Structure and Function of the PET18 Locus of Saccharomyces Cerevisiae
The pet18 locus is located 12.5 centimorgans (cM) centromere proximal to the MAT locus (12). Three independently isolated pet18 mutants (4,11, 14) display the same phenotypes, which are (a) respiratory deficiency, (b) inability to maintain killer phenotype, and (c) temperature sensitivity in growth. Leibowitz and Wickner (8) showed that the pet18 mutation resulted in the complete loss of mitochondrial DNA. Most strains of Saccharomyces cerevisiae carry double-stranded RNA (dsRNA) encapsidated in virus-like particles (VLPs) (6). Some strains, called killer yeasts (K+ phenotype), secrete a protein toxin and carry a dsRNA of 1.8 kb (M1) or 1.5 kb (M2) [see Wickner (18) for a summary of the killer system], in addition to a larger dsRNA of 4.5 kb (L). M dsRNA encodes the protein toxin secreted by killer cells (1). All these dsRNAs are genetic determinants localized in the cytoplasm; thus, they show non-Mendelian inheritance. Mutations in any of 30 chromosomal genes, including 28 mak genes, spe2 (3), and pet18, result in the loss of the M dsRNA. In contrast, L dsRNA can be found irrespective of the killer phenotype. L dsRNA consists of a mixture of at least two different species of dsRNA of similar size (L-A and L-BC) (15). The L-A dsRNA encodes a major coat protein (88 kDa) of the VLPs (7).
KeywordsTemperature Sensitivity Autonomously Replicate Sequence Tetrad Analysis Diploid Yeast Major Coat Protein
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