S. cerevisiae cells: control vs positive supercoiling accumulation in absence of telomere silencing after 0 and 120 min

Positive DNA helical stress accumulates in vivo by the unbalanced relaxation of positive and negative DNA supercoils in Δtop1, top2ts, pGPD:TopA yeast cells. The resulting overwinding of DNA greatly diminishes overall RNA synthesis. Here we show that whereas most genes reduce their transcript levels by several fold, genes situated at less than 100 kb from the chromosomal ends (near 15% of the genome) are gradually unaffected. This positional effect denotes that chromosomal ends are topologically open, thus precluding the accumulation of DNA helical stress in telomere-proximal regions. The progressive escape from the transcription stall observed in all the chromosome extremities indicates also that friction restrictions to DNA twist diffusion, rather than tight topological boundaries, suffice to confine DNA helical tension along eukaryotic chromatin. Keywords: Time course of positive helical tension accumulation in absence of telomere silencing. Two-condition experiment: delta top1 delta sir3-TOPA vs delta top1 delta sir3 top2-ts-TOPA. 2 time points: 0min, 120min. 3 biological replicates per condition and time point, independently grown in leu- selective media and harvested.