Linear Plasmids in Yeasts

Yeast linear plasmids, originally detected in K. lactis (Gunge et al. 1981), are known to occur in a panoply of ascomycetous species belonging to different genera (such as Pichia, Candida, Debaryomyces, Saccharomycopsis, Schwanniomyces, Botryoascus) and also in the basidiomycetous representative Trichosporon (see Table 2; Kitada and Hishinuma 1987; Ligon et al. 1989; Worsham and Bolen 1990; Hayman and Bolen 1991; Bolen et al. 1992; Cong et al. 1994; Fukuhara 1995; Chen et al. 2000). A systematic screening among isolates deposited in the CBS-type culture collection revealed linear plasmids in 1.8% of the strains analyzed (Fukuhara 1995). At first sight—compared to filamentous fungi—few specimens seemed to harbor such genetic elements; however, plasmid occurrence in isolates of the same species is remarkably frequent, e.g., 16% for the pGKL system in K. lactis and 54% for pTP1 in Trichosporon pullulans (see also Table 2). The latter is so far the only basid-iomycetous yeast species known to harbor linear plasmids and it remains to be elucidated whether this is an exception.

A striking difference between linear plasmids of filamentous fungi and yeasts concerns their localization (Table 2; Gunge et al. 1982; Ligon et al. 1989; Cong et al. 1994; Fukuhara 1995). There are only two mitochondrial yeast linear plasmids known (pPH1 and pPK1 from Pichia heedi and P. kluyveri; Blaissoneau et al. 1999), whereas almost all elements in filamentous fungi are mitochondrially associated (see above; Griffiths 1995). Based on the sequence data available for a number of cytoplasmic yeast linear plasmids and the mitochondrial pPH1, three types can be distinguished (Fig. 2): (1) mitochondrial elements, which resemble linear plasmids of filamentous fungi—indeed, genes encoding TP-DNA and RNA polymerase of pPH1 are arranged as for maranhar; (2) cytoplasmic autonomous elements, which are relatively large linear plasmids spanning at least 12 kb; in addition to TP-DNA and RNA poly-merase other functions, apparently required for cytoplasmic inheritance, are encoded; and (3) cytoplasmic nonautonomous elements; smaller linear plasmids (i.e., < 12 kb), depending strictly on an aforementioned autonomous element; nonessential functions, such as a killer protein may be encoded.

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