The ribonuclease activity of Csm6 is required for anti-plasmid immunity by Type III-A CRISPR-Cas systems

CRISPR-Cas systems provide prokaryotes with RNA-based adaptive immunity against viruses and plasmids. A unique feature of Type III CRISPR-Cas systems is that they selectively target transcriptionally-active invader DNA, and can cleave both the expressed RNA transcripts and source DNA. The Type III-A effector crRNP (CRISPR RNA-Cas protein complex), which contains Cas proteins Csm1-5, recognizes and degrades invader RNA and DNA in a crRNA-guided, manner. Interestingly, Type III-A systems also employ Csm6, an HEPN family ribonuclease that does not stably associate with the Type III-A effector crRNP, but nevertheless contributes to defense via mechanistic details that are still being determined. Here, we investigated the mechanism of action of Csm6 in Type III-A CRISPR-Cas systems from Lactococcus lactis, Staphylococcus epidermidis, and Streptococcus thermophilus expressed in Escherichia coli. We found that L. lactis and S. epidermidis Csm6 cleave RNA specifically after purines in vitro, similar to the activity reported for S. thermophilus Csm6. Moreover, L. lactis Csm6 functions as a divalent metal-independent, single strand-specific endoribonuclease that depends on the conserved HEPN domain. In vivo, we show that deletion of csm6 or expression of an RNase-defective form of Csm6 disrupts crRNA-dependent loss of plasmid DNA in all three systems expressed in E. coli. Mutations in the Csm1 palm domain, which are known to deactivate Csm6 ribonuclease activity, also prevent plasmid loss in the three systems. The results indicate that Csm6 ribonuclease activity rather than Csm1-mediated DNase activity effects anti-plasmid immunity by the three Type III-A systems investigated.

Small nucleolar RNAs: versatile trans-acting molecules of ancient evolutionary origin

The small nucleolar RNAs (snoRNAs) are an abundant class of trans-acting RNAs that function in ribosome biogenesis in the eukaryotic nucleolus. Elegant work has revealed that most known snoRNAs guide modification of pre-ribosomal RNA (pre-rRNA) by base pairing near target sites. Other snoRNAs are involved in cleavage of pre-rRNA by mechanisms that have not yet been detailed. Moreover, our appreciation of the cellular roles of the snoRNAs is expanding with new evidence that snoRNAs also target modification of small nuclear RNAs and messenger RNAs. Many snoRNAs are produced by unorthodox modes of biogenesis including salvage from introns of pre-mRNAs. The recent discovery that homologs of snoRNAs as well as associated proteins exist in the domain Archaea indicates that the RNA-guided RNA modification system is of ancient evolutionary origin. In addition, it has become clear that the RNA component of vertebrate telomerase (an enzyme implicated in cancer and cellular senescence) is related to snoRNAs. During its evolution, vertebrate telomerase RNA appears to have co-opted a snoRNA domain that is essential for the function of telomerase RNA in vivo. The unique properties of snoRNAs are now being harnessed for basic research and therapeutic applications.

Programmable type III-A CRISPR-Cas DNA targeting modules

The CRISPR-Cas systems provide invader defense in a wide variety of prokaryotes, as well as technologies for many powerful applications. The Type III-A or Csm CRISPR-Cas system is one of the most widely distributed across prokaryotic phyla, and cleaves targeted DNA and RNA molecules. In this work, we have constructed modules of Csm systems from 3 bacterial species and heterologously expressed the functional modules in E. coli. The modules include a Cas6 protein and a CRISPR locus for crRNA production, and Csm effector complex proteins. The expressed modules from L. lactis, S. epidermidis and S. thermophilus specifically eliminate invading plasmids recognized by the crRNAs of the systems. Characteristically, activation of plasmid targeting activity depends on transcription of the plasmid sequence recognized by the crRNA. Activity was not observed when transcription of the crRNA target sequence was blocked, or when the opposite strand or a non-target sequence was transcribed. Moreover, the Csm module can be programmed to recognize plasmids with novel target sequences by addition of appropriate crRNA coding sequences to the module. These systems provide a platform for investigation of Type III-A CRISPR-Cas systems in E. coli, and for introduction of programmable transcription-activated DNA targeting into novel organisms.

Bipartite recognition of target RNAs activates DNA cleavage by the Type III-B CRISPR-Cas system

Here, Elmore et al. investigate how the Type III-B Cmr complex, which cleaves invader RNAs recognized by the CRISPR RNA (crRNA), functions. The findings demonstrate that the Cmr complex is a novel DNA nuclease activated by invader RNAs containing a crRNA target sequence and a protospacer-adjacent motif (rPAM).

CRISPR–Cas systems eliminate nucleic acid invaders in bacteria and archaea. The effector complex of the Type III-B Cmr system cleaves invader RNAs recognized by the CRISPR RNA (crRNA ) of the complex. Here we show that invader RNAs also activate the Cmr complex to cleave DNA. As has been observed for other Type III systems, Cmr eliminates plasmid invaders in Pyrococcus furiosus by a mechanism that depends on transcription of the crRNA target sequence within the plasmid. Notably, we found that the target RNA per se induces DNA cleavage by the Cmr complex in vitro. DNA cleavage activity does not depend on cleavage of the target RNA but notably does require the presence of a short sequence adjacent to the target sequence within the activating target RNA (rPAM [RNA protospacer-adjacent motif]). The activated complex does not require a target sequence (or a PAM) in the DNA substrate. Plasmid elimination by the P. furiosus Cmr system also does not require the Csx1 (CRISPR-associated Rossman fold [CARF] superfamily) protein. Plasmid silencing depends on the HD nuclease and Palm domains of the Cmr2 (Cas10 superfamily) protein. The results establish the Cmr complex as a novel DNA nuclease activated by invader RNAs containing a crRNA target sequence and a rPAM.

