Summary: A temperature-sensitive mutant defective for the ability to utilize L-asparagine as a sole nitrogen source was isolated after -methyl-'-nitro--nitrosoguanidine mutagenesis. The mutation () produces two distinct phenotypic effects. (1) Mutant strains grow poorly at high temperature on minimal plates containing asparagine as the sole nitrogen source; this effect is greatly exacerbated by the presence of methionine. (2) Mutant strains utilize L-asparagine as a nitrogen source three to four times more efficiently at permissive temperatures than the wild-type strains. The mutation maps at 32·4 min on the chromosome, within the cotransduction gap. Mutant strains produce normal amounts of thermo-stable L-asparaginase I activity. The mutation therefore affects a component of the asparagine utilization system other than the catabolism of asparagine within the cell; it probably affects asparagine uptake.


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