Yeast strains killer

C c lasmic inheritance transfer of genetic information in eukaryotic sexual reproduction, which is not carried by the chromosomes of the nucleus. C.i. is due to extrachromosomal genetic carriers, e.g. mitochrondrial and plastid DNA. C. i. does not obey Mendelian rules, and it permits mixing of cytoplasmic genetic factors during mitosis. Certain petite mutations of yeast, the killer property of certain strains of Paramecium and leaf pigmentation in Antirrhinum majus are examples of properties transmitted by Ci. 

Certain yeast strains, known as killer strains (K), secrete proteinic toxins into their environment that are capable of killing other, sensitive strains (S). The killer strains are not sensitive to their toxin but can be killed by a toxin that they do not produce. Neutral strains (N) do not produce a toxin but are resistant. The action of a killer strain on a sensitive strain is easy to demonstrate in the laboratory on an agar culture medium at pH 4.2-4.7 at 20°C. The sensitive strain is inoculated into the mass of agar before it solidifies then the strain to be tested is inoculated in streaks on the solidified medium. If it is a killer strain, a clear zone in which the sensitive strain cannot grow encircles the inoculum streaks (Figure 1.17). 

Vaughan-Martini, A., Cardinali, G., Martini, A. Differential killer sensitivity as a tool for fingerprinting wine-yeast strains of Sacchammyces cerevisiae. Journal of Industrial Microbiology 1996,17(2), 124-127. 

Not all hereditary traits follow the Mendelian patterns expected for chromosomal genes. Some are inherited directly from the maternal cell because their genes are carried in the cytoplasm rather than the nucleus. There are three known locations for cytoplasmic genes the mitochondria, the chloroplasts, and certain other membrane-associated sites.285 286 An example of the last is found in "killer" strains of yeast. Cells with the killer trait release a toxin that kills sensitive cells but are themselves immune. The genes are carried in double-stranded RNA rather than DNA, but are otherwise somewhat analogous to the colicin factors of enteric bacteria (Box 8-D). Similar particles (kfactors) are found in Paramecium.287... 

Similar techniques were used by Shinohara et al (71) to develop hybrids with increased production of fusel alcohols and esters. Protoplast fusion techniques have been used to confer amylolytic activity to brewery yeasts (22) and ethanol tolerance to wine yeasts (70) Farris et al (72) used protoplast fusion to produce hybrids with killer factor that is, the ability to secrete proteinic toxins. Kunkee and coworkers (25) utilized a leucine auxotrophic mutant strain of S. cerevisiae (UCD Montrachet 522) to produce base wine for brandy production the mutant strain produces less isoamyl alcohol, reducing the quantity of fusel alcohols in the subsequent brandy. And Thornton (48) discussed the progress in utilizing plasmid vectors to introduce new genes into wine yeasts he cautioned, however, that until the yeast genome is better understood that direct gene manipulation techniques will be of limited value. 

Certain strains of yeast including S, cerevisiae and S, carlsbergensis have developed the ability to secrete protein material which, over a particular range of pH values, is capable of killing sensitive strains of yeast (Chapter 16). Under certain circumstances therefore these killer yeasts are able to dominate fermentation. Several killer strains, otherwise suitable for brewing, are characterized by the production of unacceptable medicinal flavours [96]. 

This phenomenon, the killer factor, was discovered in S. cerevisiae but killer strains also exist in other yeast genera such as Hansemla, Candida, Kloeckem, Hanseniaspora, Pichia, Tom-lopsis, Kluyveromyces and Debaryomyces. Killer yeasts have been classified into 11 groups according to the sensitivity reaction between strains as well as the nature and properties of the toxins involved. The killer factor is a cellular interaction model mediated by the proteinic toxin excreted. It has given rise to much fundamental research (Tipper and Bostian, 1984 Young, 1987). Barre (1984, 1992), Radler (1988) and Van Vuuren and... 

The determinants of the killer factor are both cytoplasmic and nuclear. In S. cerevisiae, the killer phenomenon is associated with the presence of double-stranded RNA particles, virus-Uke particles (VLP), in the cytoplasm. They are in the same category as non-infectious mycovirus. There are two kinds of VLP M and L. The M genome (1.3-1.9 kb) codes for the K toxin and for the immunity factor (R). The L genome (4.5 kb) codes for an RNA polymerase and the proteinic capsid that encapsulates the two genomes. Killer strains (K+R+) secrete the toxin and are immune to it. The sensitive cells (K R ) do not possess M VLP but most of them have L VLP. The two types of viral particles are necessary for the yeast cell to express the killer phenotype (K+R+), since the L mycovirus is necessary for the maintenance of the M type. 

Fig. 1.18. Yeast growth and survival curves in a grape juice medium containing killer toxin (Barre, 1992) +, 10% K2 strain active culture supernatant O, 10% supernatant inactivated by heat treatment, (a) White juice, pH 3.4 cells in exponential phase introduced at time = 0. (b) Same juice, cells in stationary phase introduced at time = 0. (c) Red juice extracted by heated maceration, pH 3.4 cells in exponential phase introduced at time = 0...

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