Sequences regulating temporal poly(A) site switching in the adenovirus major late transcription unit

Novel Pol II fusion promoter directs human immunodeficiency virus type 1-inducible coexpression of a short hairpin RNA and protein

We demonstrate a novel approach for coexpression of a short hairpin RNA (shRNA) with an open reading frame which exploits transcriptional read-through of a minimal polyadenylation signal from a Pol II promoter. We first observed efficient inducible expression of enhanced green fluorescent protein along with an anti-rev shRNA. We took advantage of this observation to test coexpression of the transdominant negative mutant (humanized) of human immunodeficiency type 1 (HIV-1) Rev (huRevM10) along with an anti-rev shRNA via an HIV-1-inducible fusion promoter. The coexpression of the shRNA and transdominant protein resulted in potent, long-term inhibition of HIV-1 gene expression and suppression of shRNA-resistant mutants. This dual expression system has broad-based potential for other shRNA applications, such as cases where simultaneous knockdown of mutant and wild-type transcripts must be accompanied by replacement of the wild-type protein.

Activation of the early-late switch in adenovirus type 5 major late transcription unit expression by L4 gene products

The adenovirus major late transcription unit (MLTU) encodes multiple proteins from five regions, L1 to L5, through differential splicing and polyadenylation. MLTU expression is temporally regulated; only a single product from L1 (52/55K) is expressed prior to replication, but a subsequent switch, the mechanism of which has not been defined, leads to full expression that encompasses L1 IIIa and all L2 to L5 products. Transfection of a plasmid containing the complete MLTU gave a full array of proteins in proportions similar to those in a late infection, and in a time course, the temporal pattern of expression in a natural infection was reproduced. However, a plasmid truncated after the L3 poly(A) site exclusively expressed the L1 52/55K protein and was defective in the switch to full gene expression from L1 to L3. The L4 33K protein, supplied in trans, was sufficient to upregulate cytoplasmic mRNA for MLTU products characteristic of the late pattern of expression to levels comparable to those produced by the full-length MLTU. There was a corresponding increase in expression of the L1 IIIa, L2, and L3 proteins, except hexon. Hexon protein expression additionally required both the L4 100K protein in trans and sequences downstream of the L3 poly(A) site in cis. These results indicate that induction of L4 protein expression is a key event in the early-late switch in MLTU expression, which we propose is precipitated by small amounts of L4 expression in a feed-forward activation mechanism.

Formation of mRNA 3' ends in eukaryotes: mechanism, regulation, and interrelationships with other steps in mRNA synthesis

Formation of mRNA 3' ends in eukaryotes requires the interaction of transacting factors with cis-acting signal elements on the RNA precursor by two distinct mechanisms, one for the cleavage of most replication-dependent histone transcripts and the other for cleavage and polyadenylation of the majority of eukaryotic mRNAs. Most of the basic factors have now been identified, as well as some of the key protein-protein and RNA-protein interactions. This processing can be regulated by changing the levels or activity of basic factors or by using activators and repressors, many of which are components of the splicing machinery. These regulatory mechanisms act during differentiation, progression through the cell cycle, or viral infections. Recent findings suggest that the association of cleavage/polyadenylation factors with the transcriptional complex via the carboxyl-terminal domain of the RNA polymerase II (Pol II) large subunit is the means by which the cell restricts polyadenylation to Pol II transcripts. The processing of 3' ends is also important for transcription termination downstream of cleavage sites and for assembly of an export-competent mRNA. The progress of the last few years points to a remarkable coordination and cooperativity in the steps leading to the appearance of translatable mRNA in the cytoplasm.