The CRISPR-associated Csx1 protein of Pyrococcus furiosus is an adenosine-specific endoribonuclease

Prokaryotes are frequently exposed to potentially harmful invasive nucleic acids from phages, plasmids, and transposons. One method of defense is the CRISPR-Cas adaptive immune system. Diverse CRISPR-Cas systems form distinct ribonucleoprotein effector complexes that target and cleave invasive nucleic acids to provide immunity. The Type III-B Cmr effector complex has been found to target the RNA and DNA of the invader in the various bacterial and archaeal organisms where it has been characterized. Interestingly, the gene encoding the Csx1 protein is frequently located in close proximity to the Cmr1-6 genes in many genomes, implicating a role for Csx1 in Cmr function. However, evidence suggests that Csx1 is not a stably associated component of the Cmr effector complex, but is necessary for DNA silencing by the Cmr system in Sulfolobus islandicus. To investigate the function of the Csx1 protein, we characterized the activity of recombinant Pyrococcus furiosus Csx1 against various nucleic acid substrates. We show that Csx1 is a metal-independent, endoribonuclease that acts selectively on single-stranded RNA and cleaves specifically after adenosines. The RNA cleavage activity of Csx1 is dependent upon a conserved HEPN motif located within the C-terminal domain of the protein. This motif is also key for activity in other known ribonucleases. Collectively, the findings indicate that invader silencing by Type III-B CRISPR-Cas systems relies both on RNA and DNA nuclease activities from the Cmr effector complex as well as on the affiliated, trans-acting Csx1 endoribonuclease.

DNA targeting by the type I-G and type I-A CRISPR-Cas systems of Pyrococcus furiosus

CRISPR–Cas systems silence plasmids and viruses in prokaryotes. CRISPR–Cas effector complexes contain CRISPR RNAs (crRNAs) that include sequences captured from invaders and direct CRISPR-associated (Cas) proteins to destroy corresponding invader nucleic acids. Pyrococcus furiosus (Pfu) harbors three CRISPR–Cas immune systems: a Cst (Type I-G) system with an associated Cmr (Type III-B) module at one locus, and a partial Csa (Type I-A) module (lacking known invader sequence acquisition and crRNA processing genes) at another locus. The Pfu Cmr complex cleaves complementary target RNAs, and Csa systems have been shown to target DNA, while the mechanism by which Cst complexes silence invaders is unknown. In this study, we investigated the function of the Cst as well as Csa system in Pfu strains harboring a single CRISPR–Cas system. Plasmid transformation assays revealed that the Cst and Csa systems both function by DNA silencing and utilize similar flanking sequence information (PAMs) to identify invader DNA. Silencing by each system specifically requires its associated Cas3 nuclease. crRNAs from the 7 shared CRISPR loci in Pfu are processed for use by all 3 effector complexes, and Northern analysis revealed that individual effector complexes dictate the profile of mature crRNA species that is generated.

An updated evolutionary classification of CRISPR-Cas systems

The evolution of CRISPR-cas loci, which encode adaptive immune systems in archaea and bacteria, involves rapid changes, in particular numerous rearrangements of the locus architecture and horizontal transfer of complete loci or individual modules. These dynamics complicate straightforward phylogenetic classification, but here we present an approach combining the analysis of signature protein families and features of the architecture of cas loci that unambiguously partitions most CRISPR-cas loci into distinct classes, types and subtypes. The new classification retains the overall structure of the previous version but is expanded to now encompass two classes, five types and 16 subtypes. The relative stability of the classification suggests that the most prevalent variants of CRISPR-Cas systems are already known. However, the existence of rare, currently unclassifiable variants implies that additional types and subtypes remain to be characterized.

Three CRISPR-Cas immune effector complexes coexist in Pyrococcus furiosus

CRISPR-Cas immune systems function to defend prokaryotes against potentially harmful mobile genetic elements including viruses and plasmids. The multiple CRISPR-Cas systems (Types I, II, and III) each target destruction of foreign nucleic acids via structurally and functionally diverse effector complexes (crRNPs). CRISPR-Cas effector complexes are comprised of CRISPR RNAs (crRNAs) that contain sequences homologous to the invading nucleic acids and Cas proteins specific to each immune system type. We have previously characterized a crRNP in Pyrococcus furiosus (Pfu) that contains Cmr (Type III-B) Cas proteins associated with one of two size classes of crRNAs and cleaves complementary target RNAs. Here, we have isolated and characterized two additional native Pfu crRNPs containing either Csa (Type I-A) or Cst (Type I-G) Cas proteins and distinct profiles of associated crRNAs. For each complex, the Cas proteins were identified by mass spectrometry and immunoblotting and the crRNAs by RNA sequencing and Northern blot analysis. The crRNAs associated with both the Csa and Cst complexes originate from all seven Pfu CRISPR loci and contain identical 5' ends (8-nt repeat-derived 5' tag sequences) but heterogeneous 3' ends (containing variable amounts of downstream repeat sequences). These crRNA forms are distinct from Cmr-associated crRNAs, indicating different 3' end processing pathways following primary cleavage of common pre-crRNAs. Like other previously characterized Type I CRISPR-Cas effector complexes, we predict that the newly identified Pfu Csa and Cst crRNPs each function to target invading DNA, adding an additional layer of protection beyond that afforded by the previously characterized RNA targeting Cmr complex.

