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Previous Article | Next Article 
Molecular and Cellular Biology, May 2004, p. 4196-4206, Vol. 24, No. 10
0270-7306/04/$08.00+0 DOI: 10.1128/MCB.24.10.4196-4206.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
and Allan Jacobson*
Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655
Received 1 November 2003/ Returned for modification 12 December 2003/ Accepted 18 February 2004
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ABSTRACT |
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and pbp1 cells and from cells lacking the Fir1p interactor, Ref2p, demonstrated that Pbp1p, Fir1p, and Ref2p are all required for the formation of a normal-length poly(A) tail on precleaved CYC1 pre-mRNA. Kinetic analyses of the respective polyadenylation reactions indicated that Pbp1p is a negative regulator of poly(A) nuclease (PAN) activity and that Fir1p and Ref2p are, respectively, a positive regulator and a negative regulator of poly(A) synthesis. We suggest a model in which these three factors and Ufd1p are part of a regulatory complex that exploits Pab1p to link cleavage and polyadenylation factors of CFIA and CFIB (cleavage factors IA and IB) to the polyadenylation factors of CPF (cleavage and polyadenylation factor). |
INTRODUCTION |
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The core sequence requirements for 3'-end formation in yeast include the polyadenylation-cleavage site (PyAn), an "efficiency element" (UAUAUA, or repeats thereof) located a variable number of nucleotides 5' to the poly(A) site, and a "positioning element" (AAUAAA, AAAAAA, and related sequences) located approximately 20 nucleotides (nt) 5' to the poly(A) site. Unlike their mammalian counterparts, the yeast elements tend to be degenerate and redundant, a fact that explains the minimal effects on gene expression of some deletions that encompass normal 3'-processing signals (28, 47).
Cleavage and polyadenylation can be reproduced in vitro and appear to be coupled mechanistically, but such coupling is not required (i.e., precleaved substrates can be polyadenylated). While a detailed mechanistic understanding of the roles of the different factors remains to be established, current models for yeast polyadenylation (28, 30, 45, 47) suggest that the CPF complex recognizes the poly(A) site via interactions of Yhh1p, Ydh1p, and Yth1p with sequences surrounding the site of cleavage (3, 15, 16, 30) and that Hrp1p binds to the efficiency element while its ability to bind Rna14p acts to recruit CFIA to the positioning element (10, 20, 29). Subsequent to cleavage (the subunit that catalyzes the cleavage reaction is unknown), the Fip1p subunit of CPF recruits poly(A) polymerase and poly(A) synthesis on the newly created 3' end ensues (22, 35). The extent of polyadenylation is regulated at least in part by Pab1p action on Pap1p and by its recruitment of PAN (8, 22, 32, 43, 48).
PAN is a yeast enzyme that trims newly added poly(A) tails after their default polyadenylation to lengths of 70 to 90 A's. Poly(A) trimming by PAN is mRNA specific, and analyses of three different mRNAs indicate that such trimmed tails have lengths ranging from 55 to 71 A's (8). PAN is a Pab1p-dependent 3'-to-5' poly(A) exoribonuclease comprised of two subunits, Pan2p and Pan3p, that respectively, appear to have catalytic and regulatory roles (7, 9, 31). Pan3p, the positive regulator of PAN activity, interacts with Pab1p, thus providing substrate specificity for this nuclease (32a). Additional 3'-processing regulatory activities of Pab1p, as well as Pap1p, have been suggested by their two-hybrid interactions. For example, Pbp1p (Pab1p-binding protein 1) was identified in a two-hybrid screen using the carboxy terminus of Pab1p as bait (32). In the absence of Pbp1p, 3' termini of pre-mRNAs are properly cleaved but lack full-length poly(A) tails (32). Since Pbp1p may also interact with Pan2p, it has been suggested that its role in poly(A) tail maturation is realized by negative regulation of PAN (32).
