Methylcitrate synthase from Aspergillus nidulans: implications for propionate as an antifungal agent

Aspergillus nidulans was used as a model organism to investigate the fungal propionate metabolism and the mechanism of growth inhibition by propionate . The fungus is able to grow slowly on propionate as sole carbon and energy source. Propionate is oxidized to pyruvate via the methylcitrate cycle. Th e key enzyme methylcitrate synthase was purified and the corresponding gene mcsA, which contains two introns, was cloned, sequenced and overexpressed in A. nidulans. The derived amino acid sequence of the enzyme shows more th an 50% identity to those of most eukaryotic citrate synthases, but only 14% identity to the sequence of the recently detected bacterial methylcitrate synthase from Escherichia coli. A mcsA deletion strain was unable to grow o n propionate. The inhibitory growth effect of propionate on glucose medium was enhanced in this strain, which led to the assumption that trapping of t he available CoA as propionyl-CoA and/or the accumulating propionyl-CoA its elf interferes with other biosynthetic pathways such as fatty acid and poly ketide syntheses. In the wild-type strain, however, the predominant inhibit or may be methylcitrate. Propionate (100 mM) not only impaired hyphal growt h of A. nidulans but also synthesis of the green polyketide-derived pigment of the conidia, whereas in the mutant pigmentation was abolished with 20 m M propionate.