Argonaute of the archaeon Pyrococcus furiosus is a DNA-guided nuclease that targets cognate DNA

Functions of prokaryotic Argonautes (pAgo) have long remained elusive. Recently, Argonautes of the bacteria Rhodobacter sphaeroides and Thermus thermophilus were demonstrated to be involved in host defense. The Argonaute of the archaeon Pyrococcus furiosus (PfAgo) belongs to a different branch in the phylogenetic tree, which is most closely related to that of RNA interference-mediating eukaryotic Argonautes. Here we describe a functional and mechanistic characterization of PfAgo. Like the bacterial counterparts, archaeal PfAgo contributes to host defense by interfering with the uptake of plasmid DNA. PfAgo utilizes small 5′-phosphorylated DNA guides to cleave both single stranded and double stranded DNA targets, and does not utilize RNA as guide or target. Thus, with respect to function and specificity, the archaeal PfAgo resembles bacterial Argonautes much more than eukaryotic Argonautes. These findings demonstrate that the role of Argonautes is conserved through the bacterial and archaeal domains of life and suggests that eukaryotic Argonautes are derived from DNA-guided DNA-interfering host defense systems.

Cas9 function and host genome sampling in Type II-A CRISPR-Cas adaptation

Wei et al. found that Cas9, previously identified as the nuclease responsible for ultimate invader destruction, is also essential for adaptation in Streptococcus thermophilus. Cas9 nuclease activity is dispensable for adaptation. Wei et al. also revealed extensive, unbiased acquisition of the self-targeting host genome sequence by the CRISPR–Cas system that is masked in the presence of active target destruction.

To acquire the ability to recognize and destroy virus and plasmid invaders, prokaryotic CRISPR–Cas systems capture fragments of DNA within the host CRISPR locus. Our results indicate that the process of adaptation by a Type II-A CRISPR–Cas system in Streptococcus thermophilus requires Cas1, Cas2, and Csn2. Surprisingly, we found that Cas9, previously identified as the nuclease responsible for ultimate invader destruction, is also essential for adaptation. Cas9 nuclease activity is dispensable for adaptation. In addition, our studies revealed extensive, unbiased acquisition of the self-targeting host genome sequence by the CRISPR–Cas system that is masked in the presence of active target destruction.

Sequences spanning the leader-repeat junction mediate CRISPR adaptation to phage in Streptococcus thermophilus

CRISPR-Cas systems are RNA-based immune systems that protect prokaryotes from invaders such as phages and plasmids. In adaptation, the initial phase of the immune response, short foreign DNA fragments are captured and integrated into host CRISPR loci to provide heritable defense against encountered foreign nucleic acids. Each CRISPR contains a ∼100-500 bp leader element that typically includes a transcription promoter, followed by an array of captured ∼35 bp sequences (spacers) sandwiched between copies of an identical ∼35 bp direct repeat sequence. New spacers are added immediately downstream of the leader. Here, we have analyzed adaptation to phage infection in Streptococcus thermophilus at the CRISPR1 locus to identify cis-acting elements essential for the process. We show that the leader and a single repeat of the CRISPR locus are sufficient for adaptation in this system. Moreover, we identified a leader sequence element capable of stimulating adaptation at a dormant repeat. We found that sequences within 10 bp of the site of integration, in both the leader and repeat of the CRISPR, are required for the process. Our results indicate that information at the CRISPR leader-repeat junction is critical for adaptation in this Type II-A system and likely other CRISPR-Cas systems.

Target RNA capture and cleavage by the Cmr type III-B CRISPR-Cas effector complex

The effector complex of the Cmr/type III-B CRISPR–Cas system cleaves RNAs recognized by the crRNA of the complex and includes six protein subunits of unknown functions. Hale et al. used reconstituted Pyrococcus furiosus Cmr complexes and found that Cmr3 recognizes the signature crRNA tag sequence (and depends on protein–protein interactions with Cmr2, Cmr4, and Cmr5), each Cmr4 subunit mediates a target RNA cleavage, and Cmr1 and Cmr6 mediate an essential interaction between the 3′ region of the crRNA and the target RNA.

The effector complex of the Cmr/type III-B CRISPR (clustered regularly interspaced short palindromic repeat)–Cas (CRISPR-associated) system cleaves RNAs recognized by the CRISPR RNA (crRNA) of the complex and includes six protein subunits of unknown functions. Using reconstituted Pyrococcus furiosus Cmr complexes, we found that each of the six Cmr proteins plays a critical role in either crRNA interaction or target RNA capture. Cmr2, Cmr3, Cmr4, and Cmr5 are all required for formation of a crRNA-containing complex detected by native gel electrophoresis, and the conserved 5′ repeat sequence tag and 5′-OH group of the crRNA are essential for the interaction. Interestingly, capture of the complementary target RNA additionally requires both Cmr1 and Cmr6. In detailed functional studies, we determined that P. furiosus Cmr complexes cleave target RNAs at 6-nucleotide (nt) intervals in the region of complementarity, beginning 5 nt downstream from the crRNA tag and continuing to within ∼14 nt of the 3′ end of the crRNA. Our findings indicate that Cmr3 recognizes the signature crRNA tag sequence (and depends on protein–protein interactions with Cmr2, Cmr4, and Cmr5), each Cmr4 subunit mediates a target RNA cleavage, and Cmr1 and Cmr6 mediate an essential interaction between the 3′ region of the crRNA and the target RNA.

