ISOLATION AND CHARACTERIZATION OF GENES FOR SULFATE ACTIVATION AND REDUCTION IN ASPERGILLUS-NIDULANS - IMPLICATIONS FOR EVOLUTION OF AN ALLOSTERIC CONTROL REGION BY GENE DUPLICATION

A region of the Aspergillus nidulans genome carrying the sA and sC gen es, encoding PAPS reductase and ATP sulphurylase, respectively, was is olated by transformation of an sA mutant with a cosmid library. The ge nes were subcloned and their functions confirmed by retransformation a nd complementation of A. nidulans strains carrying sA and sC mutations . The physical distance of 2 kb between the genes corresponds to a gen etic distance of 1 cM. While the deduced amino acid sequence of the sA gene product shows homology with the equivalent MET16 gene product of Saccharomyces cerevisiae, the sC gene product resembles the equivalen t MET3 yeast gene product at the N-terminal end, but differs markedly from it at the C-terminal end, showing homology to the APS kinases of several microorganisms. It is proposed that this C-terminal region doe s not encode a functional APS kinase, but is responsible for allosteri c regulation by PAPS of the sulphate assimilation pathway in A. nidula ns, and that the ATP sulphurylase encoding-gene (sC) of filamentous as comycetes may have evolved from a bifunctional gene similar to the nod Q gene of Rhizobium meliloti.