Figure 6.

The dual pausing model of RNAP II and nucleosome occupancy patterns during regulation of the usage of different polyA sites and transcription termination. A nucleosome upstream of a proximal site, which is adjustable and related to the site usage, is the first barrier pausing RNAP II. Meanwhile, it signals to RNAP II to prepare for a 3′ end processing event, by slowing down transcription speed. A nucleosome downstream of distal or constitutive polyA site, which is intrinsically well positioned and regulated by transcription, is the second barrier pausing RNAP II to ensure transcription termination and recognition of the last polyadenylation sites if previous sites were missed.