Essential structural and functional roles of the Cmr4 subunit in RNA cleavage by the Cmr CRISPR-Cas complex

The Cmr complex is the multisubunit effector complex of the type III-B clustered regularly interspaced short palindromic repeats (CRISPR)-Cas immune system. The Cmr complex recognizes a target RNA through base pairing with the integral CRISPR RNA (crRNA) and cleaves the target at multiple regularly spaced locations within the complementary region. To understand the molecular basis of the function of this complex, we have assembled information from electron microscopic and X-ray crystallographic structural studies and mutagenesis of a complete Pyrococcus furiosus Cmr complex. Our findings reveal that four helically packed Cmr4 subunits, which make up the backbone of the Cmr complex, act as a platform to support crRNA binding and target RNA cleavage. Interestingly, we found a hook-like structural feature associated with Cmr4 that is likely the site of target RNA binding and cleavage. Our results also elucidate analogies in the mechanisms of crRNA and target molecule binding by the distinct Cmr type III-A and Cascade type I-E complexes.

The three major types of CRISPR-Cas systems function independently in CRISPR RNA biogenesis in Streptococcus thermophilus

CRISPR-Cas systems are small RNA-based immune systems that protect prokaryotes from invaders such as viruses and plasmids. We have investigated the features and biogenesis of the CRISPR (cr)RNAs in Streptococcus thermophilus (Sth) strain DGCC7710, which possesses four different CRISPR-Cas systems including representatives from the three major types of CRISPR-Cas systems. Our results indicate that the crRNAs from each CRISPR locus are specifically processed into divergent crRNA species by Cas proteins (and non-coding RNAs) associated with the respective locus. We find that the Csm Type III-A and Cse Type I-E crRNAs are specifically processed by Cas6 and Cse3 (Cas6e), respectively, and retain an 8-nucleotide CRISPR repeat sequence tag 5' of the invader-targeting sequence. The Cse Type I-E crRNAs also retain a 21-nucleotide 3' repeat tag. The crRNAs from the two Csn Type II-A systems in Sth consist of a 5'-truncated targeting sequence and a 3' tag; however, these are distinct in size between the two. Moreover, the Csn1 (Cas9) protein associated with one Csn locus functions specifically in the production of crRNAs from that locus. Our findings indicate that multiple CRISPR-Cas systems can function independently in crRNA biogenesis within a given organism - an important consideration in engineering coexisting CRISPR-Cas pathways.

The three major types of CRISPR-Cas systems function independently in CRISPR RNA biogenesis in Streptococcus thermophilus

CRISPR-Cas systems are small RNA-based immune systems that protect prokaryotes from invaders such as viruses and plasmids. We have investigated the features and biogenesis of the CRISPR (cr)RNAs in Streptococcus thermophilus (Sth) strain DGCC7710, which possesses four different CRISPR-Cas systems including representatives from the three major types of CRISPR-Cas systems. Our results indicate that the crRNAs from each CRISPR locus are specifically processed into divergent crRNA species by Cas proteins (and non-coding RNAs) associated with the respective locus. We find that the Csm Type III-A and Cse Type I-E crRNAs are specifically processed by Cas6 and Cse3 (Cas6e), respectively, and retain an 8-nucleotide CRISPR repeat sequence tag 5' of the invader-targeting sequence. The Cse Type I-E crRNAs also retain a 21-nucleotide 3' repeat tag. The crRNAs from the two Csn Type II-A systems in Sth consist of a 5'-truncated targeting sequence and a 3' tag; however, these are distinct in size between the two. Moreover, the Csn1 (Cas9) protein associated with one Csn locus functions specifically in the production of crRNAs from that locus. Our findings indicate that multiple CRISPR-Cas systems can function independently in crRNA biogenesis within a given organism - an important consideration in engineering coexisting CRISPR-Cas pathways.

Programmable plasmid interference by the CRISPR-Cas system in Thermococcus kodakarensis

CRISPR-Cas systems are RNA-guided immune systems that protect prokaryotes against viruses and other invaders. The CRISPR locus encodes crRNAs that recognize invading nucleic acid sequences and trigger silencing by the associated Cas proteins. There are multiple CRISPR-Cas systems with distinct compositions and mechanistic processes. Thermococcus kodakarensis (Tko) is a hyperthermophilic euryarchaeon that has both a Type I-A Csa and a Type I-B Cst CRISPR-Cas system. We have analyzed the expression and composition of crRNAs from the three CRISPRs in Tko by RNA deep sequencing and northern analysis. Our results indicate that crRNAs associated with these two CRISPR-Cas systems include an 8-nucleotide conserved sequence tag at the 5' end. We challenged Tko with plasmid invaders containing sequences targeted by endogenous crRNAs and observed active CRISPR-Cas-mediated silencing. Plasmid silencing was dependent on complementarity with a crRNA as well as on a sequence element found immediately adjacent to the crRNA recognition site in the target termed the PAM (protospacer adjacent motif). Silencing occurred independently of the orientation of the target sequence in the plasmid, and appears to occur at the DNA level, presumably via DNA degradation. In addition, we have directed silencing of an invader plasmid by genetically engineering the chromosomal CRISPR locus to express customized crRNAs directed against the plasmid. Our results support CRISPR engineering as a feasible approach to develop prokaryotic strains that are resistant to infection for use in industry.