Encapsidation of viral DNA requires the adenovirus L1 52/55-kilodalton protein

Previous work demonstrated that the adenovirus L1 52/55-kDa protein is required for assembly of viral particles, although its exact role in the assembly process is unclear. The 52/55-kDa protein's early expression, however, suggests that it might have other roles at earlier times during infection. To uncover any role the 52/55-kDa protein might have at early times and to better characterize its role in assembly, a mutant adenovirus incapable of expressing the 52/55-kDa protein was constructed (H5pm8001). Analysis of the onset and extent of DNA replication and late protein synthesis revealed that H5pm8001-infected 293 cells entered the late stage of infection at the same time as did adenovirus type 5 (Ad5)-infected cells. Interestingly, H5pm8001-infected cells displayed slightly lower levels of replicated viral DNA and late proteins, suggesting that although not required, the 52/55-kDa protein does augment these activities during infection. Analysis of transcripts produced from the major late and IVa2 promoters indicated a slight reduction in H5pm8001-infected compared to Ad5-infected cells at 18 h postinfection that was not apparent at later times. Analysis of particles formed in H5pm8001 cells revealed that empty capsids could form, suggesting that the 52/55-kDa protein does not function as a scaffolding protein. Subsequent characterization of these particles demonstrated that they lacked any associated viral DNA. These findings indicate that the 52/55 kDa-protein is required to mediate stable association between the viral DNA and empty capsid and suggest that it functions in the DNA encapsidation process.

Alternative poly(A) site selection in complex transcription units: means to an end?

Many genes have been described and characterized which result in alternative polyadenylation site use at the 3'-end of their mRNAs based on the cellular environment. In this survey and summary article 95 genes are discussed in which alternative polyadenylation is a consequence of tandem arrays of poly(A) signals within a single 3'-untranslated region. An additional 31 genes are described in which polyadenylation at a promoter-proximal site competes with a splicing reaction to influence expression of multiple mRNAs. Some have a composite internal/terminal exon which can be differentially processed. Others contain alternative 3'-terminal exons, the first of which can be skipped in some cells. In some cases the mRNAs formed from these three classes of genes are differentially processed from the primary transcript during the cell cycle or in a tissue-specific or developmentally specific pattern. Immunoglobulin heavy chain genes have composite exons; regulated production of two different Ig mRNAs has been shown to involve B cell stage-specific changes in trans -acting factors involved in formation of the active polyadenylation complex. Changes in the activity of some of these same factors occur during viral infection and take-over of the cellular machinery, suggesting the potential applicability of at least some aspects of the Ig model. The differential expression of a number of genes that undergo alternative poly(A) site choice or polyadenylation/splicing competition could be regulated at the level of amounts and activities of either generic or tissue-specific polyadenylation factors and/or splicing factors.

Promoter-proximal poly(A) sites are processed efficiently, but the RNA products are unstable in the nucleus

The presence of two polyadenylation signals in the primary transcript of the human immunodeficiency virus type 1 (HIV-1) provirus leads to a requirement for regulation of 3'-end processing. To ensure that viral genome replication and gene expression occur, polyadenylation must occur at the poly(A) site transcribed from the 3' long terminal repeat (LTR) but not the 5' LTR. Models that have been proposed to explain this regulation include (i) inhibition of the 5' site as a result of proximity to the promoter and (ii) enhancement of the 3' site by U3 sequences that are transcribed upstream of only the 3' poly(A) site. In previous studies designed to investigate these models, a reduction in the levels of steady-state RNA was observed when the HIV-1 poly(A) site was placed within 500 nucleotides of the cap site. Although these findings were interpreted to be the result of promoter proximity effects on 3'-end processing, in vitro studies demonstrated that the HIV-1 poly(A) site was equally functional in promoter-proximal and promoter-distal positions. These results led to the hypothesis that, in vivo, the poly(A) site is fully active at this close distance but the short transcripts produced are highly unstable in the nucleus and undergo rapid degradation, precluding their appearance as abundant mRNAs in the steady-state pool. To investigate the biogenesis of these short RNAs in vivo, experiments were performed to examine directly the nuclear processing rates of the HIV-1 poly(A) site in intact cells. By using recombinant adenoviruses as expression vectors, it is now demonstrated conclusively that the HIV-1 poly(A) site is indeed processed at equivalent levels independent of its distance from the promoter. Although transcripts containing the promoter-proximal poly(A) site are processed efficiently, most undergo degradation in the nucleus instead of nucleocytoplasmic transport.