In an effort to further clarify the role of Pbp1p in poly(A) tail synthesis and maturation, we have carried out a two-hybrid screen using the PBP1 gene as bait. Among the Pbp1p-interacting proteins that were identified in this screen were Fir1p and Ufd1p, factors that have been previously shown to interact with Pap1p (13). Functional analyses of extracts prepared from pbp1 and fir1 cells and from cells lacking the Fir1p interactor, Ref2p, all demonstrate an inability to polyadenylate pre-mRNAs to normal lengths. Kinetic analyses of the aberrant polyadenylation carried out in vitro by all three extracts distinguish three types of defects. Collectively, these studies lend credence to the hypothesis that Pbp1p is a negative regulator of PAN activity and that Fir1p and Ref2p are, respectively, a positive regulator and a negative regulator of poly(A) synthesis.
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MATERIALS AND METHODS |
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. The new gene name, PBP4 (Pab1p-binding protein 4), was reserved for locus YDL053C in accordance with the naming guidelines of the Saccharomyces Genome Database (http://www.yeastgenome.org/). Oligonucleotides. The oligonucleotides used in this study were prepared by Operon, Inc., and are listed in Table 1.
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), PBP2-5/PBP2-3 (for pbp2), PBP4-5/PBP4-3 (for pbp4), REF2-5/REF2-3 (for ref2), PAN2-5/PAN2-3 (for pan2), and PAN3-5/PAN3-3 (for pan3). Construction of the fir1/ref2 strain, yDM428, was achieved by PCR disruption of REF2 in the fir1 strain, yDM376, as described above. The pan2/pbp1 and pan3/pbp1 strains yDM432 and yDM434, respectively, were generated by integration of a pbp1::TRP1 fragment isolated from pDM114 at the wild-type PBP1 locus in pan2 and pan3 strains yME40 and YME43, respectively.
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Plasmid pDM114, used to generate pbp1::TRP1 strains, was constructed as follows. Initially, a 0.7-kb ClaI/BamHI fragment of PBP1 was inserted into the pBluescript II SK+ phagemid forming plasmid pDM98. This plasmid was then digested with EcoRV, and a SmaI/PvuII fragment from pJJ281 (27) was inserted to create pDM114. A SacI/ClaI fragment from pDM114 was used in yeast transformations to make the pbp1::TRP1 strains.
Two-hybrid screening. Yeast strain L40 (23) (Table 2) harboring the lexA(DB)-PBP1 FL plasmid (pDM125) was transformed with GAL4(AD)-fused yeast genomic DNA libraries (25) (generously provided by Philip James and Elizabeth Craig, University of Wisconsin Medical School, Madison) and plated on synthetic complete (SC) medium without Leu, Trp, or His. Transformants were screened as described previously (32). The extent of protein interaction was assessed by monitoring the level of HIS3 reporter activity using 3-aminotriazole (3-AT) as a competitive inhibitor of His3p (i.e., imidazole glycerol-phosphate dehydratase). The amount of 3-AT on which cells can grow correlates with the level of HIS3 expression, thus indicating the relative strength of protein-protein interaction. This was assayed by growing colonies from each transformation in SC –Leu, –Trp broth, serially diluting them in water, and applying them as spots to SC –Leu, –Trp and SC –Leu, –Trp, –His plates containing 0, 5, 10, 20, 40, 60, 80, or 100 mM 3-AT.
In vitro 3'-end processing assays.
Whole-cell yeast extracts were prepared from stationary-phase cells (A600 of 
4.0) as described previously (32). CYC1 precleaved precursors were generated for in vitro polyadenylation assays by using MEGAscript T7 transcription kits (Ambion, Inc.) to produce runoff transcripts of NdeI-cut plasmid pDM335. All reactions were performed at 25°C with 30 µg of extract in a final volume of 25 µl. The resulting products were analyzed on 6% polyacrylamide-7 M urea gels and visualized by autoradiography. Poly(A) tail lengths were measured against molecular weight markers using densitometry and are reported as the peak amount of polyadenylation in a given reaction. The data shown are representative of multiple independent determinations.
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RESULTS |
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PBP4 (YDL053C) was the only gene encoding a Pbp1p interactor that had not been characterized previously. Fifteen PBP4 clones were isolated (Table 3) and encompassed four unique N-terminal fusions to GAL4(AD) (Fig. 1B). In each instance, the Pbp1p-Pbp4p interaction mediated resistance to 80 mM 3-AT, suggesting that these two proteins can associate tightly. PBP4 is predicted to encode a 20-kDa polypeptide that has no significant homology to other known proteins. This gene is not required for cell viability, and its deletion did not result in any defect in growth rate (Table 3) (data not shown). This result is consistent with those obtained in the systematic analysis of all Saccharomyces cerevisiae deletion alleles (17).
