HIGH-RESOLUTION NUCLEOSOME MAPPING REVEALS TRANSCRIPTION-DEPENDENT PROMOTER PACKAGING

Abstract

Genome-wide mapping of nucleosomes has revealed a great deal about the relationships between chromatin structure and control of gene expression, and has lead to mechanistic hypotheses regarding the rules by which chromatin structure is established. High-throughput sequencing has recently become the technology of choice for chromatin mapping studies, yet analysis of these experiments is still in its infancy. Here, we introduce a model for calling nucleosome positions from deep sequencing maps of chromatin structure and apply it to maps from S. cerevisiae. We analyze digestion series where nucleosomes are isolated from under- and over-digested chromatin. We find that certain classes of nucleosomes are unusually susceptible or resistant to overdigestion, with promoter nucleosomes easily digested and mid-coding region nucleosomes being quite stable. We find evidence for highly sensitive nucleosomes located within nucleosome-free regions suggesting that these regions are not always completely naked but instead are bound very loosely by easily-digested nucleosomes. Finally, since RNA polymerase is the dominant energy-consuming machine that operates on the chromatin template, we analyze changes in chromatin structure when RNA polymerase is inactivated via a temperature-sensitive mutation. We find evidence that RNA polymerase plays a role in nucleosome eviction at promoters, and is also responsible for retrograde shifts in nucleosomes during transcription. Loss of RNA polymerase results in a relaxation of chromatin structure to more closely match in vitro nucleosome positioning preferences. Together, these results provide analytical tools and experimental guidance for nucleosome mapping experiments, and help disentangle the interlinked processes of transcription and chromatin packaging.