Sequence-mediated regulation of adenovirus gene expression by repression of mRNA accumulation

Gene expression in complex transcription units can be regulated at virtually every step in the production of mature cytoplasmic mRNA, including transcription initiation, elongation, termination, pre-mRNA processing, nucleus-to-cytoplasm mRNA transport, and alterations in mRNA stability. We have been characterizing alternative poly(A) site usage in the adenovirus major late transcription unit (MLTU) as a model for regulation at the level of pre-mRNA 3'-end processing. The MLTU contains five polyadenylation sites (L1 through L5). The promoter proximal site (L1) functions as the dominant poly(A) site during the early stage of adenovirus infection and in plasmid transfections when multiple poly(A) sites are present at the 3' end of a reporter plasmid. In contrast, stable mRNA processed at all five poly(A) sites is found during the late stage of adenovirus infection, after viral DNA replication has begun. Despite its dominance during early infection, L1 is a comparatively poor substrate for 3'-end RNA processing both in vivo and in vitro. In this study we have investigated the basis for the early L1 dominance. We have found that mRNA containing an unprocessed L1 poly(A) site is compromised in its ability to enter the steady-state pool of stable mRNA. This inhibition, which affects either the nuclear stability or nucleus-to-cytoplasm transport of the pre-mRNA, requires a cis-acting sequence located upstream of the L1 poly(A) site.

A common mechanism for the enhancement of mRNA 3' processing by U3 sequences in two distantly related lentiviruses

The protein coding regions of all retroviral pre-mRNAs are flanked by a direct repeat of R-U5 sequences. In many retroviruses, the R-U5 repeat contains a complete core poly(A) site-composed of a highly conserved AAUAAA hexamer and a GU-rich downstream element. A mechanism that allows for the bypass of the 5' core poly(A) site and the exclusive use of the 3' core poly(A) site must therefore exist. In human immunodeficiency virus type 1 (HIV-1), sequences within the U3 region appear to play a key role in poly(A) site selection. U3 sequences are required for efficient 3' processing at the HIV-1 poly(A) site both in vivo and in vitro. These sequences serve to promote the interaction of cleavage and polyadenylation specificity factor (CPSF) with the core poly(A) site. We have now demonstrated the presence of a functionally analogous 3' processing enhancer within the U3 region of a distantly related lentivirus, equine infectious anemia virus (EIAV). U3 sequences enhanced the processing of the EIAV core poly(A) site sevenfold in vitro. The U3 sequences also enhanced the stability of CPSF binding at the core poly(A) site. Optimal processing required the TAR RNA secondary structure that resides within the R region 28 nucleotides upstream of the AAUAAA hexamer. Disruption of TAR reduced processing, while compensatory changes that restored the RNA structure also restored processing to the wild-type level, suggesting a position dependence of the U3-encoded enhancer sequences. Finally, the reciprocal exchange of the EIAV and HIV U3 regions demonstrated the ability of each of these sequences to enhance both 3' processing and the binding of CPSF in the context of the heterologous core poly(A) site. The impact of U3 sequences upon the interaction of CPSF at the core poly(A) site may therefore represent a common strategy for retroviral poly(A) site selection.

Sequences regulating poly(A) site selection within the adenovirus major late transcription unit influence the interaction of constitutive processing factors with the pre-mRNA

The adenovirus major late transcription unit (MLTU) encodes five families of mRNAs, L1 to L5, each distinguished by a unique poly(A) site. Use of the promoter-proximal L1 poly(A) site predominates during early infection, whereas poly(A) site choice shifts to the promoter-distal sites during late infection. A mini-MLTU containing only the L1 and L3 poly(A) sites has been shown to reproduce this processing switch. In vivo analysis has revealed that sequences extending 5' and 3' of the L1 core poly(A) site are required for efficient processing as well as for regulated expression. By replacement of the L1 core poly(A) site with that of the ground squirrel hepatitis virus poly(A) site, we now demonstrate that the L1 flanking sequences can enhance the processing of a heterologous poly(A). Upon recombination of the chimeric L1-ground squirrel hepatitis virus poly(A) site onto the viral chromosome, the L1 flanking sequences were also found to be sufficient to reproduce the processing switch during the course of viral infection. Subsequent in vitro analysis has shown that the L1 flanking sequences function to enhance the stability of binding of cleavage and polyadenylation specificity factor to the core poly(A) site. The impact of L1 flanking sequences on the binding of cleavage and polyadenylation specificity factor suggests that the regulation of the MLTU poly(A) site selection is mediated by the interaction of constitutive processing factors.