Our previous studies had shown that Pbp1p is capable of self-interaction (32), and we thus anticipated the isolation of PBP1 fragments in this screen. This prediction was borne out with the isolation of six PBP1(339-722) clones that promoted resistance to 20 mM 3-AT (data not shown). This observation was consistent with data from a deletion analysis in which we demonstrated that a lexA-PBP1 fusion protein including Pbp1p amino acids 357 to 722 was also resistant to 20 mM 3-AT (32a). Surprisingly, the interacting fragments identified here did not, however, include a fragment approximating PBP1(199-722), the smallest PBP1 fragment that our deletion analyses showed was capable of multimerization and high-level (80 mM) 3-AT resistance (32a).
DIG1, a gene required for regulation of the mating and filamentous growth responses, was unanticipated as a potential PBP1 interactor. This gene encodes one of two highly homologous proteins that interact with the Fus3p and Kss1p mitogen-activated protein (MAP) kinases to repress the Ste12p transcriptional activator (12, 44). Eight DIG1 clones were isolated (Table 3), from which we identified two unique N-terminal fusions to GAL4(AD) (Fig. 1B). In each instance, Pbp1p/Dig1p interaction mediated resistance to 40 mM 3-AT. The lack of independent evidence suggesting a role for Dig1p in mRNA 3'-end processing led us to minimize further studies of this protein.
Mapping domains of protein-protein interactions. Directed two-hybrid analyses with PBP1 fragments were utilized to identify the Pbp1p domains involved in specific protein-protein interactions (Fig. 1A). Three of the four factors identified in our screen were shown to interact with the Pbp1p C terminus. For Fir1p, the Pbp1p(475-722) fragment was necessary and sufficient to promote interaction. For Ufd1p and Dig1p, the strength of interaction increased when the Pbp1p N terminus was removed. For example, the enhancement of Pbp1p-Dig1p interaction promoted by deletion of Pbp1p amino acids 1 to 199 increased 3-AT resistance of the relevant clones from 40 mM to 100 mM (Fig. 1A). Similarly, deletion of Pbp1p amino acids 1 to 199 marginally strengthened association of Pbp1p and Ufd1p, but further truncation (of amino acids 1 to 475) enhanced interaction markedly, resulting in an increase in 3-AT resistance from 10 mM to 60 mM (Fig. 1A). Both of these sets of interaction patterns suggest that the Pbp1p N-terminal domain may fold in such a way as to inhibit or regulate a subset of this protein's interactions. The association of Pbp1p with Pbp4p was unique in that it was the only interaction requiring the Pbp1p N terminus. This was manifested by the ability of Pbp1p(1-443) to interact with Pbp4p, while Pbp1p(199-722) and Pbp1p(475-722) were unable to bind the same factor (Fig. 1A).
The isolation of multiple fragments of some genes encoding Pbp1p interactors allowed for partial mapping of the respective interacting domains on these proteins. Since the original screen was conducted with genomic DNA libraries, the results are biased to the identification of genes encoding factors whose interaction with Pbp1p does not require the N terminus of the protein. Fir1p, Ufd1p, and Dig1p all utilize domains within the C-terminal two-thirds of the respective proteins for interaction with Pbp1p (Fig. 1B). Pbp4p, which is composed of only 185 amino acids, requires only its C-terminal half for interaction with Pbp1p, thus delineating a relatively small binding site of 93 amino acids (Fig. 1B).
Pbp1p-interacting factors regulate the extent of polyadenylation.
