Multiple-copy integration in the yeast Yarrowia lipolytica

Using an EcoRI-BglII fragment of the G unit of the rDNA of Y. lipolytica and a set of 11 deletions in the URA3 promoter, we have constructed several plasmids to test gene amplification in the rDNA. These plasmids contain the rDNA fragment for integration, defective versions of the URA3 gene, the XPR2 gene encoding alkaline extracellular protease (AEP) as a reporter gene, and part of the pBR322 plasmid for selection and replication in E. coli. Among these plasmids, one corresponds to a deletion which allows multiple integration into the rDNA (plasmid pINA773). Two other plasmids (pINA767 and pINA772) give multiple integration only with a mutated URA3 gene. Transformants carrying these three plasmids were tested for copy number, stability, chromosomal localization and AEP secretion. Transformants containing plasmids pINA767, 772 and 773 displayed an average copy number of 5, 12 and 25–60 copies respectively of the plasmid, as estimated by PCR and DNA hybridization. Integrations occurred in only one chromosome except for transformants containing 60 copies where copies were observed at least in two different chromosomes. Multiple integrations were found both as tandem repeats and as dispersed copies. Plasmid copy number was stable in both minimum and rich media, for strains containing less than ten copies per cells. However, for higher copy number, multiple integrations were stable only when AEP synthesis was not induced, while in inducing medium stability of the multiple integrations was dramatically affected.