Folded chromosomes in non-cycling yeast cells

Folded chromosomes from stationary phase or ammonia-starved yeast (Saccharomyces cerevisiae) cells can be isolated as compact structures, distinct and separable by sedimentation from the folded chromosomes of pre-replicative (G₁) and post-replicative (G₂) nuclei. Such cells are in a dormant or non-cycling (G₀) stage. The folded genome from such cells is referred to as theg ₀ form and has a sedimentation velocity of about 1700S. Sedimentation analysis of mixed G₀ and G₁ and G₂ lysates indicates that theg ₀ structure is not an artifactual breakdown product of theg ₁ org ₂ structures. A comparison of the proteins fromg ₀ versusg ₁ andg ₂ structures by gel electrophoresis has revealed differences in about 10–11 non-histone and perhaps 2 histone proteins. Entry into the G₀ stage, and emergence into G₁ after G₀ arrest, are accompanied by an ordered transition fromg ₂ tog ₁ tog ₀, and fromg ₀ tog ₁ tog ₂ forms, respectively. Hence, entry into G₀ and re-emergence from G₀ can be considered as differentiative processes, not normally part of the cell cycle, and accompanied by specific changes in the tertiary organization of the genome.