Микология и фитопатология, 2020, T. 54, № 2, стр. 150-152

Some strains of many ascomycetous and basidiomycetous yeast species secrete (glyco)proteins (mycocins, killer toxins) having fungicidal or fungistatic action. A molecular mass of mycocins may range from 10 to more 100 kDa. The mechanisms of their actions are different: they cause membrane damage, an arrest in the G1 phase of the cell cycle, an inhibition of DNA or glucan synthesis. The genetic determinants of mycocins can be either chromosomal or cytoplasmical. In the second case, dsRNA viruses or linear dsDNA plasmids are responsible for mycocinogeny. In spite of these varieties, one characteristic remains the same, namely, sensitivity to mycocins is restricted to organisms phylogenetically related to the mycocinogenic strains (Golubev, 2006). The degree of relatedness may be different owing to the diversity of cell-wall receptors involved in binding of mycocins that can be both unique and common for certain taxa.

From this viewpoint the communications about activity against organisms distantly related to mycocinogenic strains cast doubt. As an example, killer activity of Schizosaccharomyces pombe Lindner against Candida glabrata (Anderson) Meyer et Yarrow and Saccharomyces cerevisiae Meyen ex Hansen has been reported (Bonillia-Salinas et al., 1995), though budding yeasts belong to Saccharomycotina subphylum whereas fission yeasts do to “Archiascomycotina” (Kuramae et al., 2006). This inconsistency initiated a search of mycocinogenic strains of Schizosaccharomyces.

Strains from All-Russian Collection of Microorganisms (VKM) (http://www.vkm.ru) and Raiffeisen-Bio-Forschung (RBF, Austria) were used in the present work. Sensitivity of three day cultures grown on malt agar was determined at room temperature using the “culture against culture” method. Water suspensions (0.05 mL, 10⁵ cells/mL) were spread over the surface of a buffered (with citrate-phosphate buffer) medium containing the following (g/L): glucose, 5.0; peptone, 2.5; yeast extract, 5.0; agar, 20.0. The cultures of Schizosaccharomyces pombe were then streak inoculated. The plates were incubated until cell lawns developed. The strains forming growth inhibition zones several mm wide, zones ~1 mm wide, and no zones were recorded as sensitive, weakly sensitive, and insensitive, respectively.

When screening 39 strains of the genus Schizosaccharomyces (three species) for mycocin secretion, two strains of S. pombe, VKM Y-1876 and VKM Y-1912, received as S. acidodevoratus, were found that they show the activity (Table 1). They exhibited antagonistic activity at pH from 3.5 to 5.5 (citrate-phosphate buffer). At pH 6.0 the activity did not revealed. Growth inhibition zones were the most pronounced at pH 4.0. Zones became broader on the media supplemented with glycerol (100 ml/l). Five strains of S. pombe were sensitive, the rest strains of this species as well as S. japonicus and S. octosporus have neutral phenotype. Blue edging has not developed on the lawns of sensitive strains along the streaks of mycocinogenic S. pombe strains on the medium with methylene blue (0.03 g/l) that indicated fungistatic action of secreted agents.

The two mycocinogenic strains are not active against any representatives of Saccharomycotina belonging to 13 species of 12 genera but they act against Protomyces macrosporus, Taphrina bergeniae, T. carnea and T. tosquinetii (Table 2). All strains of Schizosaccharomyces spp. examined are insensitive to mycocin produced by type strain of Lachancea (= Kluyveromyces) waltii (Kono, Himeno, 1997).

The differences between yeast fungi in mycocin sensitivity patterns are correlated with whole range of taxonomic and phylogenetic markers, such as cell ultrastructure, composition of polysacchardes, sequence similarity of small and large subunit ribosomal RNAs (Golubev, 2012). In most cases, the strains of the same species have identical responses to specific mycocins. As a rule, teleomorphic taxa are homogenous, unlike the anamorphic taxa in which heterogeneity in mycocin sensitivity patterns is much more widespread.

The data presented further support placement of Protomyces, Schizosaccharomyces and Taphrina at the base of Ascomycota as the “Archiascomycetes” (the subdivision Taphrinomycotina).