A new plasmid was found in the yeast Kluyoeromyces wdtii. This high-copy-number plasmid, named pKW1, is a double-stranded circular DNA plasmid of 5619 bp. It has several features characteristic of the 2ptype plasmids: presence of two inverted repeats and four open reading frames, as well as the interconversion of two isomeric forms. However, the nucleotide sequence shows little homology with known yeast plasmids. An ARS function was localized within a segment of 545 bp near one of the inverted repeats. Chimeric plasmids carrying this segment efficiently transformed K. wdtii. A strain of K. waltii cured of the plasmid (ciro) was also obtained. In the pKWl sequence, a functionally neutral region was found at which foreign DNA can be inserted with little effect on plasmid stability. Such constructions carrying the full sequence of pKWl replicated autonomously in a cir" host and were particularly stable. pK W 1-derived full-sequence plasmids also transformed K. thermotolerans, but not K. lactis.
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