Amplification of plasmid copy number by thymidine kinase expression in Saccharomyces cerevisiae

A 2 micron circle-based chimaeric plasmid containing the yeast LEU2 and the Herpes Simplex Virus type 1 thymidine kinase (HSV-1 TK) genes was constructed. Transformants grown under selective conditions for the LEU2 gene harboured the plasmid at about 15 copies per cell whilst selection for the HSV-1 TK gene led to an increase to about 100 copies per cell. Furthermore, the plasmid copy number could be controlled by the stringency of selection for the TK gene, and the increase in TK gene dosage was reflected in an increase in intracellular thymidine kinase activity. The mitotic stability of the plasmid in "high-copy" and "low-copy" number cells was determined. "High-copy" number cells showed a greater mitotic stability. The relationship of TK expression to plasmid copy number may be useful for the isolation of plasmid copy number mutants in yeast and the control of heterologous gene expression.