Mutants of Saccharomyces cerevisiae partially defective in the last steps of the haem biosynthetic pathway: isolation and genetical characterization

Role of heme in the antifungal activity of the azaoxoaporphine alkaloid sampangine

Sampangine, a plant-derived alkaloid found in the Annonaceae family, exhibits strong inhibitory activity against the opportunistic fungal pathogens Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus. In the present study, transcriptional profiling experiments coupled with analyses of mutants were performed in an effort to elucidate its mechanism of action. Using Saccharomyces cerevisiae as a model organism, we show that sampangine produces a transcriptional response indicative of hypoxia, altering the expression of genes known to respond to low-oxygen conditions. Several additional lines of evidence obtained suggest that these responses could involve effects on heme. First, the hem1Delta mutant lacking the first enzyme in the heme biosynthetic pathway showed increased sensitivity to sampangine, and exogenously supplied hemin partially rescued the inhibitory activity of sampangine in wild-type cells. In addition, heterozygous mutants with deletions in genes involved in five out of eight steps in the heme biosynthetic pathway showed increased susceptibility to sampangine. Furthermore, spectral analyses of pyridine extracts indicated significant accumulation of free porphyrins in sampangine-treated cells. Transcriptional profiling experiments were also performed with C. albicans to investigate the response of a pathogenic fungal species to sampangine. Taking into account the known differences in the physiological responses of C. albicans and S. cerevisiae to low oxygen, significant correlations were observed between the two transcription profiles, suggestive of heme-related defects. Our results indicate that the antifungal activity of the plant alkaloid sampangine is due, at least in part, to perturbations in the biosynthesis or metabolism of heme.

Identification of amino acid changes affecting yeast uroporphyrinogen decarboxylase activity by sequence analysis of hem12 mutant alleles

The molecular basis of the uroporphyrinogen decarboxylase defect in eleven yeast 'uroporphyric' mutants was investigated. Uroporphyrinogen decarboxylase, an enzyme of the haem-biosynthetic pathway, catalyses the decarboxylation of uroporphyrinogen to coproporphyrinogen and is encoded by the HEM12 gene in the yeast Saccharomyces cerevisiae. The mutations were identified by sequencing the mutant hem12 alleles amplified in vitro from genomic DNA extracted from the mutant strains. Four mutations leading to the absence of enzyme protein were found: one mutation caused the substitution of the translation initiator Met to Ile, a two-base deletion created a frameshift at codon 247 and two nonsense mutations were found at codons 50 and 263. Four different point mutations were identified in seven 'leaky' mutants with residual modified uroporphyrinogen decarboxylase activity; each of three mutations was found in two independently isolated mutants. The nucleotide transitions resulted in the amino acid substitutions Ser-59 to Phe, Thr-62 to Ile, Leu-107 to Ser, or Ser-215 to Asn, all located in or near highly conserved regions. The results suggest that there is a single active centre in uroporphyrinogen decarboxylase, the geometry of which is affected in the mutant enzymes.

The effects in vivo of mutationally modified uroporphyrinogen decarboxylase in different hem12 mutants of baker's yeast (Saccharomyces cerevisiae)

Nine new hem12 haploid mutants of baker's yeast (Saccharomyces cerevisiae), totally or partially deficient in uroporphyrinogen decarboxylase activity, were subjected to both genetic and biochemical analysis. The mutations sites studied are situated far apart within the HEM12 gene located on chromosome IV. Uroporphyrinogen decarboxylase activity in the cell-free extracts of the mutants was decreased by 50-100%. This correlated well with the decrease of haem formation and the increased accumulation and excretion of porphyrins observed in vivo. The pattern of porphyrins (uroporphyrin and its decarboxylation products) accumulated in the cells of mutants partially deficient in uroporphyrinogen decarboxylase activity did not differ significantly, although differences in vitro were found in the relative activity of the mutant enzyme at the four decarboxylation steps. The excreted porphyrins comprised mainly dehydroisocoproporphyrin or pentacarboxyporphyrin. In heterozygous hem12-1/HEM12 diploid cells, a 50% decrease in decarboxylase activity led to an increased accumulation of porphyrins as compared with the wild-type HEM12/HEM12 diploid, which points to the semi-dominant character of the hem12-1 mutation. The biochemical phenotypes of both the haploid and the heterozygous diploid resembles closely the situation encountered in porphyria cutanea tarda, the most common human form of porphyria.