Molecular Cloning and Analysis of the Regulation of Cys-14+, a Structural Gene of the Sulfur Regulatory Circuit of Neurospora Crassa

The cys-14+ gene encodes sulfate permease II, which is primarily expressed in mycelia. cys-14+ is one of a set of sulfur-related structural genes under the control of cys-3+ and scon+, the regulatory genes of the sulfur control circuit. We have cloned cys-14+ from a cosmid library of Neurospora crassa DNA. A restriction fragment length polymorphism analysis showed that this clone maps to the region of chromosome IV corresponding to the cys-14+ locus. Northern blot analyses were used to examine the regulated expression of the cys-14+ gene. In the wild type, a 3-kilobase cys-14+ transcript was highly expressed under sulfur-derepressing conditions but completely absent during sulfur repression. A cys-3 mutant, which cannot synthesize any of the sulfur-controlled enzymes, including sulfate permease II, did not possess any cys-14+ transcript under either condition. A cys-3 temperature-sensitive revertant completely lacked any cys-14+ mRNA at the conditional temperature but expressed the cys-14+ transcript upon derepression at the permissive temperature. Mutation of a second sulfur regulatory gene, scon(c), causes the expression of sulfur-related enzymes in a constitutive fashion; the scon(c) mutant showed a corresponding constitutive expression of cys-14+ mRNA, such that it was present even in cells subjected to sulfur repression conditions. These results show that the cys-14+ gene is regulated through the modulation of message content by the cys-3+ and scon(c) control genes in response to the sulfur levels of the cells.