Novel biosynthetic pathway for sulfur amino acids in <italic>Cryptococcus neoformans</italic>.

We elucidated a unique feature of sulfur metabolism in <italic>Cryptococcus neoformans. C. neoformans</italic> produces cysteine solely by the <italic>O</italic>-acetylserine pathway that consists of serine-<italic>O</italic>-acetyl transferase and cysteine synthase. We designated the gene encoding the former enzyme <italic>CYS2</italic> (locus tag CNE02740) and the latter enzyme <italic>CYS1</italic> (locus tag CNL05880). The <italic>cys1</italic>Δmutant strain was found to be avirulent in a murine infection model. Methionine practically does not support growth of the <italic>cys1</italic>Δ strain, and cysteine does not serve as a methionine source, indicating that the transsulfuration pathway does not contribute to sulfur amino acid synthesis in <italic>C. neoformans</italic>. Among the genes encoding enzymes catalyzing the reactions from homoserine to methionine, the gene corresponding to the <italic>Saccharomyces cerevisiae MET17</italic> encoding <italic>O</italic>-acetylhomoserine sulfhydrylase (Met17p) had remained to be identified in <italic>C. neoformans</italic>. By genetic analysis of Met− mutants obtained by <italic>Agrobacterium tumefaciens</italic>-mediated mutagenesis, we concluded that Cnc01220, most similar to Str2p (36% identity), cystathionine-γ-synthase, in the <italic>Saccharomyces</italic> genome, is the <italic>C. neoformans</italic> version of <italic>O</italic>-acetylhomoserine sulfhydrylase. We designated <italic>CNC01220</italic> as <italic>MET17</italic>. The <italic>C. neoformans met3</italic>Δ mutant defective in the first step of the sulfate assimilation pathway, sulfate adenylyltransferase, barely uses methionine as a sulfur source, whereas it uses cysteine efficiently. The poor utilization of methionine by the <italic>met3</italic>Δ mutant is most probably due to the absence of the transsulfuration pathway, causing an incapability of <italic>C. neoformans</italic> to produce cysteine and hydrogen sulfide from methionine. When cysteine is used as a sulfur source, methionine is likely produced de novo by using hydrogen sulfide derived from cysteine via an unidentified pathway. Altogether, the unique features of sulfur amino acid metabolism in <italic>C. neoformans</italic> will make this fungus a valuable experimental system to develop anti-fungal agents and to investigate physiology of hydrogen sulfide. [ABSTRACT FROM AUTHOR]