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Killer DNA Plasmids of the Yeast Kluyveromyces lactis : I. Mutations Affecting the Killer Phenotype
M Wésolowski1, A Algeri, P Goffrini
1Institut Curie - Biologic, Centre Universitaire, Bâtiment 110, 91405, Orsay, France.
Abstract:
The killer character of Kluyveromyces lactis is associated with a cytoplasmic genetic element which confers on the host cells: a) the capacity to kill sensitive cells of various yeast by producing a diffusible toxin, and b) the immunity to that toxin. The killing activity is associated with the presence of two linear DNA plasmids that we call kl and k2. Both plasmids seem to be required for the expression of the killing activity. Mutants defective for the killing activity have been isolated by mutagenesis and are either cytoplasmic or nuclear. Four classes of cytoplasmic mutations have been found: 1) loss of kl ; 2) loss of both k1 and k2; 3) internal deletion in kl and 4) probable point mutations in the plasmids. Among the nuclear mutants, many conserved the normal plasmids, but expression of the killer phenotype was blocked. In some respects, the organisation of this plasmid system resembles a DNA version of the double-stranded RNA killer system of Saccharomyces cerevisiae.
Insights
The killer trait in Kluyveromyces lactis yeast is linked to two DNA plasmids (kl and k2). These plasmids enable toxin production and immunity, with mutations affecting this killer phenotype.
Area of Science:
- Microbiology
- Molecular Biology
- Yeast Genetics
Background:
- Kluyveromyces lactis exhibits a 'killer' phenotype, enabling it to secrete a toxin that kills sensitive yeast strains.
- This killer trait is conferred by a cytoplasmic genetic element, providing both toxin production and self-immunity.
- The genetic basis of this killer character was previously less understood, particularly concerning the role of plasmids.
Purpose of the Study:
- To investigate the molecular basis of the killer character in Kluyveromyces lactis.
- To identify and characterize the genetic elements responsible for toxin production and immunity.
- To compare the Kluyveromyces lactis killer system with other known yeast killer systems.
Main Methods:
- Isolation and characterization of mutants defective in killer activity through mutagenesis.
- Analysis of cytoplasmic and nuclear mutations affecting the killer phenotype.
- Investigation of the role of specific DNA plasmids (kl and k2) in killer activity.
- Comparative analysis of the plasmid system's organization with other yeast killer systems.
Main Results:
- The killer activity is directly associated with the presence of two linear DNA plasmids, designated kl and k2.
- Both kl and k2 plasmids are essential for the expression of the killer phenotype.
- Cytoplasmic mutations involved loss of kl, loss of both plasmids, internal deletions in kl, or point mutations within the plasmids.
- Nuclear mutations could block killer phenotype expression even when the plasmids were present, indicating regulatory roles.
Conclusions:
- The killer character of Kluyveromyces lactis is mediated by a system involving two linear DNA plasmids, kl and k2.
- Both plasmids are necessary for both toxin production and immunity.
- The organization of this plasmid system shares similarities with the double-stranded RNA killer system found in Saccharomyces cerevisiae, suggesting convergent evolution or shared ancestry.
