Sequences 5' to the polyadenylation signal mediate differential poly(A) site use in hepatitis B viruses.

Abstract

Most genetic elements that employ reverse transcription generate a terminally redundant genomic RNA that serves as the template for this reaction. Because the identical polyadenylation signal is present in each terminally redundant segment, synthesis of this RNA requires that this signal be ignored on the first pass of the transcription machinery, then recognized and used on the second pass. We have studied the mechanism of this differential poly(A) site use in one family of retroid elements, the hepatitis B viruses (hepadnaviruses). Our results indicate that two features are involved: the presence of a variant poly(A) signal (TATAAA) and the participation of multiple sequences 5' to this signal that act to increase the efficiency of its use. Deletion of these upstream elements abolishes proper poly(A) site use, despite the presence of the poly(A) signal and downstream GT-and T-rich motifs known to be required for polyadenylation. Sequences from the corresponding regions of retroviral genomes can restore proper processing to these hepadnaviral deletion mutants. Thus, functionally analogous upstream elements exist in other classes of retroid elements, including those employing the canonical AATAAA hexanucleotide signal.

3′-end-forming signals of yeast mRNA

“…In addition to the AAUAAA motif and the downstream GU-rich or U-rich element of mammalian systems, upstream sequences that contribute to the efficiency of mRNA 3Ј end formation were also identified in the mammalian viruses simian virus 40 (6,36), adenovirus (8,9), hepatitis B virus (29), and human immunodeficiency virus type 1 (3,11,12,38,39). These elements function in an orientation-and position-dependent manner, and several appear to be functionally analogous.…”

A dissection of the cauliflower mosaic virus polyadenylation signal.

“…In vertebrate systems, it has been shown that an AATAAA and a downstream element are the only necessary requirements for efficient processing and that the polyadenylation site is usually at an A residue (Kessler et al 1986;Levitt et al 1989). However, upstream elements in addition to the downstream element have been described for the SV40 late poly (A) signal (Carswell and Alwine 1989), for the adenovirus late transcription unit (DeZasso and Imperiale 1989), and for the hepatitis B virus (Russnak and Ganem 1990). In most cases, the upstream element induces the recognition of weak poly( A) signals, thereby allowing a tight regulation of mRNA 3'-end formation.…”

“…Both the AATAAA and the downstream element are required for formation of the specific cleavage and polyadenylation complexes (Skolnik-David et al 1987;Gilmartin and Nevins 1989). Additional upstream sequences were shown to improve recognition of the SV40 late poly(A) signal (Carswell and Alwine 1989) and the hepatitis B virus poly(A) signal (Russnak and Ganem 1990) and to influence poly(A) site selection in the adenovirus late transcription unit (DeZasso and Imperiale 1989).…”

Molecular and Cellular Biology

“…The deviant nature of the HBV poly(A) signal might be important in this process (33). Indeed, in vitro studies designed to determine the relationship between the sequence of the poly(A) signal and its activity have classified the HBV type of poly(A) signal sequence (TATAAA) as a poor one because it supports RNA processing with only 17% efficiency (39).…”

“…Interestingly, in spite of its remarkable inefficiency some viruses tend to prefer this deviant poly(A) signal. These include all the mammal hepadnaviruses (33), the figwart mosaic virus (34), and Epstein-Barr virus (40). Particularly puzzling is the fact that this deviant box is conserved among the different members of mammalian hepadnaviruses, raising an interesting possibility that it has a unique but yet-unidentified role.…”