Scarcity of ars sequences isolated in a morphogenesis mutant of the yeast Yarrowia lipolytica

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Abstract
Previous attempts to isolate autonomously replicating sequences (ars) from the dimorphic yeast Yarrowia lipolytica have been unsuccessful. We isolated a Fil− mutant unable to produce hyphae and growing only in a yeast form to facilitate ars isolation. This mutant was transformed with a Y. lipolytica DNA bank and several unstable clones were obtained. Extrachromosomal plasmids were evidenced in yeast recovered in Escherichia coli and characterized by restriction mapping. They were able to retransform Fil− and Fil+ yeast strains at high frequency and transformants displayed a slightly unstable phenotype. The detailed analysis of the plasmids showed that only two different ars sequences had been isolated, each of them corresponding to a unique sequence in the Y. lipolytica genome. We concluded that functional ars sequences that can be cloned on plasmids are rare in this yeast.