Following u.v.-irradiation and plating on a selective chlorate medium, about 200 chlorate-resistant isolates of were obtained. These were divided into three groups all of which grew well with ammonium as nitrogen source. One group grew well with nitrate under all conditions tested. The second, and largest group, grew well with nitrate only in the absence of acetate. The third group could not grow with nitrate as sole nitrogen source. Isolates in this group were further subdivided according to their ability to grow with hypoxanthine or nitrite as sole nitrogen source. Three mutants (14/15, 17/4 and 0/8) were examined genetically in some detail. Each differed from wild-type (strain 2192) by a mutation in a single gene. Isolates 14/15 and 17/4 could grow with hypoxanthine as a nitrogen source. The mutant genes in these two isolates were shown to be located in different linkage groups and have been designated (in 17/4) and (in 14/15). There is evidence that may be in linkage group VI. Mutant 0/8 resembles the mutants of in that it would not grow on hypoxanthine. The mutation in 0/8 is designated ; it, too, may be in linkage group VI. Strain 137c was also analysed. It appears to be unable to grow on nitrate because of mutations in two loci.


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