Reversion of aryl sulfataseless mutants of Neurospora.

I N previous publications from this laboratory, we have described two loci, called cys-3 and eth-lr, that control a family of enzymes of sulfur anabolism in Neurospora ( MARZLUF and METZENBERG 1968; METZENBERG 1968). These could be viewed as regulatory genes in search of a locus to control, since no proven structural mutants for any of the regulated enzymes were available. KERR and FLAVIN (1970) have noted that &-Ir may be the structural gene for S-adenosyl methionine synthetase, an hypothesis for which we also have some not very convincing evidence ( JACOBSON, CHEN and METZENBERG, in preparation). In any event, it is not clear that eth-1‘ has a purely regulatory function. Recently MARZLUF (1970a, 1970b) has isolated mutants which he has called cys-13 and cys-14. Each of these lacks one of the two permeases which conduct inorganic sulfate into the cell, and the double mutant lacks both permeases and is an auxotroph. It is likely that these mutations correspond to the structural genes for the permeases. Likewise, we have discussed the properties of mutants that appear to be deficient only in aryl sulfatase (METZENBERG and AHLGREN 1970a). These strains, called ars-, are thus reasonable candidates for being structural gene mutants. We now report the isolation of revertants of two of the arsalleles. We were interested in revertants on the possibility that, if ars is the structural locus for aryl sulfatase, some of these revertants might have an aryl sulfatase with properties distinguishable from those of the wild-type enzyme. The results reported below are negative: no qualitatively altered enzyme is apparent in any of the revertant strains. Some of these have low or very low levels of aryl sulfatase. One low-level suppressor mutation was detected. The rest of the reversion events apparently occurred at or near the ars locus. The failure to find altered forms of aryl sulfatase among the revertants must not be taken as proof that ars is not the structural gene for aryl sulfatase.