In vivo synthesis of sulfur containing amino acids in Candida albicans

Abstract

Growth in a supplemented minimal culture medium and chlamydospore formation in supplemented corn meal agar were the criteria used to evaluate in vivo biosynthesis of sulfur containing amino acids in auxotrophs of Candida albicans. Supplementation with L-cystathionine, L-homocysteine, L-methionine, S-adenosyl-L-methionine, or S-adenosyl-L-homocysteine resulted in growth in the minimal medium and both growth and chlamydospore formation in corn meal agar. One auxotroph (CYS) gave comparable results when inoculated into media supplemented with L-cysteine, responded minimally when O-acetyl-L-serine and sulfide comprised a dual supplement, and failed to grow or form chlamydospores when sulfide or O-acetyl-L-serine was used as a single supplement, suggesting that this organism may possess a deficiency in the synthesis or activity or cysteine synthetase. Another auxotroph (OAHS) did not grow or produce chlamydospores when O-acetyl-L-serine was added to the media in combination with sulfide, or when either L-cysteine, O-acetyl-L-serine, sulfide, or L-homoserine was used as a single supplement, but did respond to media supplemented with O-acetyl-L-homoserine indicating a deficiency in homoserine-O-transacetylase in this microorganism. Auxotroph OAHS grew and formed chlamydospores as well in media containing only supplemental O-acetyl-L-homoserine as it did when O-acetyl-L-homoserine was used in combination with sulfide S-methyl-L-cysteine or thiomethyladenosine, while (CYS) neither grew nor produced chlamydospores in any of these supplemented media. These results lend support for the hypothesis that cystathionine is an important intermediate in methionine biosynthesis in C. albicans. The positive results obtained when the auxotrophs were inoculated into media supplemented with S-adenosyl-L-homocysteine or S-adenosyl-L-methionine may reflect an alternate route for methionine biosynthesis which functions conditionally in this microorganism. The potential significance for endogenous formation of sulfur containing amino acids from S-adenosylmethionine is therefore considered.