By using the renaturation kinetics technique we tried to get informations about the maintenance of the 2 pm plasmid in yeast cells. For this purpose we determined the 2 pm plasmid copy number: in various yeast strains, in a special set of mutants, in cells treated with ethidium bromide and cycloheximide and in different yeast strains obtained by transformation with 2 pm chimeric plasmids.
According to the strain used the proportion of 2pro DNA varied from 1.1% to 3.9%, which corresponds to 24 to 88 2 pm molecules per haploid genome.
The particular multiresistant mutant, where the frequent loss of oligomycine resistance is correlated with the loss of extractible covalently closed circular DNA, contained 39 2/lm copies per haploid genome. In the partial revertant oligomycine sensitive all the 2 pm DNA sequences were lost. (Less than 0.1 copy per haploid genome.) Ethidium bromide did not affect the 2 /~m copy number while cycloheximide induces an increase of 36%.
When a strain containing 88 2 grn DNA copies per haploid genome is transformed with 2 ~m chimeric plasmids there is no significative change in the total number of plasmid: 36 copies of endogenous and 44 of chimeric plasmid per haploid genome. When 2 /am chimeric plasmids were introduced in our 2 pro-less strain despite the stability of the transformants, there is only 8 copies per haploid genome.