We previously reported the isolation of yeast mutants that seem to affect the function of certain autonomously replicating sequences (ARSs). These mutants are known as mcm for their defect in the maintenance of minichromosomes. We have now characterized in more detail one ARS-specific mutation, mcm1-1. This Mcm1 mutant has a second phenotype; MAT alpha mcm1-1 strains are sterile. MCM1 is non-allelic to other known alpha-specific sterile mutations and, unlike most genes required for mating, it is essential for growth. The alpha-specific sterile phenotype of the mcm1-1 mutant is manifested by its failure to produce a normal amount of the mating pheromone, alpha-factor. In addition, transcripts of the MF alpha 1 and STE3 genes, which encode the alpha-factor precursor and the alpha-factor receptor, respectively, are greatly reduced in this mutant. These and other properties of the mcm1-1 mutant suggest that the MCM1 protein may act as a transcriptional activator of alpha-specific genes. We have cloned, mapped and sequenced the wild-type and mutant alleles of MCM1, which is located on the right arm of chromosome XIII near LYS7. The MCM1 gene product is a protein of 286 amino acid residues and contains an unusual region in which 19 out of 20 residues are either aspartic or glutamic acid, followed by a series of glutamine tracts. MCM1 has striking homology to ARG80, a regulatory gene of the arginine metabolic pathway located about 700 base-pairs upstream from MCM1. A substitution of leucine for proline at amino acid position 97, immediately preceding the polyanionic region, was shown to be responsible for both the alpha-specific sterile and minichromosome-maintenance defective phenotypes of the mcm1-1 mutant.