ISOLATION AND CHARACTERIZATION OF EXTRAGENIC MUTATIONS AFFECTING THE EXPRESSION OF THE UROPORPHYRINOGEN DECARBOXYLASE GENE (HEM12) IN SACCHAROMYCES-CEREVISIAE

Uroporphyrinogen decarboxylase (Uro-d; EC 4.1.1.37), the fifth enzyme in the heme biosynthetic pathway, which catalyzes the sequential decar boxylation of uroporphyrinogen to coproporphyrinogen, is encoded by th e HEM12 gene in Saccharomyces cerevisiae. The HEM12 gene is transcribe d into a major short mRNA and a minor longer one, approximately 1.35 a nd 1.55 kb, respectively, in size, and that differ in the 5' untransla ted region. ''Uroporphyric'' mutants, which have no mutations in the H EMI 2 gene but accumulate uroporphyrinogen, a phenotype chracteristic of partial Uro-d deficiency, were investigated. Genetic analysis showe d that the mutant phenotype depends on the combined action of two unli nked mutations, udtl and either ipa1, ipa2, or ipa3. ipa1 is tightly l inked to HEM12. The mutation udt1 apparently acts specifically on the HEM12 gene, and causes a six to tenfold decrease in the levels of the short HEM12 mRNA, in the beta-galactosidase activity of a HEM12-lacZ f usion, in immunodetectable protein and enzyme activity. But heme synth esis is normal and porphyrin accumulation was modest. The mutations ip a1, ipa2, and ipa3 had no phenotype on their own, but they caused an i ncrease in porphyrin accumulation in a udt1 background. This multiplic ity of genetic factors leading to uroporphyric yeast cells closely res embles the situation in human porphyria cutanea tarda.