Complex Growth Requirement of an Arginineless Mutant of Neurospora

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Abstract
Mutant strain 30820 of N. crassa represents an instance of genetic block in the conversion of ornithine into citrulline, since the mutant fails to grow on the minimal medium for wild type Neurospora unless it is supplemented with arginine or citrulline. Ornithine does not have growth-factor activity. The locus of the mutant gene is different from that of either of 2 other genes previously reported as being immediately concerned in the biosynthesis of citrulline from ornithine. Although the mutant growth requirement of strain 30820 is inherited as a single-gene difference from wild type, no single compound has yet been found which will entirely fulfill the requirement. In addition to citrulline or arginine, supplements of adenine, guanine, or certain other purine or pyrimidine compounds are required before the mutant strain can grow at an approximation of wild-type growth rate. Urea markedly stimulates growth of this strain also, but only in the presence of citrulline or arginine. However, urea is much less active than, for example, adenine or guanylic acid in its growth-stimulus effect. Studies with an experimentally created double-mutant strain indicate that the adenine portion of the double growth factor requirement of strain 30820 is not due to inhibition effects of some ornithine derivative which accumulates when ornithine cannot be converted into citrulline.

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