Spreading of gene silencing through yeast minichromosomes

Chromatin silencers are regulatory DNA signals required to initiate gene silencing that represses and spreads to nearby genes. Barrier elements protect genes from being silenced by establishing boundaries that prevents heterochromatin spreading. Here we dissected functional topography of silencers, nucleosomes and barrier elements using circular minichromosomes in Saccharomyces cerevisiae . We found that HML, E and I silencers can efficiently repress the URA3 reporter on multi‐copy yeast minichromosomes. In contrast to what had been observed in yeast genome, we report that two distinct heterochromatin barrier elements, STAR and TEF2‐UASrpg, control spreading of gene silencing through a position‐independent desilencing activity. We have also examined spreading of gene silencing through an array of strong stably positioned clone 601 nucleosomes with long 207 bp repeats that form heterochromatin in higher eukaryotes and short 167 bp repeats predominantly found in S. cerevisiae chromatin. Though both long and short repeats were able to conduct silencing, we found that the short yeast nucleosomal repeats were more efficient in conducting silencing in vivo and were also more condensed in vitro . We suggest that the shorter nucleosomal repeats in yeast are more suitable for conducting silencing and chromatin condensation in absence of the linker histone. Source of research support: NSF, TSF and AAUW.