Wine Saccharomyces cerevisiae strains producing a new killer toxin (Klus) were isolated. They killed all the previously known S. cerevisiae killer strains, in addition to other yeast species, including Kluyveromyces lactis and Candida albicans. The Klus phenotype is conferred by a medium-size double-stranded RNA (dsRNA) virus, Saccharomyces cerevisiae virus Mlus (ScV-Mlus), whose genome size ranged from 2.1 to 2.3 kb. ScV-Mlus depends on ScV-L-A for stable maintenance and replication. We cloned and sequenced Mlus. Its genome structure is similar to that of M1, M2, or M28 dsRNA, with a 5-terminal coding region followed by two internal A-rich sequences and a 3-terminal region without coding capacity. Saccharomyces cerevisiae killer strains produce and secrete protein toxins that are lethal to sensitive strains of the same or related yeast species. These toxins have been grouped into three types, K1, K2, or K28, based on their killing profiles and lack of cross-immunity. Members of each group can kill nonkiller yeasts as well as killer yeasts belonging to the other types. They are immune, however, to their own toxin or to toxins produced by strains of the same killer type (for reviews, see references 21, 32, 33, and 47).K1, K2, and K28 killer toxins are genetically encoded by medium-size double-stranded RNA (dsRNA) viruses grouped into three types, M1, M2, and M28, of 1.6, 1.5, and 1.8 kb, respectively. Only one strand (the positive strand) has coding capacity. In each case, the 5Ј-end region contains an open reading frame (ORF) that codes for the toxin precursor, or preprotoxin (pptox), which also provides immunity. The three toxin-coding M dsRNAs show no sequence homology to each other (35). M viruses depend on a second large (4.6-kb) dsRNA helper virus, L-A, for maintenance and replication. L-A provides the capsids in which both L-A and M dsRNAs are separately encapsidated (reviewed by Schmitt and Breinig [33]). L-BC virus is an L-A-related virus, with a similar 4.6-kb genome size, which coexists with L-A in most killer and nonkiller S. cerevisiae strains (1, 37). L-BC shows no sequence homology with L-A, and it has no known helper activity. L-A and L-BC, however, share the same genomic organization. They code for two proteins, the major coat protein Gag and a minor Gag-Pol fusion protein translated by a Ϫ1 ribosomal frameshifting mechanism (7,10,17,26). These viruses, called Saccharomyces cerevisiae viruses (ScVs), belong to the Totiviridae family and are cytoplasmically inherited, spreading horizontally by cell-cell mating or by heterokaryon formation (47). In addition to the M dsRNA-encoded killer toxins, other S. cerevisiae killer toxins, named KHR and KHS, showing weak killer activity, are encoded on chromosomal DNA (13,14).The positive strands of both L-A and M viruses contain cis signals in their 3Ј-terminal regions essential for packaging and replication (46). The signal for transcription initiation has been proposed to be present in the first 25 nucleotides (nt) of L-A, probably in the 5Ј-terminal sequ...

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Yeasts within the Saccharomyces sensu stricto cluster can produce different killer toxins. Each toxin is encoded by a medium size (1.5–2.4 Kb) M dsRNA virus, maintained by a larger helper virus generally called L-A (4.6 Kb). Different types of L-A are found associated to specific Ms: L-A in K1 strains and L-A-2 in K2 strains. Here, we extend the analysis of L-A helper viruses to yeasts other than S. cerevisiae, namely S. paradoxus, S. uvarum and S. kudriavzevii. Our sequencing data from nine new L-A variants confirm the specific association of each toxin-producing M and its helper virus, suggesting co-evolution. Their nucleotide sequences vary from 10% to 30% and the variation seems to depend on the geographical location of the hosts, suggesting cross-species transmission between species in the same habitat. Finally, we transferred by genetic methods different killer viruses from S. paradoxus into S. cerevisiae or viruses from S. cerevisiae into S. uvarum or S. kudriavzevii. In the foster hosts, we observed no impairment for their stable transmission and maintenance, indicating that the requirements for virus amplification in these species are essentially the same. We also characterized new killer toxins from S. paradoxus and constructed “superkiller” strains expressing them.