Two of the Pbp1p interactors we identified appear to play a role in mediating proper mRNA 3'-end formation. Deletion of FIR1 causes less efficient use of cryptic 3'-end cleavage sites in vivo, while loss of Ufd1p results in a substantial decrease in the amount of polyadenylation in vitro (13, 38). Since our earlier studies demonstrated that loss of Pbp1p diminishes the extent of polyadenylation in vitro (32), we sought to determine whether the nonessential factors identified in our two-hybrid screens with Pbp1p and Pab1p (32) behave similarly. To that end, we prepared multiple independent 3'-end processing extracts from wild-type, pbp1, fir1, pbp2 (32), and pbp4 strains and tested their ability to synthesize poly(A) tails in vitro on precleaved substrates. As described previously (32) and as illustrated in Fig. 2A, pbp1 extracts routinely synthesized poly(A) tails that were approximately 15 to 30 nt shorter than those synthesized by wild-type extracts (compare lanes 1 and 2). Similarly, extracts from fir1 cells consistently generated tails that were 5 to 7 nt shorter than those produced by wild-type extracts (compare lanes 1 and 3). In contrast, extracts from pbp2 and pbp4 cells polyadenylated the precursor transcript to the same extent as that derived from wild-type cells (compare lanes 1 with lanes 4 and 5).
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cells and observed that the poly(A) tails synthesized in those extracts were routinely 5 to 15 nt longer than those synthesized by extracts from wild-type cells (Fig. 2B, compare lanes 1 and 2).
Complementation of defects in poly(A) tail synthesis and maturation.
To demonstrate that the changes in poly(A) tail lengths observed in pbp1, fir1, and ref2 extracts were specific, we sought to test for biochemical complementation of the respective defects. Since we and others have been unable to purify functional recombinant Pbp1p, Fir1p, and Ref2p, we tested the effects of mixing extracts from the various deletion strains with extract prepared from wild-type cells (Fig. 3). Mixing wild-type extract with either pbp1 or fir1 extracts reversed the respective defects and restored polyadenylation to near-wild-type levels (Fig. 3, compare lanes 1 and 2 with lane 3 and lanes 1 and 4 with lane 5), but assays containing wild-type and ref2 extracts showed only marginal effects (compare lane 1 with lanes 6 and 7). Further experiments, combining extracts from pbp1 and ref2 cells, fir1 and ref2 cells, and fir1 and pbp1 cells, led to full restoration of wild-type poly(A) lengths (data not shown). These results demonstrate that the mutant extracts have specific defects in poly(A) tail synthesis or maturation.
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and ref2 cells fail to polyadenylate pre-mRNAs to normal lengths. To assess whether these effects are mediated at the level of poly(A) addition or trimming, we analyzed the respective time courses of polyadenylation in vitro. If Fir1p or Ref2p is required for poly(A) synthesis, poly(A) tail lengths in extracts lacking these factors should differ from those produced in wild-type extracts throughout the course of the reaction. However, if either of these factors regulates poly(A) trimming, the initial lengths of poly(A) tails produced in wild-type and mutant extracts should be very similar but then change as the rate of poly(A) shortening is affected. The results of this type of experiment are shown in Fig. 4 and 5, where the A panels are autoradiographs of the poly(A) tail length distributions and the B panels are graphic depictions of the peak amount of polyadenylation observed at each time point of the reaction. In these experiments, extracts derived from fir1 and ref2 cells displayed differences from wild-type extracts in the sizes of poly(A) tails initially added to substrate pre-mRNAs. In fir1 extracts, poly(A) tail lengths were initially 3 to 6 nt shorter than those generated by wild-type extracts and this difference was maintained throughout the reaction (Fig. 4). This result suggests that the difference in the extent of polyadenylation observed in fir1 extracts results from the loss of Fir1p stimulation of Pap1p activity. In extracts made from ref2 cells, the poly(A) tail lengths were initially longer than those produced by wild-type extracts (Fig. 5). This difference continued to increase during the polyadenylation phase of the reaction and peaked after 10 min of incubation. Beyond that point, the reaction switched to a deadenylation phase where the rates of poly(A) removal were roughly equivalent in both the wild-type and ref2 extracts. This result suggests that the increase in the extent of polyadenylation observed in the ref2 extracts is attributable to a loss of Ref2p inhibition of Pap1p activity. Since fir1 and ref2 extracts had opposing effects, we examined the effects on polyadenylation activity engendered by the combined loss of both genes. As shown in Fig. 6, extracts prepared from fir1/ref2 cells demonstrate that the ref2-like polyadenylation activity is dominant, i.e., poly(A) tails are longer than those produced by wild-type extracts.
