Association of the 2-micron DNA plasmid with yeast folded chromosomes

The 2-micron DNA plasmid cosedimented in sucrose gradients with the folded chromosome on centrifugation of lysates from logarithmically growing yeast cells. It banded with both the slow-sedimenting g1 form derived from prereplicative cells and the fast-sedimenting g2 form from postreplicative cells. When cells were induced to withdraw from the cell cycle by nitrogen starvation, the folded chromosome was converted to the g0 form, which sedimented more slowly than the g1 form, and only 1/10th the amount of 2-micron DNA cosedimented with this g0 form as compared to the logarithmic-phase forms, g1 and g2. Expression of the temperature-sensitive cell division cycle (cdc) 28 mutation, which defines the "start" of the cell cycle, resulted in a rapid alteration in the folded chromosome sedimentation pattern, and less than 20% of the 2-micron DNA cosedimented with the folded chromosome. These results raise the possibility that association of the 2-micron DNA plasmid with structures in the cell corresponding to the folded chromosome is subject to cell division cycle control.