A functional mRNA polyadenylation signal is required for transcription termination by RNA polymerase II.

Abstract

Polyadenylation of pre-mRNAs requires the conserved hexanucleotide AAUAAA, as well as sequences located downstream from the poly(A) addition site. The role of these sequences in the production of functional mRNAs was studied by analyzing a series of mutants containing deletions or substitutions in the SV40 early region poly(A) site. As expected, both a previously defined GU-rich downstream element and an AAUAAA sequence were required for efficient usage of the wild-type poly(A) addition site. However, when either of these elements was deleted, greatly increased levels of SV40-specific RNA were detected in the nuclei of transfected cells' Evidence is presented that this accumulation of RNA resulted from a failure of transcription termination, leading to multiple rounds of transcription of the circular templates. We conclude that the sequences required for efficient cleavage/polyadenylation of the SV40 early pre-mRNA also constitute an important element of an RNA polymerase II termination signal. A model proposing a mechanism by which the act of pre-mRNA 3' end formation is signaled to the elongating RNA polymerase, resulting in termination, is presented.

Transcription termination by nuclear RNA polymerases

“…Allosteric/anti-terminator or torpedo: a unified model for transcription termination Two models were proposed more than 20 years ago to explain the role of 39-end processing in RNAPII transcription termination (Logan et al 1987;Connelly and Manley 1988;for review, see Proudfoot 2004;Buratowski 2005;Rosonina et al 2006). The first model, known as the allosteric or anti-terminator model, proposes that transcription through the poly(A) site leads to conformational changes of the elongation complex (EC) by dissociation of elongation factors and/or association of termination factors (Logan et al 1987).…”

“…In mammals, termination can occur anywhere from a few base pairs to several kilobases downstream from the 39-end of the mature RNA (Proudfoot 1989). RNAPII transcription termination is coupled to 39-end processing of the pre-mRNA (Birse et al 1998;Hirose and Manley 2000;Yonaha and Proudfoot 2000;Proudfoot 2004;Buratowski 2005), and an intact polyadenylation signal has long been known to be necessary for transcription termination of protein-coding genes in human and yeast cells (Whitelaw and Proudfoot 1986;Logan et al 1987;Connelly and Manley 1988).…”

An end in sight? Xrn2 and transcriptional termination by RNA polymerase II

“…The most important sequence element for transcriptional termination on protein-coding genes is the polyadenylation signal (PAS), which consists of an AAUAAA hexamer (or a variant thereof) followed by a U or GU rich element [1,2]. The former is bound by the cleavage and polyadenylation specificity factor (CPSF) complex and the latter by cleavage stimulation factors (CstF).…”

“…This might be in RNA polymerase II (Pol II) itself or via an exchange of its associated factors. Another is that RNA cleavage promotes termination by generating a Pol II-associated RNA, which is degraded by a 5ʹ-3ʹ exonuclease [2,5]. This torpedo model posits that the degrading exonuclease chases down Pol II and somehow signals termination.…”

“…The torpedo model envisages that Xrn2 chases Pol II and then signals termination by a process that is still not understood [2,5]. We have discussed how paused Pol II may constitute a frequent target for Xrn2 and data from purified systems shows that prone polymerases are more effectively terminated by 5ʹ-3ʹ exonucleases [15].…”