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cells. The data in Fig. 7 demonstrate that the polyadenylation reaction has at least two phases, an initial poly(A) synthesis phase followed by a phase in which the poly(A) tail is then subjected to shortening. Initially, the products produced by both the wild-type and pbp1 extracts are identical in size (Fig. 7A, compare lanes 3 and 4). However, by 6 min of reaction, the poly(A) tails produced by the pbp1 extract are smaller than those of the wild-type extract, and this difference becomes more pronounced over time (Fig. 7B). In the wild-type extract, poly(A) tail lengths increase in size for 20 min and then are gradually trimmed. In contrast, the polyadenylation phase of the pbp1 extract is over by 10 min of incubation and is followed by poly(A) trimming that occurs at a considerably faster rate than that observed in the wild-type extract. Consistent with the results of earlier experiments (Fig. 2) (32), the poly(A) tails produced by pbp1 and wild-type extracts by 60 min of incubation differed by 27 nt (Fig. 7B). This kinetic analysis indicates that pbp1 extracts are unable to properly regulate poly(A) maturation. We conclude that Pbp1p is most likely a negative regulator of PAN, with its loss allowing for both premature initiation and a more rapid rate of poly(A) shortening.
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extracts to degrade fully adenylated poly(A) tails. Fully adenylated, radiolabeled substrate was prepared by incubating our standard CYC1 precleaved 3' RNA fragment in wild-type extract for 20 min (Fig. 3 to 7). Purified substrate was then added to wild-type and pbp1 extracts, and the kinetics of poly(A) removal was analyzed. Figure 8 shows that poly(A) shortening rates are approximately sevenfold faster in pbp1 extracts than in wild-type extracts, implying that the presence of Pbp1p somehow inhibits the activity of a poly(A) nuclease.
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, and pan2/pbp1 cells. As shown in Fig. 9A and B, pan2 extracts accumulate significantly longer poly(A) tails than wild-type extracts. Interestingly, this result reflects the combined effect of an early increase in poly(A) synthesis as well as a reduction in the rate of poly(A) trimming. Since extracts of pan2/pbp1 and pan2 cells exhibit the same characteristics (Fig. 9), we conclude that Pbp1p does indeed regulate PAN. This conclusion is supported by additional experiments demonstrating similar results with extracts prepared from pan3 cells and pan3/pbp1 cells (data not shown).
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DISCUSSION |
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The creation of a binding site for these cis-acting effectors of gene expression, i.e., the process of polyadenylation, is an elaborate mechanism involving a large complex of factors. These factors must interact with components of the transcription and splicing machineries, recognize several distinct sequence elements near the 3' end of a transcript, cleave endonucleolytically with high precision, add and trim a fairly homogenous length of adenylate residues to different templates, and then remodel the resulting mRNP for efficient passage to and through the nuclear pore (47). In yeast, the factors other than Pab1p responsible for these steps have been identified by a combination of biochemical and genetic methods and include CFIA, CFIB, CPF, and PAN (47). Almost all of the factors present in these complexes have been identified, with most encoding proteins essential for cell viability (30, 47). Although these factors have been categorized by their associated biochemical function, these complexes are interconnected. For example, CPF and CFIA appear to be linked via several interactions, including Fip1p/Rna14p, Psf2p/Rna14p, as well as multiple contacts between Ydh1p, Yhh1p, Ysh1p, Pta1p and Clp1p, Pcf11p, and Rna14p (30, 34, 35).
