Functions encoded by the yeast plasmid pSB3 were analysed in and . The autonomously replicating sequence (ARS) and partitioning mechanisms of pSB3 worked as satisfactorily in ME3 as in the native host IFO 1730 (formerly var. ). The ARS in ME3 was located within a 168 bp II–III fragment spanning part of the inverted repeat (IR) and a unique region contiguous to it. The FLP enzyme (responsible for the intramolecular recombination at IRs) of pSB3 was functional in ME3. In spite of the similarity of the putative recognition site for the FLP enzyme in pSB3 and pSR1, the FLP enzyme of pSB3 did not recognize the recombination site of pSR1, and the FLP enzyme of pSR1 did not use the recombination site of pSB3. Three transcripts of 3·4, 1·8 and 1·1 kb from pSB3 in ME3 were identified by Northern blotting; they encompassed the and genes, respectively. pSB3 contained a region from which no poly(A)-containing RNA was transcribed.