Mechanism of high-copy-number integration of pMIRY-type vectors into the ribosomal DNA of Saccharomyces cerevisiae.

Targeted integration of the yeast plasmid pMIRY2 into the ribosomal DNA (rDNA) of Saccharomyces cerevisiae by homologous recombination results in transformants carrying 100-200 copies of the plasmid per cell which are stably maintained over a large number of generations [Lopes et al., Gene 79 (1989) 199-206]. These properties make pMIRY2 an attractive vector for high-level production of (heterologous) proteins by yeast cells. We have investigated the mechanism underlying high-copy-number (hcn) integration of pMIRY-type plasmids and show that either targeting to a location outside the rDNA locus or use of the wild-type LEU2, instead of the deficient LEU2d gene, as selection marker reduces the copy number to the low value characteristic of standard integrating (YIp-type) yeast plasmids. Further experiments demonstrate that the hcn of pMIRY-type plasmids is achieved by amplification of a small number of copies initially integrated into the rDNA locus. Amplification depends upon the strong selection pressure created by the extremely low expression of the deficient LEU2d gene, but not on the presence of this gene per se. The hcn integration also occurs when either the TRP1 or URA3 gene is used as the selection marker, provided expression of the marker gene is severely curtailed, e.g., by removal of most of its 5'-flanking region.