Extranuclear gene expression in yeast: evidence for a plasmid-encoded RNA polymerase of unique structure

Strains of the yeast Kluyveromyces lactis that produce killer-toxin have been found to contain two linear dsDNA plasmids, k1 (8.9 Kb) and k2 (13.4 Kb). The four transcribed open reading frames of plasmid k1 contain no recognisable yeast nuclear expression signals. Moreover, a toxin subunit gene fused with the lacZ gene of Escherichia coli is not detectably expressed when introduced to K.lactis or Saccharomyces cerevisiae on a nuclear vector, even when native k1 and k2 are present in the cell. This and other evidence is consistent with the hypothesis that k1 and k2 reside in an extranuclear location, and do not utilise the nuclear RNA polymerases I, II or III for transcription of their genes. Sequencing of plasmid k2, which is thought to encode factors necessary for the maintenance or expression of k1, reveals an open reading frame predicted to encode a 974 amino acid polypeptide with homology to several DNA-directed RNA polymerases. We suggest that this is a component of a novel plasmid-specific extranuclear gene expression system.