["Killer" plasmid mutants obtained by exposure to 5-fluorouracil]

Area of Science:

• Microbiology
• Yeast Genetics
• Molecular Biology

Background:

• Certain strains of Saccharomyces cerevisiae possess a killer phenotype mediated by a cytoplasmic determinant (KIL-k).
• This killer trait is associated with specific double-stranded RNA (dsRNA) molecules.
• Understanding the genetic basis of killer activity and its stability is crucial for yeast research.

Purpose of the Study:

• To investigate the genetic alterations leading to instability in the killer phenotype of Saccharomyces cerevisiae.
• To characterize the nature of mutations affecting the cytoplasmic killer determinant (KIL-k).
• To analyze the relationship between dsRNA composition and killer instability.

Main Methods:

• Treatment of a k2 killer strain of Saccharomyces cerevisiae with 5-fluorouracil to induce mutations.
• Selection and characterization of nonstable killer (Knst) mutants exhibiting segregation of sensitive clones.
• Analysis of dsRNA composition in wild-type and mutant strains.

Main Results:

• 5-fluorouracil treatment yielded nonstable killer (Knst) mutants of Saccharomyces cerevisiae, segregating sensitive clones at 2-20%.
• Killer activity alteration in these mutants results from mutations in the cytoplasmic killer determinant (KIL-k), forming KIL-nst.
• KIL-nst partially suppresses the wild-type determinant and generally maintains dsRNA composition, though M dsRNA reduction is observed in some mutants.

Conclusions:

• The instability of the killer phenotype in these mutants is linked to mutations in the KIL-k determinant.
• The mutant KIL-nst determinant exhibits partial suppression of the wild-type determinant.
• Segregation of sensitive clones in mitotic divisions is likely due to secondary alterations of the KIL-nst determinant.