Dynamic regulation of alternative splicing by silencers that modulate 5' splice site competition.

Alternative splicing makes a major contribution to proteomic diversity in higher eukaryotes with approximately 70% of genes encoding two or more isoforms. In most cases, the molecular mechanisms responsible for splice site choice remain poorly understood. Here, we used a randomization-selection approach in vitro to identify sequence elements that could silence a proximal strong 5' splice site located downstream of a weakened 5' splice site. We recovered two exonic and four intronic motifs that effectively silenced the proximal 5' splice site both in vitro and in vivo. Surprisingly, silencing was only observed in the presence of the competing upstream 5' splice site. Biochemical evidence strongly suggests that the silencing motifs function by altering the U1 snRNP/5' splice site complex in a manner that impairs commitment to specific splice site pairing. The data indicate that perturbations of non-rate-limiting step(s) in splicing can lead to dramatic shifts in splice site choice.

Kaposi's Sarcoma-Associated Herpesvirus Utilizes and Manipulates RNA N6-Adenosine Methylation To Promote Lytic Replication.

N6-adenosine methylation (m6A) is the most common posttranscriptional RNA modification in mammalian cells. We found that most transcripts encoded by the Kaposi's sarcoma-associated herpesvirus (KSHV) genome undergo m6A modification. The levels of m6A-modified mRNAs increased substantially upon stimulation for lytic replication. The blockage of m6A inhibited splicing of the pre-mRNA encoding the replication transcription activator (RTA), a key KSHV lytic switch protein, and halted viral lytic replication. We identified several m6A sites in RTA pre-mRNA crucial for splicing through interactions with YTH domain containing 1 (YTHDC1), an m6A nuclear reader protein, in conjunction with serine/arginine-rich splicing factor 3 (SRSF3) and SRSF10. Interestingly, RTA induced m6A and enhanced its own pre-mRNA splicing. Our results not only demonstrate an essential role of m6A in regulating RTA pre-mRNA splicing but also suggest that KSHV has evolved a mechanism to manipulate the host m6A machinery to its advantage in promoting lytic replication.IMPORTANCE KSHV productive lytic replication plays a pivotal role in the initiation and progression of Kaposi's sarcoma tumors. Previous studies suggested that the KSHV switch from latency to lytic replication is primarily controlled at the chromatin level through histone and DNA modifications. The present work reports for the first time that KSHV genome-encoded mRNAs undergo m6A modification, which represents a new mechanism at the posttranscriptional level in the control of viral replication.

Expansion of the eukaryotic proteome by alternative splicing.

The collection of components required to carry out the intricate processes involved in generating and maintaining a living, breathing and, sometimes, thinking organism is staggeringly complex. Where do all of the parts come from? Early estimates stated that about 100,000 genes would be required to make up a mammal; however, the actual number is less than one-quarter of that, barely four times the number of genes in budding yeast. It is now clear that the 'missing' information is in large part provided by alternative splicing, the process by which multiple different functional messenger RNAs, and therefore proteins, can be synthesized from a single gene.

Enzyme engineering through evolution: thermostable recombinant group II intron reverse transcriptases provide new tools for RNA research and biotechnology.

Current investigation of RNA transcriptomes relies heavily on the use of retroviral reverse transcriptases. It is well known that these enzymes have many limitations because of their intrinsic properties. This commentary highlights the recent biochemical characterization of a new family of reverse transcriptases, those encoded by group II intron retrohoming elements. The novel properties of these enzymes endow them with the potential to revolutionize how we approach RNA analyses.

Establishment of an in vitro trans-splicing system in Trypanosoma brucei that requires endogenous spliced leader RNA.

In trypanosomes a 39 nucleotide exon, the spliced leader (SL) is donated to all mRNAs from a small RNA, the SL RNA, by trans-splicing. Since the discovery of trans-splicing in trypanosomes two decades ago, numerous attempts failed to reconstitute the reaction in vitro. In this study, a crude whole-cell extract utilizing the endogenous SL RNA and synthetic tubulin pre-mRNA were used to reconstitute the trans-splicing reaction. An RNase protection assay was used to detect the trans-spliced product. The reaction was optimized and shown to depend on ATP and intact U2 and U6 snRNPs. Mutations introduced at the polypyrimidine tract and the AG splice site reduced the reaction efficiency. To simplify the assay, RT-PCR and quantitative real-time PCR assays were established. The system was used to examine the structural requirements for SL RNA as a substrate in the reaction. Interestingly, synthetic SL RNA assembled poorly to its cognate particle and was not utilized in the reaction. However, SL RNA synthesized in cells lacking Sm proteins, which is defective in cap-4 modification, was active in the reaction. This study is the first step towards further elucidating the mechanism of trans-splicing, an essential reaction which determines the trypanosome transcriptome.

