Folded chromosomes in non-cycling yeast cells

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Abstract
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.