1. Decapping Goes Nuclear
Daniel Reines 
Molecular Cell 
Volume 46, Issue 3, Pages (May 2012)
DOI: /j.molcel
Copyright © 2012 Elsevier Inc. Terms and Conditions

2. Figure 1
Alternative Mechanisms of Termination of RNA Polymerization by Pol II
(A) Abortive initiation: Promoter (bent arrow)-bound Pol II (circle) autonomously spits out small RNA oligomers (in red) which are too small to be capped, when it fails to enter productive elongation.
(B) Polyadenylation-coupled termination: In the torpedo model, cleavage of the primary transcript generates a long polyadenylated mRNA that bears a 7-methyl guanine cap () linked through a 5′-5′ phosphodiester bond. This cut leaves a “stump” of RNA in the elongation complex. A 5′ to 3′ exonuclease such as Xrn2 engages the uncapped 5′ end of this RNA and degrades it. In an as yet unknown manner, pol II is evicted from the template and transcription is terminated.
(C) Termination of short, noncoding RNAs: The hnRNP-like proteins Nrd1, Nab3, and the helicase Sen1 associate with the transcript and pol II to release them from DNA.
(D) Termination of divergent pol II: A back-to-back pair of elongation complexes, each with a capped transcript (in red), have distinct fates. The elongation complex on the right, the “sense” complex, is elongation competent. That on the left is subject to the termination mechanism described by Brannon et al. The 7-methyl guanine cap is hydrolyzed off by a decapping enzyme complex (Dcp). The RNA is degraded by Xrn2 starting at this new 5′ end. Termination factor Ttf2 removes pol II from the DNA by an as yet unknown mechanism.
Molecular Cell  , DOI: ( /j.molcel )
Copyright © 2012 Elsevier Inc. Terms and Conditions