Bidirectional barrier model to account for phasing in wild-type and Rsc8-depleted cells. (A) Nucleosome phasing is due to the presence of a putative sequence-specific barrier complex located in the NDR at the promoter, which acts through steric occlusion of nucleosomes. Its boundaries are the phasing points. The TSS is actually located within the +1 nucleosome. To explain its phasing properties, the TSS must be located at a fixed distance downstream from the barrier. In Rsc8-depleted cells, we propose that the barrier complex is smaller (reduced footprint) such that its boundaries direct phasing farther upstream of the TSS, accounting for narrowing of the NDR. Genes with filled promoters have no barrier and so phasing here will depend on the nearest barriers. (B) Interference between arrays beginning at neighboring barriers will be constructive or destructive, depending on whether their separation is an integral number of nucleosomal repeats. Destructive interference will contribute to “fuzzy positioning.”
