetic TechnologyRestriction Enzyme Recognition SequenceThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.