Genetical analysis of proline mutants and their suppressors in Aspergillus nidulans

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Abstract
1.Complementation between thirteen proline auxotrophs in A. nidulans was studied. Two groups of mutants with different complementation pattern were found. These two groups could also be distinguished on the basis of recombination tests.2. The spontaneous reversion rate of proline mutants was established. In all cases studied the reversions were due to suppressor mutations. Dominant, semi-dominant and recessive suppressors were distinguished.3. Complementation between recessive suppressors was studied. Only a few of the suppressors obtained could be located in different complementation groups.4. Three suppressor loci were mapped, two of them, su-2 and su-6 in chromosome III linked to the phen-2 locus, respectively 22 and 26 map units distant, and the third in chromosome I, linked to the ad-9 locus (1·9 map units). su-2 is a mutant at the Su-4 pro locus already identified by Forbes (1956).5. The action of these suppressors is thought to consist in affecting the pathway of arginine synthesis by one of three mechanisms: (1) accumulation of an inter mediate (ornithine); (2) increased activity of ornithine δ-transaminase; and (3) a third, as yet, unclear process possibly involving feed-back regulation of arginine synthesis or the regulation of arginine breakdown to ornithine.