CLONING AND CHARACTERIZATION OF THE ASPERGILLUS-NIDULANS CYSB GENE ENCODING CYSTEINE SYNTHASE

The cysB gene of A. nidulans was cloned by complementation of a cysB m utation. This is the first cloned eukaryotic genomic sequence coding f or cysteine synthase. The gene contains one 71-bp intron and codes for a protein of 370 amino acids. Its N-terminal region has characteristi c features of transit peptides. suggesting mitochondrial localisation of the enzyme. The protein shows homology with bacterial and plant cys teine synthases among which it occupies a remote phylogenetic position and apparently represents a distinct subfamily. Transcription of the cysB gene is not appreciably regulated by the concentration of methion ine in the growth medium.