Structure of the Cmr2-Cmr3 subcomplex of the Cmr RNA silencing complex

The Cmr complex is an RNA-guided effector complex that cleaves invader RNA in the prokaryotic immune response mediated by the CRISPR (Clustered Regularly Interspaced Short Palindromic Repeat)-Cas system. Here, we report the crystal structure of a Cmr subcomplex containing Cmr2 (Cas10) and Cmr3 subunits at 2.8 Å resolution. The structure revealed a dual ferredoxin fold and glycine-rich loops characteristic of previously known repeat-associated mysterious proteins and two unique insertion elements in Cmr3 that mediate its interaction with Cmr2. Surprisingly, while mutation of both insertion elements significantly weakened Cmr3-Cmr2 interaction, they exhibit differential effects on Cmr-mediated RNA cleavage by the Cmr complex, suggesting stabilization of Cmr2-Cmr3 interactions by other subunits. Further mutational analysis of the two conserved (but non-Cmr2-binding) glycine-rich loops of Cmr3 identified a region that is likely involved in assembly or the RNA cleavage function of the Cmr complex.

Structure of the Cmr2 subunit of the CRISPR-Cas RNA silencing complex

Cmr2 is the largest and an essential subunit of a CRISPR RNA-Cas protein complex (the Cmr complex) that cleaves foreign RNA to protect prokaryotes from invading genetic elements. Cmr2 is thought to be the catalytic subunit of the effector complex because of its N-terminal HD nuclease domain. Here, however, we report that the HD domain of Cmr2 is not required for cleavage by the complex in vitro. The 2.3Å crystal structure of Pyrococcus furiosus Cmr2 (lacking the HD domain) reveals two adenylyl cyclase-like and two α-helical domains. The adenylyl cyclase-like domains are arranged as in homodimeric adenylyl cyclases and bind ADP and divalent metals. However, mutagenesis studies show that the metal- and ADP-coordinating residues of Cmr2 are also not critical for cleavage by the complex. Our findings suggest that another component provides the catalytic function and that the essential role by Cmr2 does not require the identified ADP- or metal-binding or HD domains in vitro.

Essential features and rational design of CRISPR RNAs that function with the Cas RAMP module complex to cleave RNAs

Small RNAs target invaders for silencing in the CRISPR-Cas pathways that protect bacteria and archaea from viruses and plasmids. The CRISPR RNAs (crRNAs) contain sequence elements acquired from invaders that guide CRISPR-associated (Cas) proteins back to the complementary invading DNA or RNA. Here, we have analyzed essential features of the crRNAs associated with the Cas RAMP module (Cmr) effector complex, which cleaves targeted RNAs. We show that Cmr crRNAs contain an 8 nucleotide 5' sequence tag (also found on crRNAs associated with other CRISPR-Cas pathways) that is critical for crRNA function and can be used to engineer crRNAs that direct cleavage of novel targets. We also present data that indicate that the Cmr complex cleaves an endogenous complementary RNA in Pyrococcus furiosus, providing direct in vivo evidence of RNA targeting by the CRISPR-Cas system. Our findings indicate that the CRISPR RNA-Cmr protein pathway may be exploited to cleave RNAs of interest.

Processive and distributive extension of human telomeres by telomerase under homeostatic and nonequilibrium conditions

Specific information about how telomerase acts in vivo is necessary for understanding telomere dynamics in human tumor cells. Our results imply that, under homeostatic telomere length-maintenance conditions, only one molecule of telomerase acts at each telomere during every cell division and processively adds ∼60 nt to each end. In contrast, multiple molecules of telomerase act at each telomere when telomeres are elongating (nonequilibrium conditions). Telomerase extension is less processive during the first few weeks following the reversal of long-term treatment with the telomerase inhibitor Imetelstat (GRN163L), a time when Cajal bodies fail to deliver telomerase RNA to telomeres. This result implies that processing of telomerase by Cajal bodies may affect its processivity. Overexpressed telomerase is also less processive than the endogenously expressed telomerase. These findings reveal two major distinct extension modes adopted by telomerase in vivo.

Visualization of human telomerase localization by fluorescence microscopy techniques

Human telomerase is a ribonucleoprotein (RNP) that synthesizes DNA repeats at the ends of chromosomes and maintains telomere length and genome stability. The enzyme comprises telomerase RNA (hTR) (which provides the template for telomere addition) and several protein subunits, including telomerase reverse transcriptase (hTERT) (the catalytic component). Intracellular trafficking of the enzyme has emerged as an important factor in the regulation of telomerase activity. Telomerase trafficking between nuclear Cajal bodies (proposed sites of telomerase biogenesis and regulation) and telomeres (sites of action) is regulated by the cell cycle in concordance with telomere synthesis during S phase. Here, we describe fluorescence microscopy approaches to visualize the subcellular localization of the essential RNA component of hTR relative to Cajal bodies and telomeres in cultured human cells. These methods include fluorescence in situ hybridization (to detect hTR and telomeric DNA) and immunofluorescence (IF) (to detect Cajal bodies and telomere-binding proteins). Because telomerase localization to telomeres is normally restricted to S phase, we also describe methods to synchronize and analyze cells within this phase of the cell cycle.

CRISPR-based adaptive immune systems

CRISPR-Cas systems are recently discovered, RNA-based immune systems that control invasions of viruses and plasmids in archaea and bacteria. Prokaryotes with CRISPR-Cas immune systems capture short invader sequences within the CRISPR loci in their genomes, and small RNAs produced from the CRISPR loci (CRISPR (cr)RNAs) guide Cas proteins to recognize and degrade (or otherwise silence) the invading nucleic acids. There are multiple variations of the pathway found among prokaryotes, each mediated by largely distinct components and mechanisms that we are only beginning to delineate. Here we will review our current understanding of the remarkable CRISPR-Cas pathways with particular attention to studies relevant to systems found in the archaea.