3'-end-forming signals of yeast mRNA

It was previously shown that three distinct but interdependent elements are required for 3' end formation of mRNA in the yeast Saccharomyces cerevisiae: (i) the efficiency element TATATA and related sequences, which function by enhancing the efficiency of positioning elements; (ii) positioning elements, such as TTAAGAAC and AAGAA, which position the poly(A) site; and (iii) the actual site of polyadenylation. In this study, we have shown that several A-rich sequences, including the vertebrate poly(A) signal AATAAA, are also positioning elements. Saturated mutagenesis revealed that optimum sequences of the positioning element were AATAAA and AAAAAA and that this element can tolerate various extents of replacements. However, the GATAAA sequence was completely ineffective. The major cleavage sites determined in vitro corresponded to the major poly(A) sites observed in vivo. Our findings support the assumption that some components of the basic polyadenylation machinery could have been conserved among yeasts, plants, and mammals, although 3' end formation in yeasts is clearly distinct from that of higher eukaryotes.

Sequences homologous to 5' splice sites are required for the inhibitory activity of papillomavirus late 3' untranslated regions

Expression of bovine papillomavirus type 1 (BPV-1) late genes is limited to terminally differentiated keratinocytes in an infected epithelium. We have previously shown that although the BPV-1 late polyadenylation site is functional in nonpermissive cells, a 53-nucleotide (nt) fragment of the late 3' untranslated region acts posttranscriptionally to reduce polyadenylated cytoplasmic RNA levels. This 53-nt fragment does not appear to function by destabilizing polyadenylated cytoplasmic RNA (P. A. Furth and C. C. Baker, J. Virol. 65:5806-5812, 1991). In this study, we used site-directed mutagenesis and deletion analysis to demonstrate that the sequence AAG/GUAAGU, which is identical to the consensus 5' splice site sequence, was both necessary and sufficient for the inhibitory activity of the 53-nt fragment. Furthermore, base pairing between the 5' end of the U1 small nuclear RNA and this 5' splice site-like sequence was shown to be required for the inhibitory activity in vivo. We have also further mapped the human papillomavirus type 16 late 3' inhibitory element (I. M. Kennedy, J. K. Haddow, and J. B. Clements, J. Virol. 65:2093-2097, 1991) to a 51-nt region containing four overlapping sequence motifs with partial homology to 5' splice sites. Mutation of each of these motifs demonstrated that only one of these motifs is required for the inhibitory activity. However, the presence of the other motifs may contribute to the full inhibitory activity of the element. No BPV-1 or human papillomavirus type 16 mRNAs which are spliced by using the potential 5' splice sites present in the viral late 3' untranslated regions have been identified. This suggests that the primary function of these 5' splice site-like sequences is the inhibition of late gene expression. The most likely mechanism of action of these elements is reduction of polyadenylation efficiency, perhaps through interference with 3'-terminal exon definition.

Sequences homologous to 5' splice sites are required for the inhibitory activity of papillomavirus late 3' untranslated regions

Expression of bovine papillomavirus type 1 (BPV-1) late genes is limited to terminally differentiated keratinocytes in an infected epithelium. We have previously shown that although the BPV-1 late polyadenylation site is functional in nonpermissive cells, a 53-nucleotide (nt) fragment of the late 3' untranslated region acts posttranscriptionally to reduce polyadenylated cytoplasmic RNA levels. This 53-nt fragment does not appear to function by destabilizing polyadenylated cytoplasmic RNA (P. A. Furth and C. C. Baker, J. Virol. 65:5806-5812, 1991). In this study, we used site-directed mutagenesis and deletion analysis to demonstrate that the sequence AAG/GUAAGU, which is identical to the consensus 5' splice site sequence, was both necessary and sufficient for the inhibitory activity of the 53-nt fragment. Furthermore, base pairing between the 5' end of the U1 small nuclear RNA and this 5' splice site-like sequence was shown to be required for the inhibitory activity in vivo. We have also further mapped the human papillomavirus type 16 late 3' inhibitory element (I. M. Kennedy, J. K. Haddow, and J. B. Clements, J. Virol. 65:2093-2097, 1991) to a 51-nt region containing four overlapping sequence motifs with partial homology to 5' splice sites. Mutation of each of these motifs demonstrated that only one of these motifs is required for the inhibitory activity. However, the presence of the other motifs may contribute to the full inhibitory activity of the element. No BPV-1 or human papillomavirus type 16 mRNAs which are spliced by using the potential 5' splice sites present in the viral late 3' untranslated regions have been identified. This suggests that the primary function of these 5' splice site-like sequences is the inhibition of late gene expression. The most likely mechanism of action of these elements is reduction of polyadenylation efficiency, perhaps through interference with 3'-terminal exon definition.

