Genetic modifiers of streptomycin resistance in Pneumococcus.

Bryan, Betty E. (University of Rochester, Rochester, N. Y.). Genetic modifiers of streptomycin resistance in pneumococcus. J. Bacteriol. 82:461-470. 1961.-Streptomycin-resistant mutants arising spontaneously in pneumococcus vary widely with respect to the maximal concentrations of streptomycin which they resist. A small random sample of these mutations tested for linkage are allelic or closely linked. One single-step mutant resisting a relatively low level of streptomycin undergoes the following changes: (i) replacement (by a secondary mutation) of the original mutation by an allele conferring a high level of resistance; (ii) reversion to complete sensitivity by back mutation at the originally mutated site; (iii) enhancement by an unlinked modifier gene which confers no resistance by itself but raises the level of resistance of the mutant strain approximately 10-fold. This modifier may have some specificity in its action on the different mutations occurring at a gene locus.The possibility is discussed that multi-step resistance may often originate by acquisition of modifiers.