Interaction of the Cas6 riboendonuclease with CRISPR RNAs: recognition and cleavage

The CRISPRs (Clustered Regularly Interspaced Short Palindromic Repeats) found in prokaryotic genomes confer small RNA-mediated protection against viruses and other invaders. CRISPR loci contain iterations of a short repeat sequence alternating with small segments of varying invader-derived sequences. Distinct families of CRISPR-associated Cas proteins function to cleave within the repeat sequence of CRISPR transcripts and produce the individual invader-targeting crRNAs. Here, we report the crystal structure of Pyrococcus furiosus Cas6 bound with a repeat RNA at 3.2 Å resolution. In contrast to other Cas families of endonucleases, Cas6 clasps nucleotides 2-9 of the repeat RNA using its two ferredoxin-like domains, and the enzyme-anchored 5' end tethers the distal cleavage site of the RNA between nucleotides 22 and 23 to the predicted enzyme active site on the opposite side of the ferrodoxin-like domains. Our findings suggest a wrap-around mechanism for CRISPR RNA recognition and cleavage by Cas6 and related processing endonucleases.

Telomerase trafficking and assembly in Xenopus oocytes

The core components of telomerase are telomerase RNA (TR) and telomerase reverse transcriptase (TERT). In vertebrate cells, TR and TERT have been reported to associate with intranuclear structures, including Cajal bodies and nucleoli as well as telomeres. Here, we examined the time course of both TR localization and assembly of TR with TERT in Xenopus oocytes. The major trafficking pathway for microinjected TR is through Cajal bodies into the nucleoplasm, with a fraction of TR found in nucleoli at later time points. Telomerase assembly precedes nucleolar localization of TR, and TR mutants that do not localize to nucleoli form active enzyme, indicating that localization of TR to nucleoli is not required for assembly with TERT. Assembly of telomerase coincides with Cajal-body localization; however, assembly is also unaffected by a CAB-box mutation (which significantly reduces association with Cajal bodies), suggesting that Cajal-body localization is not important for assembly. Our results suggest that assembly of TR with TERT occurs in the nucleoplasm. Unexpectedly, however, our experiments reveal that disruption of the CAB box does not eliminate early targeting to Cajal bodies, indicating that a role for Cajal bodies in telomerase assembly cannot be excluded on the basis of existing knowledge.

Binding and cleavage of CRISPR RNA by Cas6

The CRISPR-Cas system provides many prokaryotes with acquired resistance to viruses and other mobile genetic elements. The core components of this defense system are small, host-encoded prokaryotic silencing (psi)RNAs and Cas (CRISPR-associated) proteins. Invader-derived sequences within the psiRNAs guide Cas effector proteins to recognize and silence invader nucleic acids. Critical for CRISPR-Cas defense is processing of the psiRNAs from the primary transcripts of the host CRISPR (clustered regularly interspaced short palindromic repeat) locus. Cas6, a previously identified endoribonuclease present in a wide range of prokaryotes with the CRISPR-Cas system, binds and cleaves within the repeat sequences that separate the individual invader targeting elements in the CRISPR locus transcript. In the present study, we investigated several key aspects of the mechanism of function of Cas6 in psiRNA biogenesis. RNA footprinting reveals that Pyrococcus furiosus Cas6 binds to a 7-nt (nucleotide) sequence near the 5' end of the CRISPR RNA repeat sequence, 14 nt upstream of the Cas6 cleavage site. In addition, analysis of the cleavage activity of P. furiosus Cas6 proteins with mutations at conserved residues suggests that a triad comprised of Tyr31, His46, and Lys52 plays a critical role in catalysis, consistent with a possible general acid-base RNA cleavage mechanism for Cas6. Finally, we show that P. furiosus Cas6 remains stably associated with its cleavage products, suggesting additional roles for Cas6 in psiRNA biogenesis.

Structural basis for substrate placement by an archaeal box C/D ribonucleoprotein particle

Box C/D small nucleolar and Cajal body ribonucleoprotein particles (sno/scaRNPs) direct site-specific 2'-O-methylation of ribosomal and spliceosomal RNAs and are critical for gene expression. Here we report crystal structures of an archaeal box C/D RNP containing three core proteins (fibrillarin, Nop56/58, and L7Ae) and a half-mer box C/D guide RNA paired with a substrate RNA. The structure reveals a guide-substrate RNA duplex orientation imposed by a composite protein surface and the conserved GAEK motif of Nop56/58. Molecular modeling supports a dual C/D RNP structure that closely mimics that recently visualized by electron microscopy. The substrate-bound dual RNP model predicts an asymmetric protein distribution between the RNP that binds and methylates the substrate RNA. The predicted asymmetric nature of the holoenzyme is consistent with previous biochemical data on RNP assembly and provides a simple solution for accommodating base-pairing between the C/D guide RNA and large ribosomal and spliceosomal substrate RNAs.

A Cajal body-independent pathway for telomerase trafficking in mice

The intranuclear trafficking of human telomerase involves a dynamic interplay between multiple nuclear sites, most notably Cajal bodies and telomeres. Cajal bodies are proposed to serve as sites of telomerase maturation, storage, and assembly, as well as to function in the cell cycle-regulated delivery of telomerase to telomeres in human cells. Here, we find that telomerase RNA does not localize to Cajal bodies in mouse cells, and instead resides in separate nuclear foci throughout much of the cell cycle. However, as in humans, mouse telomerase RNA (mTR) localizes to subsets of telomeres specifically during S phase. The localization of mTR to telomeres in mouse cells does not require coilin-containing Cajal bodies, as mTR is found at telomeres at similar frequencies in cells from wild-type and coilin knockout mice. At the same time, we find that human TR localizes to Cajal bodies (as well as telomeres) in mouse cells, indicating that the distinct trafficking of mTR is attributable to an intrinsic property of the RNA (rather than a difference in the mouse cell environment such as the properties of mouse Cajal bodies). We also find that during S phase, mTR foci coalesce into short chains, with at least one of the conjoined mTR foci co-localizing with a telomere. These findings point to a novel, Cajal body-independent pathway for telomerase biogenesis and trafficking in mice.

