Genetic characterisation of dsRNA plasmids in Italian Saccharomyces cerevisiae wine yeasts

Yeasts are responsible for the production of fermented foods and, in partic ular Saccharomyces cerevisiae is considered as the principal wine yeast. Th e existence of dsRNA plasmids in the cytoplasm of yeast has been correlated with the production of a killer toxin lethal to other strains of the same species, making this character of great interest in enology. In the present study we have analysed a total of 56 strains of Sacch. cerevisiae from two different geographical areas of Italy: 27 of them came from the Basilicata region (South) while the remaining 29 strains were from Northern regions. These strains were assayed for the presence of dsRNA plasmids in their cyto plasm. The same strains were also tested for the production of the killer t oxin. On the basis of length of dsRNAs, most strains were classified as K-2 -K-3. The K-2-K-"3" type was much less common and K-1 killer teas absent. These results were corroborated by plate assay and have been explained as a consequence of the pH of the fermenting musts. The existence of neutral st rains that showed immunity to killer toxins has been mainly correlated to v ariation in length in the M genome of dsRNAs. However, the absence of viral particles of the M genome characterised most sensitive strains, while in s ome cases small M genomes were present. In those cases in which the M genom e was present and the killer activity was lost, insertions/deletions affect ing the killer preprotoxin gene has been suggested.