The mapping of genetic loci affecting streptomycin resistance in Pneumococcus.

HE term “complex locus” implies a series of closely linked genetic deterTminants all of which produce similar or identical modifications in the phenotype of an organism. Much of the experimental work to be described is concerned with a series of this type conferring streptomycin resistance in Pneumococcus. BRYAN (1 961 ) has isolated spontaneous streptomycin-resistant mutants of Pneumococcus which show a wide variation in the maximum concentration of streptomycin they are able to resist. A small random sample (four) of these spontaneous mutations was tested for linkage in transformation studies, and they were all found, by the criterion of genetic recombination, to be either allelic (at the same site) or closely linked (at different sites). While this finding could have been due to the small number of mutants tested, it suggested that all streptomycin-resistant mutations occur in the same region (a cistron?) of a DNA molecule. However, analysis of additional markers was necessary to obtain a more complete picture. With this aim in mind the present studies have been undertaken; they have shown that the remainder of the spontaneous streptomycin-resistant mutations isolated by BRYAN (1961) , as well as two spontaneous mutations originally isolated by SCHAEFFER (1956) , represent either allelic or closely linked markers on the same molecule of transforming DNA. In addition, a nitrous acid-induced mutation to streptomycin resistance has been examined. This mutation appears to belong to a different locus; it does not exhibit the degree of linkage to the spontaneous mutations that the latter exhibit to each other.