In this and a prior study (32), we have identified at least three factors (Pbp1p, Ref2p, and Fir1p) that influence the final size of a yeast poly(A) tail synthesized in vitro. While Ref2p has previously been shown to be a copurifying CPF component (14), evidence for the linkage of Pbp1p and Fir1p to the polyadenylation apparatus has depended largely on their genetic interactions (i.e., Fir1p's interactions with Pap1p and Ref2p's and Pbp1p's interactions with Pab1p) (32, 38). By extending the list of Pbp1p's interactors to include Ufd1p, Fir1p, and Pan2p (32a) and by obtaining more extensive evidence of RNA processing defects in extracts of pbp1, ref2, and fir1 cells, it now appears likely that Pbp1p, Ref2p, and Fir1p are bona fide regulators of yeast polyadenylation. Fir1p and Ref2p appear to affect poly(A) synthesis directly, since the effects on poly(A) tail lengths engendered by their loss are largely evident at the onset of polyadenylation (Fig. 4 to 6). Initial poly(A) tail lengths were decreased by the loss of Fir1p and increased by the loss of Ref2p, leading us to conclude that these two proteins can either act as respective positive and negative regulators of poly(A) polymerase (Pap1p) or mediate their effects indirectly through other components of CPF. The epistasis of the ref2 phenotype over that seen with fir1 extracts (Fig. 6) is intriguing in light of the known interactions of Fir1p and Pap1p and suggests that regulatory interactions within CPF are multifaceted.
Whereas Fir1p and Ref2p regulate poly(A) synthesis, several independent observations indicate that Pbp1p negatively regulates PAN during poly(A) tail maturation. The collective evidence includes our recent demonstration of interactions between Pbp1p and Pan2p (32a) and experiments here showing that pbp1 extracts have faster rates of poly(A) trimming than wild-type extracts (Fig. 7 and 8) and that this enhanced nucleolytic activity is dependent on functional Pan2p and Pan3p (Fig. 9) (data not shown). Since pbp1 extracts also pass prematurely from the synthesis to the maturation phase of the polyadenylation reaction (Fig. 7B), it is possible that Pbp1p may also help control an early step in poly(A) synthesis. However, Pbp1p does not appear to participate in the switch in Pap1p activity from processive to distributive activity (48), so its absence may simply allow for premature PAN recruitment and poly(A) trimming that would normally follow the binding of Pab1p to the elongating poly(A) tail. The extremely large poly(A) lengths observed in vivo in pab1, pan2, and pan3 mutants (9, 39) suggest that the latter step may also play a role in terminating the synthesis reaction.
The experiments presented here and elsewhere (32a) define a new set of interactions linking CPF and CFIA that help to coordinate control of poly(A) tail synthesis and maturation. A model that incorporates these new regulators and some of their interactions into the general scheme for polyadenylation in yeast is shown in Fig. 10. It should be noted that many of the interactions portrayed as direct have only been inferred from one experimental approach (e.g., two-hybrid analysis) and may thus be bridged. Moreover, although the model implies the existence of multiple simultaneous interactions, sequential dynamic interactions are more likely. As befits a putative scaffolding protein (33), Pab1p is shown playing a central role, interacting with CFIA and -B, Pbp1p, and Pan3p to orchestrate poly(A) tail maturation (by Pan2p, Pan3p, and Pbp1p) (8) while simultaneously bridging the CFI factors to those associated with Pap1p via its interaction with Pbp1p. This arrangement, which also depicts the principal Pap1p interactors, takes into account the linkage that two of them (Fir1p and Ufd1p) have to Pbp1p as well as the observed positive and negative regulatory effects of Fir1p and Ref2p on polyadenylation. In addition, Fip1p is positioned to carry out its role as a mediator of Pap1p's transition from processive to distributive activity (22, 35, 48) and Ufd1p is included in this model by virtue of its interactions with Pap1p (13) and Pbp1p (Table 1 and Fig. 1). Ufd1p's status as the product of an essential gene precluded the possibility of our testing the consequences of its absence by analysis of cell extracts from a deletion strain, but subsequent analyses using other methodologies will allow more definitive tests of its possible regulatory roles. While the lack of in vitro polyadenylation phenotypes in pbp2 and pbp4 extracts suggested that Pbp2p and Pbp4p did not have roles in poly(A) tail synthesis or maturation, we cannot exclude the possibility that these factors have a function in pre-mRNA cleavage or mRNA export.
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ACKNOWLEDGMENTS |
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We thank Neptune Mizrahi and Claire Moore for plasmids and strains and members of our laboratory for discussions of the experiments and comments on the manuscript.
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FOOTNOTES |
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Present address: Tufts University School of Medicine, Boston, MA 02111.
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