Multiple roles for polypyrimidine tract binding (PTB) proteins in trypanosome RNA metabolism.

Trypanosomatid genomes encode for numerous proteins containing an RNA recognition motif (RRM), but the function of most of these proteins in mRNA metabolism is currently unknown. Here, we report the function of two such proteins that we have named PTB1 and PTB2, which resemble the mammalian polypyrimidine tract binding proteins (PTB). RNAi silencing of these factors indicates that both are essential for life. PTB1 and PTB2 reside mostly in the nucleus, but are found in the cytoplasm, as well. Microarray analysis performed on PTB1 and PTB2 RNAi silenced cells indicates that each of these factors differentially affects the transcriptome, thus regulating a different subset of mRNAs. PTB1 and PTB2 substrates were categorized bioinformatically, based on the presence of PTB binding sites in their 5' and 3' flanking sequences. Both proteins were shown to regulate mRNA stability. Interestingly, PTB proteins are essential for trans-splicing of genes containing C-rich polypyrimidine tracts. PTB1, but not PTB2, also affects cis-splicing. The specificity of binding of PTB1 was established in vivo and in vitro using a model substrate. This study demonstrates for the first time that trans-splicing of only certain substrates requires specific factors such as PTB proteins for their splicing. The trypanosome PTB proteins, like their mammalian homologs, represent multivalent RNA binding proteins that regulate mRNAs from their synthesis to degradation.

Evidence that microRNAs are associated with translating messenger RNAs in human cells.

MicroRNAs (miRNAs) regulate gene expression post-transcriptionally by binding the 3' untranslated regions of target mRNAs. We examined the subcellular distribution of three miRNAs in exponentially growing HeLa cells and found that the vast majority are associated with mRNAs in polysomes. Several lines of evidence indicate that most of these mRNAs, including a known miRNA-regulated target (KRAS mRNA), are actively being translated.

Mechanisms of microRNA-mediated gene regulation in animal cells.

MicroRNAs are a large family of regulatory molecules found in all multicellular organisms. Even though their functions are only beginning to be understood, it is evident that microRNAs have important roles in a wide range of biological processes, including developmental timing, growth control, and differentiation. Indeed, recent bioinformatic and experimental evidence suggests that a remarkably large proportion of genes (>30%) are subject to microRNA-mediated regulation. Although it is clear that microRNAs function by suppressing protein production from targeted mRNAs, there is, at present, no consensus about how such downregulation is accomplished. In this review, I describe the evidence that there are multiple mechanisms of microRNA-mediated repression and discuss the possible connections between these mechanisms.

Cotranslational microRNA mediated messenger RNA destabilization.

MicroRNAs are small (22 nucleotide) regulatory molecules that play important roles in a wide variety of biological processes. These RNAs, which bind to targeted mRNAs via limited base pairing interactions, act to reduce protein production from those mRNAs. Considerable evidence indicates that miRNAs destabilize targeted mRNAs by recruiting enzymes that function in normal mRNA decay and mRNA degradation is widely thought to occur when mRNAs are in a ribosome free state. Nevertheless, when examined, miRNA targeted mRNAs are invariably found to be polysome associated; observations that appear to be at face value incompatible with a simple decay model. Here, we provide evidence that turnover of miRNA-targeted mRNAs occurs while they are being translated. Cotranslational mRNA degradation is initiated by decapping and proceeds 5' to 3' behind the last translating ribosome. These results provide an explanation for a long standing mystery in the miRNA field.

Direct chemical sequencing of end-labeled RNA.

Chemical sequencing of RNA relies on the fact that each of the four bases in RNA is susceptible to chemical modification in a different way. In this protocol, end-labeled RNAs are subjected to base-specific chemical modification reactions that make the RNA strand adjacent to the modified base susceptible to cleavage. The chemical modification reaction is base-specific but limited so that not every base in every strand is modified. After cleavage, the resulting set of radioactive fragments is resolved via polyacrylamide gel electrophoresis.

Poisoned primer extension.

Poisoned primer extension is primarily used to distinguish between RNAs that are nearly identical in sequence but cannot be distinguished by standard primer extension because they are the same size (e.g., edited vs. nonedited transcripts). It is conceptually identical to the standard primer extension reaction but involves the use of a chain-terminating dideoxynucleotide (the "poison") in the presence of the other three nucleotides. A radioactively labeled primer that hybridizes a short-distance downstream from the "changed" region of interest is extended by reverse transcription into this region of sequence variation. The reactions contain three of the four substrates for extension (e.g., dATP, dGTP, and dTTP) and a chain-terminating dideoxynucleotide (e.g., ddCTP). The extension reaction stops when reverse transcriptase adds a chain-terminating dideoxynucleotide to the template (e.g., it will add ddCTP when it encounters a G in the template sequence). RNAs that differ in sequence at that position will yield different-sized extension products that can be resolved on a denaturing gel.

