Epigenomic and RNA structural correlates of polyadenylation.

Polyadenylation (poly(A)) of mRNA plays a critical role in regulating gene expression. Identifying the sequence, structural, and epigenomic determinants of poly(A) site usage is an important long term goal. Several cis elements that mediate poly(A) regulation have been identified. Highly used poly(A) sites are also known to have a greater nucleosome occupancy in the immediate downstream. However, a detailed exploration of additional epigenomic and mRNA structural correlates of poly(A) site usage has not been reported. Importantly, functional interaction between sequence, structure, and the epigenome in determining the poly(A) site usage is not known. We show that highly used poly(A) sites are positively associated with an mRNA structure that is energetically more favorable and one that better exposes a critical polyadenylation cis element. In exploring potential interplay between RNA and chromatin structure, we found that a stronger nucleosome occupancy downstream of poly(A) site strongly correlated with (1) a more favorable mRNA structure, and (2) a greater accumulation of RNA Polymerase II (PolII) at the poly(A) site. Further analysis suggested a causal relationship pointing from PolII accumulation to a stable RNA structure. Additionally, we found that distinct patterns of histone modifications characterize poly(A) sites and these epigenetic patterns alone can distinguish true poly(A) sites with ~76% accuracy and also discriminate between high and low usage poly(A) sites with ~74% accuracy. Our results suggest a causative link between chromatin structure and mRNA structure whereby a compacted chromatin downstream of the poly(A) site slows down the elongating transcript, thus facilitating the folding of nascent mRNA in a favorable structure at poly(A) site during transcription. Additionally we report hitherto unknown epigenomic correlates for poly(A) site usage.