Mutations in the Aspergillus nidulans metE gene lead to requirement for O-acetylhomoserine. The gene was cloned by complementation of the metE31 mutation. The coding sequence was found to be interrupted by two introns of 66 and 50 bp, respectively. metE codes for a peptide of 489 amino acids which belongs to the family of homoserine O-acetyltransferases and a well-defined superfamily of alpha/beta hydrolases. Transcription of the metE gene is strongly up-regulated by a severe limitation of methionine, but not of cysteine. This gene is the first sulphur metabolism gene described in A. nidulans which is not regulated by the sulphur metabolite repression system in which cysteine acts as the low-molecular-weight effector.