Sequence elements upstream of the 3' cleavage site confer substrate strength to the adenovirus L1 and L3 polyadenylation sites

The adenovirus major late transcription unit is a well-characterized transcription unit which relies heavily on alternative pre-mRNA processing to generate distinct populations of mRNA during the early and late stages of viral infection. In the early stage of infection, two major late transcription unit mRNA transcripts are generated through use of the first (L1) of five available poly(A) sites (L1 through L5). This contrasts with the late stage of infection when as many as 45 distinct mRNAs are generated, with each of the five poly(A) sites being used. In previous work characterizing elements involved in alternative poly(A) site use, we showed that the L1 poly(A) site is processed less efficiently than the L3 poly(A) site both in vitro and in vivo. Because of the dramatic difference in processing efficiency and the role processing efficiency plays in production of steady-state levels of mRNA, we have identified the sequence elements that account for the differences in L1 and L3 poly(A) site processing efficiency. We have found that the element most likely to be responsible for poly(A) site strength, the GU/U-rich downstream element, plays a minor role in the different processing efficiencies observed for the L1 and L3 poly(A) sites. The sequence element most responsible for inefficient processing of the L1 poly(A) site includes the L1 AAUAAA consensus sequence and those sequences which immediately surround the consensus hexanucleotide. This region of the L1 poly(A) site contributes to an inability to form a stable processing complex with the downstream GU/U-rich element. In contrast to the L1 element, the L3 poly(A) site has a consensus hexanucleotide and surrounding sequences which can form a stable processing complex in cooperation with the downstream GU/U-rich element. The L3 poly(A) site is also aided by the presence of sequences upstream of the hexanucleotide which facilitate processing efficiency. The sequence UUCUUUUU, present in the L3 upstream region, is shown to enhance processing efficiency as well as stable complex formation (shown by increased binding of the 64-kDa cleavage stimulatory factor subunit) and acts as a binding site for heterogeneous nuclear ribonucleoprotein C proteins.

Sequence elements upstream of the 3' cleavage site confer substrate strength to the adenovirus L1 and L3 polyadenylation sites

The adenovirus major late transcription unit is a well-characterized transcription unit which relies heavily on alternative pre-mRNA processing to generate distinct populations of mRNA during the early and late stages of viral infection. In the early stage of infection, two major late transcription unit mRNA transcripts are generated through use of the first (L1) of five available poly(A) sites (L1 through L5). This contrasts with the late stage of infection when as many as 45 distinct mRNAs are generated, with each of the five poly(A) sites being used. In previous work characterizing elements involved in alternative poly(A) site use, we showed that the L1 poly(A) site is processed less efficiently than the L3 poly(A) site both in vitro and in vivo. Because of the dramatic difference in processing efficiency and the role processing efficiency plays in production of steady-state levels of mRNA, we have identified the sequence elements that account for the differences in L1 and L3 poly(A) site processing efficiency. We have found that the element most likely to be responsible for poly(A) site strength, the GU/U-rich downstream element, plays a minor role in the different processing efficiencies observed for the L1 and L3 poly(A) sites. The sequence element most responsible for inefficient processing of the L1 poly(A) site includes the L1 AAUAAA consensus sequence and those sequences which immediately surround the consensus hexanucleotide. This region of the L1 poly(A) site contributes to an inability to form a stable processing complex with the downstream GU/U-rich element. In contrast to the L1 element, the L3 poly(A) site has a consensus hexanucleotide and surrounding sequences which can form a stable processing complex in cooperation with the downstream GU/U-rich element. The L3 poly(A) site is also aided by the presence of sequences upstream of the hexanucleotide which facilitate processing efficiency. The sequence UUCUUUUU, present in the L3 upstream region, is shown to enhance processing efficiency as well as stable complex formation (shown by increased binding of the 64-kDa cleavage stimulatory factor subunit) and acts as a binding site for heterogeneous nuclear ribonucleoprotein C proteins.

