Stability of a cloned gene in yeast grown in chemostat culture.

A study has been made of the stability of LEU2, a cloned chromosomal gene of Saccharomyces cerevisiae, when reintroduced into yeast on a number of plasmid vectors which permit a chromosomal or episomal location for the gene in either high or low copy number. Glucose-limited continuous culture was employed to ensure that there was no selection for the inserted gene. Both the rate of segregation of plasmid minus cells and the effect of the plasmid on host growth rate were found to determine plasmid stability which, in many cases, could be predicted by simple mathematical models. The presence or absence of the endogenous 2 mu plasmid of yeast was found to have an important influence on the stability of 2 mu-based vectors. This led to the discovery that, for the host strain used, the presence of 2 mu sequences represented a selective advantage for the cells.