Structure and regulation of cysD, the homocysteine synthase gene of Aspergillus nidulans.

The A. nidulans cysD gene encoding homocysteine synthase (O-acetyl-L-homoserine sulphydrylase) has been isolated by functional complementation of a cysD11 mutation. The gene contains five short introns and codes for a protein of 437 amino acids. The protein shows homology with bacterial and yeast O-acetyl- and O-succinyl-homoserine sulphydrylases, particularly from Schizosaccharomyces pombe, Saccharomyces cerevisiae and Kluyveromyces lactis. The cysD cDNA is able to complement a S. cerevisiae mutation impairing homocysteine synthase. Synthesis of the cysD mRNA is down-regulated by a high concentration of methionine in growth medium without sulphate and up-regulated under sulphur limitation. A comparison of cysD genomic and cDNA copies, derived from different A. nidulans strains, revealed a marked DNA-sequence polymorphism manifested mostly by silent point mutations. There was, however, much less polymorphism in the protein sequence.