The Aspergillus nidulans cysA gene encodes a novel type of serine O-acetyltransferase which is homologous to homoserine O-acetyltransferases

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