Pyrimidine-requiring suppressor mutations of Arginine-3 in Neurospora and their bearing on the structure of the Pyrimidine-3 locus

Summary

The acridine ICR-170 was used to study the bifunctional pyr-3 locus, which specifies carbamyl phosphate synthetase (CPS) and aspartate transcarbamylase (ATC) activities. Both spontaneous and ICR-170-induced mutants were selected as suppressors of the arginine requirement of. an arg-3 mutation. One of the ICR-170-induced CPS⁺ATC^- mutants was identified, by reversion analysis, as a frame shift. This mutant, a second CPS⁺-ATC^- frame shift, and several other new CPS⁺ATC^- alleles, were integrated into the complementation map of the locus produced by a thorough reanalysis of ten previously existing CPS⁺-ATC^- alleles. KS36, a CPS^-ATC^- frame shift allele which had previously been shown to map very close to certain CPS⁺ATC^- alleles was studied by an experiment designed to select partial revertants to CPS⁺ATC^-, as it was thought that a frame shift in the normalle ATC-specific translation-distal end of the locus might destroy both functions. Failure to recover such partial revertants indicates that the site of KS36 is unlikely to be located in the distal ATC-specific region of the locus, and that earlier recombination data showing it to be distal to the CPS⁺-ATC^- allele KS43 are probably incorrect. The general applicability of partial reversion analysis of complex loci is discussed.