Mo

RNApol SU

^^ transcription replication C ) toxin carrier toxin/immunity (type I) ^kJ toxin/immunity (type II)

Fig. 3 Schematic representation of yeast linear plasmids: mitochondrial, cytoplasmic au-tonomous/nonautonomous, and toxin type I/II encoding. Symbols as in Fig. 2. Known or assumed functions of predicted proteins are given. Colors correspond to functional categories as indicated as pairs or triplets, one of which is invariably self-sufficient with respect to replication and maintenance. Only in a few cases (e.g., Saccharomyces kluyveri pSKL, T. pullulans pTP1, and Debaryomyces polymorphus pDP1), was an autonomous element discovered without any attendance. Sequencing autonomous linear plasmids from K. lactis, S. kluyveri, D. etchellsii, and P. acaciae (Hishinuma et al. 1984; Tommasino et al. 1988; Hishinuma and Hi-rai 1991; Larsen and Meinhardt 2000; Klassen et al. 2001; Jeske and Meinhardt 2006) revealed a strictly conserved genetic organization, irrespective of the taxonomic or geographic origin. Exemplified for pPac1-1 in Fig. 3, such an illustration in principle applies for any of the autonomous cytoplasmic plas-mids sequenced thus far. pGKL2 and pSKL from K. lactis and S. kluyveri negligibly deviate as they possess an additional nonessential gene of unknown function (ORF1). Since replication and transcription is confined genetically to the nucleus, autonomous cytoplasmic elements must necessarily provide the enzymatic repertoire ensuring both replication and transcription. With the exception of the above mentioned ORF1 in pGKL2 and pSKL, nonessential additional functions, such as the killer phenotype, are invariably encoded by an accompanying smaller linear plasmid, which, however, strictly depends on an autonomous element in terms of cytoplasmic gene expression and maintenance.

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