RNA-guided RNA cleavage by a CRISPR RNA-Cas protein complex

Compelling evidence indicates that the CRISPR-Cas system protects prokaryotes from viruses and other potential genome invaders. This adaptive prokaryotic immune system arises from the clustered regularly interspaced short palindromic repeats (CRISPRs) found in prokaryotic genomes, which harbor short invader-derived sequences, and the CRISPR-associated (Cas) protein-coding genes. Here, we have identified a CRISPR-Cas effector complex that is comprised of small invader-targeting RNAs from the CRISPR loci (termed prokaryotic silencing (psi)RNAs) and the RAMP module (or Cmr) Cas proteins. The psiRNA-Cmr protein complexes cleave complementary target RNAs at a fixed distance from the 3' end of the integral psiRNAs. In Pyrococcus furiosus, psiRNAs occur in two size forms that share a common 5' sequence tag but have distinct 3' ends that direct cleavage of a given target RNA at two distinct sites. Our results indicate that prokaryotes possess a unique RNA silencing system that functions by homology-dependent cleavage of invader RNAs.

Structure of a functional ribonucleoprotein pseudouridine synthase bound to a substrate RNA

Box H/ACA small nucleolar and Cajal body ribonucleoprotein particles comprise the most complex pseudouridine synthases and are essential for ribosome and spliceosome maturation. The multistep and multicomponent-mediated enzyme mechanism remains only partially understood. Here we report a crystal structure at 2.35 A of a substrate-bound functional archaeal enzyme containing three of the four proteins, Cbf5, Nop10 and L7Ae, and a box H/ACA RNA that reveals detailed information about the protein-only active site. The substrate RNA, containing 5-fluorouridine at the modification position, is fully docked and catalytically rearranged by the enzyme in a manner similar to that seen in two stand-alone pseudouridine synthases. Structural analysis provides a mechanism for plasticity in the diversity of guide RNA sequences used and identifies a substrate-anchoring loop of Cbf5 that also interacts with Gar1 in unliganded structures. Activity analyses of mutated proteins and RNAs support the structural findings and further suggest a role of the Cbf5 loop in regulation of enzyme activity.

A human telomerase holoenzyme protein required for Cajal body localization and telomere synthesis

Telomerase is a ribonucleoprotein (RNP) complex that synthesizes telomere repeats in tissue progenitor cells and cancer cells. Active human telomerase consists of at least three principal subunits, including the telomerase reverse transcriptase, the telomerase RNA (TERC), and dyskerin. Here, we identify a holoenzyme subunit, TCAB1 (telomerase Cajal body protein 1), that is notably enriched in Cajal bodies, nuclear sites of RNP processing that are important for telomerase function. TCAB1 associates with active telomerase enzyme, established telomerase components, and small Cajal body RNAs that are involved in modifying splicing RNAs. Depletion of TCAB1 by using RNA interference prevents TERC from associating with Cajal bodies, disrupts telomerase-telomere association, and abrogates telomere synthesis by telomerase. Thus, TCAB1 controls telomerase trafficking and is required for telomere synthesis in human cancer cells.

Cas6 is an endoribonuclease that generates guide RNAs for invader defense in prokaryotes

An RNA-based gene silencing pathway that protects bacteria and archaea from viruses and other genome invaders is hypothesized to arise from guide RNAs encoded by CRISPR loci and proteins encoded by the cas genes. CRISPR loci contain multiple short invader-derived sequences separated by short repeats. The presence of virus-specific sequences within CRISPR loci of prokaryotic genomes confers resistance against corresponding viruses. The CRISPR loci are transcribed as long RNAs that must be processed to smaller guide RNAs. Here we identified Pyrococcus furiosus Cas6 as a novel endoribonuclease that cleaves CRISPR RNAs within the repeat sequences to release individual invader targeting RNAs. Cas6 interacts with a specific sequence motif in the 5' region of the CRISPR repeat element and cleaves at a defined site within the 3' region of the repeat. The 1.8 angstrom crystal structure of the enzyme reveals two ferredoxin-like folds that are also found in other RNA-binding proteins. The predicted active site of the enzyme is similar to that of tRNA splicing endonucleases, and concordantly, Cas6 activity is metal-independent. cas6 is one of the most widely distributed CRISPR-associated genes. Our findings indicate that Cas6 functions in the generation of CRISPR-derived guide RNAs in numerous bacteria and archaea.

