Genetic and accumulation studies in sulfite-requiring mutants of Aspergillus nidulans.

Abstract

Using complementation tests and genetic mapping, mutants of two new genes, sE (sulfite less) and sF (thiosulfateless) were identified among 145 nitroso-guanidine-induced mutants of the sulfur metabolism in Aspergillus nidulans. Most of the other mutants (134) turned out to be alleles of the four established 'sulfite' genes, sA - sD. The new genes were found to be unlinked to any other s-gene; sE was mapped in linkage group VIII distal to cha and sF in VII. Mapping of several mutants allelic to one or the other of the closely linked sA or sC genes showed that these loci are two cistrons 1.1 cMo units apart.Results from studies using radioactive sulfate as a substrate for the different mutants showed35S-accumulation profiles by electrophoresis consistent with the assignment of the five sulfite genes to the following steps: sB(3), sulfate transport; sC(12), sulfurylation of ATP to produce APS; sD(50 or 205), phosphorylation of APS to PAPS; and sA(6) and sE(15), PAPS reduction to sulfite. Although the last two genes appear to control the same step, their35S-accumulation profiles are distinct. In addition, it was found that mutants of all previously established genes, including sA, are epistatic to sE(15) when single and double mutants are tested for resistance on selenate medium.