| Popis: |
The double mutant strain pyr-3 arg-12s is a prototroph because a common precursor of arginine and pyrimidine is supplied by the arginine pathway. Growth of this strain is inhibited by exogenous citrulline or arginine. Citrulline-resistant mutants of this strain were selected, and they resulted from modifier mutations at other loci. Forced heterokaryons were used to study complementation among these modifiers. Since the complementation test requires the scoring of non-growth as the positive result, there was concern that variations in nuclear ratios could give erroneous results. This possibility does not seem significant, since groups of mutants established by complementation correspond with groups established by physiological, enzymatic, and recombinational measurements.-The technique has revealed that the most frequently mutated loci are arg-1 and what is probably un-3. Azg-1 mutations affect the conversion of citrulline to argininosuccinate, while un-3 mutations reduce the citrulline uptake rate. Since most of these mutations are of the intracistronic complementing type, a complementation map was constructed for most of the affected loci. The high proportion of complementors in each map can be explained by assuming that partially functioning gene products are more likely to complement with each other than are those which are nonfunctional. N Neurospora, if not in all eukaryotes, both arginine and pyrimidine synthesis I utilize carbamyl phosphate (CAP) as a precursor. Each biosynthetic pathway has its own source of CAP (Figure 1 ) , with the pyrimidine source apparently being localized in the nucleus and the arginine source in the mitochondria (DAVIS 1972). The metabolic separation of the two paths in Neurospora is nearly complete, since mutations which specifically block the synthesis of pyrimidinespecific CAP (CAPpyr) are auxotrophs under normal growth conditions. That the sequestering of CAP by the two pathways is not absolute, however, is demonstrated by the suppressor (a~gj2~) of CAPpyr-deficient mutants. The ~g-12~ mutation is a very leaky mutant affecting the utilization of CAParg but not causing an arginine requirement (DAVIS 1962). Insertion of the arg-1Z8 suppressor into a CAPpyr-deficient mutant such as KS-20 (a pyr-3 allele) results in the accumulation of CAParg and an overflow to the pyrimidine pathway. Thus the pyr-3, arg-l2* double mutant is a prototroph, but since CAParg production is subject to repression by arginine (WILLIAMS, BERNHARDT and DAVIS 1970) |
