We isolated three highly homologous genes, PIR1, PIR2 and PIR3, collectively called the PIR genes. The remarkable feature of their putative amino acid sequence is that they contain a sequence consisting of 18-19 amino acid residues repeated tandemly seven to ten times. Genes homologous to PIR were found in Kluyveromyces lactis and Zygosaccharomyces rouxii but not in Schizosaccharomyces pombe, suggesting that a set of PIR genes plays some role in budding yeast. Bias of codon usage seen in each of the PIR translation products suggests that they are expressed abundantly. The fact that disruption of each gene is viable indicates that none of them is essential. The double disruptants, pir1 pir2, were viable under various conditions, such as higher temperature (37 degrees C) or high salt concentration, but showed a slow-growing phenotype on an agar slab. Furthermore, they were sensitive to heat shock. Addition of a pir3 disruption to the pir1 pir2 double disruptant brought about no phenotypic difference from the original double mutant. PIR1 and PIR3 are closely linked to each other and are on chromosome XI.

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The Arg870His mutation was suggested to cause HCM. In contrast, the gene with the nonsense mutation would encode for a cardiac beta-MHC protein of only 53 amino acid residues, which may be too short to be incorporated into the thick filament assembly of cardiac myosin chains and showed no dominant phenotype of heart disease. This is the first report of a nonsense mutation in the human cardiac beta-MHC gene.

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