Yeast plasmids have fewer superhelical turns than predicted by the nucleosome repeat of yeast DNA

Abstract

The superhelical density of three Saccharomyces cerevisiae plasmids was determined with respect to a defined reference state during vegetative growth and stationary phase. The levels of supercoiling determined were ~20% lower than predicted by comparisons with SV40 DNA and reconstituted minichromosomes using histones from higher eukaryotes. In two different plasmids with the ARS1 origin of replication, the level of supercoiling changed substantially as the host cells entered stationary phase. Supercoiling of the endogenous 2-μm plasmid during vegetative growth was lower than in the ARS1 -containing plasmids but did not change significantly upon entry of the cells into stationary phase.Key words: chromatin structure, nucleosome, yeast, DNA supercoiling.