Role of intron splicing in the function of the MATa1 gene of Saccharomyces cerevisiae.

The MATa1 gene of Saccharomyces cerevisiae is unique in yeast cells in that it contains two short intervening sequences (IVS1 and IVS2) 54 and 51 nucleotides long. The 3' intron is inefficiently spliced and results in the accumulation of transcript with only the first intron removed, leading to the speculation that the gene may produce different protein products by alternative splicing patterns. We have used in vitro mutagenic techniques to construct intronless MATa1 genes and have introduced point substitutions in the 5'-TACTAAC-3' internal conserved sequence of each intron to identify the protein product that is required for repression of haploid-specific genes. Analysis of these constructs for the ability to repress expression of an HO::lacZ fusion and for the ability to allow diploid cells to undergo sporulation during conditions of starvation revealed that the gene is functional with two, one, or no introns and that the only functional protein is the one produced when both introns are spliced from the mRNA.