A possible model for the structure of the Neurospora carbamoyl phosphate synthase-aspartate carbamoyl transferase complex enzyme.

Summary The pyrimidine-3 locus of Neurospora crassa specifies a multienzyme complex comprising pyrimidine-specific carbamoyl phosphate synthase (CPSpyr) and aspartate carbamoyl transferase (ACT). It appears to be divided into a translationally proximal CPS-specific region and a distal ACT-specific region.Levels of complementation for ACT activity between pairs of four pyr-3 CPS⁺ACT^- mutants showed a range from 12% to 68% of the wild-type level of the enzyme. This is interpreted as interallelic complementation, contradicting certain earlier suggestions of two dissimilar ACT subunits.Proteolysis of an extract from a heterokaryon formed from two of the above CPS⁺ACT^- alleles ( and ) did not lead to loss of ACT activity, but led to the formation of a fragment with ACT activity with a similar molecular weight (92,000 daltons) to that produced in extracts of wild type strain.The pyr-3 polar mutant 43–174, which is enzymatically CPS⁺ACT^- and which fails to complement with any other CPS⁺ACT^- alleles, thus suggesting its location towards the proximal end of the ACT region, has CPS activity associated with a form of 180,000 daltons molecular weight. These findings are used to construct a model for the structure of the native enzyme complex.This work supported in part by S.R.C. grant B/RG/2981