Upstream and downstream cis-acting elements for cleavage at the L4 polyadenylation site of adenovirus-2

A study of the cis-acting elements involved in the 3' end formation of the RNAs from the major late L4 family of adenovirus-2 was undertaken. Series of 5' or 3' end deletion mutants and mutants harboring either internal deletions or substitutions were prepared and assayed for in vitro cleavage. This first allowed the demonstration of a sequence, located at -6 to -29, relative to AAUAAA, whose deletion or substitution reduces cleavage efficiency at the L4 polyadenylation site two to three fold. This upstream efficiency element 5' AUCUUUGUUGUC/AUCUCUGUGCUG 3' is constituted of a partially repeated 12 nucleotide long, UCG rich sequence. The activities of the 2 sequence elements in cleavage are additive. We also searched for regulatory sequences downstream of the L4 polyadenylation site. We found that the deletion or substitution of a 30 nucleotide long UCG rich sequence, between nucleotides +7 and +35 relative to the cleavage site and harboring a UCCUGU repeat reduces cleavage efficiency at least ten fold. A GUUUUU sequence, starting at +35 had no influence. Thus, the usage of the L4 polyadenylation site requires down-stream sequences different from the canonical GU or U boxes and is regulated by upstream sequence elements.

Alternative poly(A) site utilization during adenovirus infection coincides with a decrease in the activity of a poly(A) site processing factor

The recognition and processing of a pre-mRNA to create a poly(A) addition site, a necessary step in mRNA biogenesis, can also be a regulatory event in instances in which the frequency of use of a poly(A) site varies. One such case is found during the course of an adenovirus infection. Five poly(A) sites are utilized within the major late transcription unit to produce more than 20 distinct mRNAs during the late phase of infection. The proximal half of the major late transcription unit is also expressed during the early phase of a viral infection. During this early phase of expression, the L1 poly(A) site is used three times more frequently than the L3 poly(A) site. In contrast, the L3 site is used three times more frequently than the L1 site during the late phase of infection. Recent experiments have suggested that the recognition of the poly(A) site GU-rich downstream element by the CF1 processing factor may be a rate-determining step in poly(A) site selection. We demonstrate that the interaction of CF1 with the L1 poly(A) site is less stable than the interaction of CF1 with the L3 poly(A) site. We also find that there is a substantial decrease in the level of CF1 activity when an adenovirus infection proceeds to the late phase. We suggest that this reduction in CF1 activity, coupled with the relative instability of the interaction with the L1 poly(A) site, contributes to the reduced use of the L1 poly(A) site during the late stage of an adenovirus infection.

Alternative poly(A) site utilization during adenovirus infection coincides with a decrease in the activity of a poly(A) site processing factor

The recognition and processing of a pre-mRNA to create a poly(A) addition site, a necessary step in mRNA biogenesis, can also be a regulatory event in instances in which the frequency of use of a poly(A) site varies. One such case is found during the course of an adenovirus infection. Five poly(A) sites are utilized within the major late transcription unit to produce more than 20 distinct mRNAs during the late phase of infection. The proximal half of the major late transcription unit is also expressed during the early phase of a viral infection. During this early phase of expression, the L1 poly(A) site is used three times more frequently than the L3 poly(A) site. In contrast, the L3 site is used three times more frequently than the L1 site during the late phase of infection. Recent experiments have suggested that the recognition of the poly(A) site GU-rich downstream element by the CF1 processing factor may be a rate-determining step in poly(A) site selection. We demonstrate that the interaction of CF1 with the L1 poly(A) site is less stable than the interaction of CF1 with the L3 poly(A) site. We also find that there is a substantial decrease in the level of CF1 activity when an adenovirus infection proceeds to the late phase. We suggest that this reduction in CF1 activity, coupled with the relative instability of the interaction with the L1 poly(A) site, contributes to the reduced use of the L1 poly(A) site during the late stage of an adenovirus infection.

Regulated adenovirus mRNA 3'-end formation in a coupled in vitro transcription-processing system

The adenovirus major late transcription unit encodes five poly(A) sites whose use during infection is regulated. Early in the infection, the 5'-most site, L1, is used preferentially, whereas late in infection, all sites are used equivalently. Previous in vivo experiments indicated that regulatory sequences flank the AAUAAA and GU-rich elements of the L1 poly(A) site. We have developed an in vitro coupled transcription-processing system for studying the function of these regulatory sequences in HeLa cell nuclear extracts. The in vitro analysis using this system shows that predominant use of the L1 poly(A) site, as mediated by the upstream regulatory sequence, is independent of transcription. Furthermore, the reaction conditions are favorable to both 3'-end processing and splicing, making this system generally useful for the study of posttranscriptional processes.