Regulation of poly(A) site use during mouse B-cell development involves a change in the binding of a general polyadenylation factor in a B-cell stage-specific manner.

During the development of mouse B cells there is a regulated shift from the production of membrane to the secretion-specific forms of immunoglobulin (Ig) mRNA, which predominate in the late-stage or plasma B cells. By DNA transfection experiments we have previously shown that there is an increase in polyadenylation efficiency accompanying the shift to secretion-specific forms of Ig mRNA (C. R. Lassman, S. Matis, B. L. Hall, D. L. Toppmeyer, and C. Milcarek, J. Immunol. 148:1251-1260, 1992). When we look in vitro at nuclear extracts prepared from early or memory versus late-stage or plasma B cells, we see cell stage-specific differences in the proteins which are UV cross-linked to the input RNAs. We have characterized one of these proteins as the 64-kDa subunit of the general polyadenylation factor cleavage-stimulatory factor (CstF) by immunoprecipitation of UV-cross-linked material. The amount of 64-kDa protein and its mobility on two-dimensional gels do not vary between the B-cell stages. However, the activity of binding of the protein to both Ig and non-Ig substrates increases four- to eightfold in the late-stage or plasma cell lines relative to the binding seen in the early or memory B-cell lines. Therefore, the binding activity of a constitutive factor required for polyadenylation is altered in a B-cell-specific fashion. The increased binding of the 64-kDa protein may lead to a generalized increase in polyadenylation efficiency in plasma cells versus early or memory B cells which may be responsible for the increased use of the secretory poly(A) site seen in vivo.

3' RNA processing efficiency plays a primary role in generating termination-competent RNA polymerase II elongation complexes.

In several mammalian transcription units, a transcription termination mechanism in which efficient termination is dependent on the presence of an intact 3' RNA processing site has been identified. The mouse beta maj-globin transcription unit is one such example, in which an intact poly(A) site is required for efficient transcription termination. It is now evident that 3' mRNA processing sites are not always processed with the same efficiency. In this study, we characterized several pre-mRNAs as substrates for the 3' mRNA processing reaction of cleavage and polyadenylation. We then determined whether poly(A) sites which vary in processing efficiency support a poly(A) site-dependent termination event. The level of processing efficiency was determined in vitro by assays measuring the efficiency of the pre-mRNA cleavage event and in vivo by the level of poly(A) site-dependent mRNA and gene product expression generated in transient transfection assays. The beta maj globin pre-mRNA is very efficiently processed. This efficient processing correlates with its function in termination assays using recombinant adenovirus termination vectors in nuclear run-on assays. When the beta maj globin poly(A) site was replaced by the L1 poly(A) site of the adenovirus major late transcription unit (Ad-ml), which is a poor processing substrate, termination efficiency decreased dramatically. When the beta maj globin poly(A) site was replaced by the Ad-ml L3 poly(A) site, which is 10- to 20-fold more efficiently processed than the Ad-ml L1 poly(A) site, termination efficiency remained high. Termination is therefore dependent on the yield of the processing event. We then tested chimeric poly(A) sites containing the L3 core AAUAAA but varied downstream GU-rich elements. The change in downstream GU-rich elements affected processing efficiency in a manner which correlated with termination efficiency. These experiments provide evidence that the efficiency of 3' processing complex formation is directly correlated to the efficiency of RNA polymerase II termination at the 3' end of a mammalian transcription unit.

A mutation in a methionine tRNA gene suppresses the prp2-1 Ts mutation and causes a pre-mRNA splicing defect in Saccharomyces cerevisiae.

The PRP2 gene in Saccharomyces cerevisiae encodes an RNA-dependent ATPase that activates spliceosomes for the first transesterification reaction in pre-mRNA splicing. We have identified a mutation in the elongation methionine tRNA gene EMT1 as a dominant, allele-specific suppressor of the temperature-sensitive prp2-1 mutation. The EMT1-201 mutant suppressed prp2-1 by relieving the splicing block at high temperature. Furthermore, EMT1-201 single mutant cells displayed pre-mRNA splicing and cold-sensitive growth defects at 18 degrees. The mutation in EMT1-201 is located in the anticodon, changing CAT to CAG, which presumably allowed EMT1-201 suppressor tRNA to recognize CUG leucine codons instead of AUG methionine codons. Interestingly, the prp2-1 allele contains a point mutation that changes glycine to aspartate, indicating that EMT1-201 does not act by classical missense suppression. Extra copies of the tRNA(Leu)(UAG) gene rescued the cold sensitivity and in vitro splicing defect of EMT1-201. This study provides the first example in which a mutation in a tRNA gene confers a pre-mRNA processing (prp) phenotype.

The binding of a subunit of the general polyadenylation factor cleavage-polyadenylation specificity factor (CPSF) to polyadenylation sites changes during B cell development.