Boundary Analysis to Determine the Minimal RNA Sequence Required for Protein Binding.

Boundary analysis is a powerful and often overlooked method for determining the sequences within an RNA molecule that are required for a specific RNA-protein interaction. In this approach, 5'- and 3'-end-labeled RNAs are fragmented randomly (usually by limited alkaline hydrolysis, but nuclease fragmentation can also be used) such that a ladder of fragments covering the whole molecule of interest is produced. This mixture of fragments is then allowed to bind to the protein (or other molecule) of interest. Bound fragments are selected by affinity or antibody binding and then displayed on a gel. The point at which banding is lost for 5'-end-labeled RNAs defines the 3' boundary of the binding site, and the point at which banding is lost for 3'-end-labeled RNAs defines the 5' boundary of the binding site.

Chemical modification interference.

Chemical modification interference is a powerful method for surveying an entire RNA molecule to identify functionally important chemical groups. The basic idea is to generate a pool of end-labeled RNAs wherein each RNA molecule is chemically modified (e.g., by diethyl pyrocarbonate [DEPC], hydrazine, dimethyl sulfate, CMCT, or kethoxal) at a different position. The pool of RNAs is then allowed to participate in the reaction of interest. The functionally important RNA molecules (e.g., those bound by protein or that successfully participate in a processing reaction) are then separated from the nonfunctional RNA molecules (e.g., those not bound by protein or unable to participate in a processing reaction). This is often achieved by straightforward gel electrophoretic analysis. In the case of protein binding, it is necessary to be able to separate bound RNA from unbound RNA, which can be accomplished using electrophoretic mobility shift assays, filter binding, or affinity approaches (e.g., by immunoprecipitation or the use of tagged proteins). None of these techniques requires that a large fraction of RNA be bound or reacted, and, as a result, they are quite sensitive. Here we describe one example of a chemical modification interference assay in which RNA is modified with DEPC or hydrazine before binding to a protein. This technique can be readily adapted for use with other chemicals.

Nucleotide analog interference mapping.

Methods collectively known as modification interference are exceptionally powerful approaches used to identify functionally important chemical groups in the phosphodiester backbone or nucleobases of an RNA. In a modification interference assay, end-labeled RNAs that have been modified at different positions are allowed to participate in a reaction of interest, and then functional RNA molecules (e.g., those bound by protein or that successfully participate in a processing reaction) are separated from nonfunctional RNA molecules (e.g., those not bound by protein or unable to participate in a processing reaction). Nucleotide analog interference mapping (NAIM) involves the incorporation of α-thionucleotides containing a modified base into the RNA molecule of interest. The sites containing the modified base are identified by cleavage with iodoethanol. NAIM is useful whenever the thiophosphate substitution on its own does not prevent or inhibit a specific reaction. To perform NAIM, it is first necessary to perform a thiophosphate interference analysis. Any positions that are not affected by thiophosphate substitution can then be analyzed by NAIM.

Preparing Cellular DNA from Nuclei or Whole Cells.

It is often desirable to have cellular DNA on hand. DNA is stable and can be maintained intact for many years. This protocol describes the preparation of DNA from nuclei after the cytoplasmic extract has been removed. The resulting DNA is suitable for polymerase chain reactions and Southern blots to determine copy number and sites of integration of plasmids in stable cell lines. Quantitation of DNA may not be exact because RNA is not completely removed. The method can also be used on whole cells, but there will be more RNA contamination.

Removal of rRNA from Deproteinized, Phenol-Extracted Total RNA by Hybrid Selection.

In this protocol, rRNAs are selectively removed from a total RNA sample by hybridizing the rRNAs to complementary biotinylated oligodeoxynucleotides that can be affinity-purified using streptavidin beads, leaving all other RNAs behind. Although commercially available kits can be used to perform this procedure, they are expensive. We recommend that investigators order species-specific oligodeoxynucleotides for their own applications. There are well-established secondary structure predictions for all rRNAs.

Measuring the length of poly(A) tails.

Adenylation status has an important role in the regulation of mRNA metabolism: mRNAs are deadenylated before degradation, microRNAs (miRNAs) can cause deadenylation, and the poly(A) length of certain mRNAs is regulated during development. This protocol describes methods that can be used to measure the poly(A) tail length of specific mRNAs. These include, in the order of increasing sensitivity, (1) northern blotting of intact and experimentally deadenylated mRNAs and (2) northern blotting of intact and experimentally deadenylated mRNA fragments that have been cleaved near the 3' end with RNase H. Highly sensitive polymerase chain reaction (PCR)-based approaches are also discussed.