REGULATORY ROLE OF ALA-S AND ALA-D IN A HEME-DEFICIENT MUTANT OF SACCHAROMYCES-CEREVISIAE

The biochemical characteristics of a haem-deficient mutant strain B231 of Saccharomyces cerevisiae isolated from D273-10B strain are describ ed. B231 strain accumulates substantial amounts of 5-aminolevulinic ac id (ALA) and uroporphyrin III (uro III). This pattern of porphyrins ac cumulation is due to both a defect in uroporphyrinogen decarboxylase ( Uro-p) activity and an enhancement of porphobilinogenase (PBGase) acti vity. ALA accumulation would indicate that feed-back control by haem i s not operating in this strain. ALA synthase (ALA-S), ALA-dehydrase (A LA-D) and PBGase activities; intracellular content of ALA (I-ALA) and porphyrins (I-porphyrins) were examined during the different phases of growth. Both accumulation of metabolites and enzyme activities reache d their maximum values at 20 hrs. of growth, when glucose concentratio n in the medium fell to zero. Evidence for negative feed-back control on ALA-S and ALA-D by heme are provided by the observations of both en hanced I-ALA accumulation and increased ALA-D activity (2.5 times) in the mutant strain. Our results would indicate that both ALA-S and ALA- D can be considered regulatory enzymes in yeast.