During the development of mouse B cells there is a regulated shift from the production of membrane (mb) to secretory-specific (sec) forms of immunoglobulin (Ig) mRNA. The mRNAs are produced from one gene that is alternatively processed at the 3' end. We have previously shown that there is an increase in polyadenylation efficiency accompanying the developmentally regulated shift to secretory-specific forms of Ig mRNA by DNA transfection experiments (1). When we look in vitro at nuclear extracts prepared from early/memory versus late stage/plasma B cells, we see cell stage-specific differences in the proteins which are crosslinked to poly(A) site-containing RNAs. Here we show that one of these proteins is the mouse homologue of 100 kDa subunit of Hela CPSF by immunoprecipitation and Western analysis of UV crosslinked material. The amount of 100 kDa protein and its mobility on two-dimensional gels do not change between the B cell stages. However, the binding of the 100 kDa polypeptide to poly(A) sites increases in the late stage/plasma cell lines relative to the binding seen in early/memory cell lines. The increased binding may reflect an increase in polyadenylation efficiency at the sec poly(A) site in plasma cells versus early/memory cells seen in vivo.

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.

Isolation and characterization of polyadenylation complexes assembled in vitro.

We developed a two-step purification of mammalian polyadenylation complexes assembled in vitro. Biotinylated pre-mRNAs containing viral or immunoglobulin poly(A) sites were incubated with nuclear extracts prepared from mouse myeloma cells under conditions permissive for in vitro cleavage and polyadenylation and the mixture was fractionated by gel filtration; complexes containing biotinylated pre-mRNA and bound proteins were affinity purified on avidin-agarose resin. Western analysis of known components of the polyadenylation complex demonstrated copurification of polyadenylation factors with poly(A) site-containing RNA but not with control RNA substrates containing either no polyadenylation signals or a point mutation of the AAUAAA polyadenylation signal. Polyadenylation complexes that were assembled on exogenous RNA eluted from the Sephacryl column in fractions consistent with their size range extending from 2 to 4 x 10(6) Mr. Complexes endogenous to the extract were of approximately the same apparent size, but more heterogeneous in distribution. This method can be used to study polyadenylation/cleavage complexes that may form upon a number of different RNA sequences, an important step towards defining which factors might differentially associate with specific RNAs.

Increase in the 64-kDa subunit of the polyadenylation/cleavage stimulatory factor during the G0 to S phase transition.

The amount of the 64-kDa subunit of polyadenylation/cleavage stimulatory factor (CstF-64) increases 5-fold during the G0 to S phase transition and concomitant proliferation induced by serum in 3T6 fibroblasts. Higher levels of CstF-64 result in an increase in CstF trimer. The rise in CstF-64 occurs at a time when the amount of poly(A)-containing RNA rose at least 5-8 fold in the cytoplasm. Primary human splenic B cells, resting in G0, show a similar 5-fold increase in CstF-64 when cultured under conditions inducing proliferation (CD40 ligand exposure). Therefore, the increase in CstF-64 is associated with the G0 to S phase transition. As B cell development progresses, RNA processing changes occur at the Ig heavy chain locus resulting in a switch from the membrane- to the upstream secretory-specific poly(A) site. Treating resting B cells with agents triggering this switch in Ig mRNA production along with proliferation (CD40 ligand plus lymphokines or Staphylococcus aureus protein A) induces no further increase in CstF-64 above that seen for proliferation alone. The rise in CstF-64 is therefore insufficient to induce secretion. After stimulation of a continuously growing B cell line with lymphokines, a switch to Ig micrometer secretory mRNA and protein occurs but without a change in the CstF-64 level. Therefore, an increase in CstF-64 levels is not necessary to mediate the differentiation-induced switch to secreted forms of Ig-micrometer heavy chain. Because augmentation of CstF-64 levels is neither necessary nor sufficient for Ig secretory mRNA production, we conclude that other lymphokine-induced factors play a role.

Dominant negative mutants of the yeast splicing factor Prp2 map to a putative cleft region in the helicase domain of DExD/H-box proteins.

The Prp2 protein of Saccharomyces cerevisiae is an RNA-dependent ATPase required before the first transesterification reaction in pre-mRNA splicing. Prp2 binds to the spliceosome in the absence of ATP and is released following ATP hydrolysis. We determined what regions in Prp2 are essential for release from the spliceosome by analyzing dominant negative mutants in vivo and in vitro. We made mutations in conserved motif II (DExH) and motif VI (QRxGR) of the helicase (H) domain. Mutations that inactivated PRP2 had a dominant negative phenotype when overexpressed in vivo. To test whether mutations outside of the H domain could confer a dominant negative phenotype, we mutagenized a GAL1-PRP2 construct and screened for mutants unable to grow on galactose-containing media. Five dominant negative mutants were characterized; three mapped within the H domain and two mapped downstream of motif VI, indicating that an extended helicase domain is required for release of Prp2 from the spliceosome. Most mutants stalled in the spliceosome in vitro. However, not all mutants that were dominant negative in vivo were dominant negative in vitro, indicating that multiple mechanisms may cause a dominant negative phenotype. Structural modeling of the H domain of Prp2 suggests that mutants map to a cleft region found in helicases of known structure.