SUMMARY: By selection with concentrations of streptomycin from 25 to 250 μg./ml., auxotrophic mutants were obtained with high frequency from strain LT-2. All of them were found to require both thiamine and nicotinic acid for their growth. The requirement for thiamine can be satisfied by either 4-methyl-5-hydroxyethyl-thiazole, cystine or cysteine but not by methionine. Nicotinic acid can be replaced by nicotinamide. No other amino acid, vitamin, purine or pyrimidine can replace these vitamins.

In addition to the nutritional requirement, all of these mutants are streptomycin resistant slow growers. These four characters, i.e. thiamine and nicotinic acid-requirement, slow growth and streptomycin resistance are converted to wild-type in a single step by transduction with phage PLT-22, grown on wild-type bacteria as well as on prototrophic one-step intermediate streptomycin resistant mutants and streptomycin indifferent mutants. Spontaneous mutants with wield phenotype were also obtained from some unstable mutants of this group, apparently in a single mutational event. the transductional findings suggest that these auxotrophic mutants are probably due not to some chromosomal aberration but to mutation within a single locus. The data from reciprocal transductions between these mutants suggest that multiple sites of the same locus are responsible. The mutations at different sites seem to have different levels of mutability and some of them have different levels of streptomycin resistance.


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