The Aspergillus nidulans cysA gene was cloned by functional complementation of the cysA1 mutation that impairs the synthesis of O:-acetylserine. The molecular nature of cysA1 and cysA103 alleles was characterized; a nucleotide substitution and a frame shift were found in the former and a deletion mutation in the latter. The CYSA protein is 525 amino acids long and is encoded by an uninterrupted open reading frame. Expression of the cysA gene appears not to be regulated by sulfur, carbon and nitrogen sources. Protein sequence analysis reveals extensive similarity to homoserine O:-acetyltransferases, particularly the bacterial ones, and no homology with known serine O:-acetyltransferases. The authors propose that the CYSA protein is analogous to serine O:-acetyltransferases, i.e. it catalyses the same reaction but has an independent evolutionary origin.