The sulfur-regulatory circuit of Neurospora crassa consists of a set of unlinked structural genes which encode sulfur-catabolic enzymes and two major regulatory genes which govern their expression. The positive-acting cys-3 regulatory gene is required to turn on the expression of the sulfur-related enzymes, whereas the other regulatory gene, scon, acts in a negative fashion to repress the synthesis of the same set of enzymes. Expression of the cys-3 regulatory gene was found to be controlled by scon and by sulfur availability. The nucleotide sequence of the cys-3 gene was determined and can be translated to yield a protein of molecular weight 25,892 which displays significant homology with the oncogene protein Fos, yeast GCN4 protein, and sea urchin histone Hl. Moreover, the putative cys-3 protein has a well-defined leucine zipper element plus an adjacent charged region which together may make up a DNA-binding site. A cys-3 mutant and a cys-3 temperature-sensitive mutant lead to substitutions of glutamine for basic amino acids within the charged region and thus may alter DNA-binding properties of the cys-3 protein.In the filamentous fungus Neurospora crassa, a high degree of genetic and metabolic regulation governs the expression of sets of enzymes within various global areas of metabolism such as nitrogen, phosphorus, and sulfur catabolism (7, 18). The sulfur control circuit of N. crassa consists of a set of unlinked structural genes which specify enzymes involved in sulfur metabolism. Synthesis of this entire family of sulfur-related enzymes, which includes aryl sulfatase, choline sulfatase, sulfate permease, a high-affinity methionine permease, and an extracellular protease occurs only when cellular levels of sulfur become limited (10,16,19,22). The expression of these sulfur-catabolic enzymes is controlled by two distinct regulatory genes. One of these, designated scon (for sulfur controller), appears to act in a negative fashion; scon mutants are insensitive to sulfur catabolite repression and thus express the sulfur-related enzymes in a constitutive fashion (4). The other sulfurregulatory gene, known as cys-3, acts in a positive manner to activate the expression of the various sulfur-related genes (17,21).Two different sulfate permease species, specified by distinct and unlinked structural genes, are both members of the sulfur circuit (16). The structural gene for sulfate permease II, cys-14, has been cloned and shown to encode an mRNA of approximately 3 kilobases (kb) whose content is highly regulated by cys-3, by scon, and by the sulfur status of the celis (11). Thus, it appears that both of the regulatory genes as well as sulfur repression act at the level of transcription or at a closely related step such as mRNA processing or stability. The cys-3 major sulfur control gene has been postulated to encode a regulatory protein which is needed to turn on the expression of the entire set of sulfur-related activities, presumably by binding at target DNA sequences adjacent to each structural gene (11). Mutants...