Prokaryotic silencing (psi)RNAs in Pyrococcus furiosus

In many prokaryotes, noncoding RNAs that arise from the clustered regularly interspaced short palindromic repeat (CRISPR) loci are now thought to mediate defense against viruses and other molecular invaders by an RNAi-like pathway. CRISPR loci contain multiple short regions of similarity to invader sequences separated by short repeat sequences, and are associated with resistance to infection by corresponding viruses. It is hypothesized that RNAs derived from these regions, termed prokaryotic silencing (psi)RNAs, guide Slicer-like complexes of partner proteins to destroy invader nucleic acids. Here we have investigated CRISPR-derived RNAs in the archaeon Pyrococcus furiosus. Northern analysis revealed multiple RNA species consistent with a proposed biogenesis pathway that includes full-length CRISPR locus transcripts and intermediates generated by endonucleolytic cleavages within the repeat sequences. However, our results identify the principal products of the CRISPR loci as small psiRNAs comprised primarily of invader-targeting sequence with perhaps only 5-10 nucleotides of CRISPR repeat sequence. These RNAs are the most abundant CRISPR RNA species in P. furiosus and are likely the guides for the effector complexes of the proposed prokaryotic RNAi (pRNAi) system. We analyzed cell-free extracts fractionated under non-denaturing conditions and found that the various CRISPR RNA species are components of distinct RNA-protein complexes, including at least two complexes that contain mature-length psiRNAs. Finally, RNAs are produced from all seven CRISPR loci present in the P. furiosus genome, and interestingly, the most recently acquired psiRNAs encoded proximal to the leader sequence of a CRISPR locus appear to be the most abundant.

Telomerase reverse transcriptase is required for the localization of telomerase RNA to cajal bodies and telomeres in human cancer cells

Telomere maintenance by telomerase is critical for the unlimited division potential of most human cancer cells. The two essential components of human telomerase, telomerase RNA (hTR) and telomerase reverse transcriptase (hTERT), are recruited from distinct subnuclear sites to telomeres during S phase. Throughout the remainder of the cell cycle hTR is found primarily in Cajal bodies. The localization of hTR to Cajal bodies and telomeres is specific to cancer cells where telomerase is active and is not observed in primary cells. Here we show that the trafficking of hTR to both telomeres and Cajal bodies depends on hTERT. RNA interference-mediated depletion of hTERT in cancer cells leads to loss of hTR from both Cajal bodies and telomeres without affecting hTR levels. In addition, expression of hTERT in telomerase-negative cells (including primary and ALT cancer cell lines) induces hTR to localize to both sites. Factors that did not stimulate hTR localization in our experiments include increased hTR RNA levels and Cajal body numbers, and expression of SV40 large T antigen and oncogenic Ras. Our findings suggest that the trafficking of telomerase to Cajal bodies and telomeres in cancer cells correlates with and depends on the assembly of the enzyme.

Determination of protein-RNA interaction sites in the Cbf5-H/ACA guide RNA complex by mass spectrometric protein footprinting

Pseudouridylation is one of the most common forms of RNA modification. In eukaryotes and archaea, these modifications are carried out by H/ACA ribonucleoprotein (RNP) complexes, composed of an H/ACA guide RNA and four proteins, including the pseudouridine synthase, Cbf5. Remarkable progress has been made toward understanding the structure and function of H/ACA RNPs, both through mapping of RNA-protein and protein-protein interactions and the availability of X-ray structures, including that of the entire RNP. The pseudouridine synthase, Cbf5, is also the protein that specifically recognizes the guide RNAs. In this work, we have investigated the molecular basis of this key interaction. A mass spectrometric protein footprinting approach was employed to determine the amino acids of archaeal Cbf5 involved in interaction with the guide RNA. We found amino acid protections along the same RNA binding track observed in the crystal structure of the fully assembled complex, indicating that this interaction is established in the subcomplex. However, in addition, we observed a set of protections in the D2 subdomain of Cbf5 that appear to represent a unique, additional interaction of the guide RNA with the protein in the subcomplex. On the basis of these results, we present a model for the Cbf5-guide RNA complex that also incorporates other recent findings. Our analysis suggests that the assembly or function of H/ACA RNPs may be accompanied by dynamic changes in RNA-protein interactions.

Substrate RNA positioning in the archaeal H/ACA ribonucleoprotein complex

The most complex RNA pseudouridylases are H/ACA ribonucleoprotein particles, which use a guide RNA for substrate capture and four proteins (Cbf5, Nop10, Gar1 and L7Ae/NHP2) for substrate modification. Here we report the three-dimensional structure of a catalytically deficient archaeal enzyme complex (including the guide RNA and three of the four essential proteins) bound to a substrate RNA. Extensive interactions of Cbf5 with one guide-substrate helix and a guide RNA stem shape the forked guide–substrate RNA complex structure and position the substrate in proximity of the Cbf5 catalytic center. Our structural and complementary fluorescence analyses also indicate that precise placement of the target uridine at the active site requires a conformation of the guide–substrate RNA duplex that is brought about by the previously identified concurrent interaction of the guide RNA with L7Ae and a composite Cbf5-Nop10 surface, and further identify a residue that is critical in this process.

Human telomerase RNA accumulation in Cajal bodies facilitates telomerase recruitment to telomeres and telomere elongation

Telomerase is required for telomere maintenance and is responsible for the immortal phenotype of cancer cells. How telomerase is assembled and reaches telomeres in the context of nuclear architecture is not understood. Recently, the telomerase RNA subunit (hTR) was shown to accumulate in Cajal bodies (CBs), subnuclear structures implicated in ribonucleoprotein maturation. However, the functional relevance of this localization for telomerase was unknown. hTR localization to CBs requires a short sequence motif called the CAB box. Here, we reconstitute telomerase in human cells and determine the effects of CAB box mutations on telomere biology. We demonstrate that mutant hTR, which fails to accumulate in CBs, is fully capable of forming catalytically active telomerase in vivo but is strongly impaired in telomere extension. The functional deficiency is accompanied by a decreased association of telomerase with telomeres. Collectively, these data identify subnuclear localization as an important regulatory mechanism for telomere length homeostasis in human cells.

Dynamic interactions within sub-complexes of the H/ACA pseudouridylation guide RNP

H/ACA RNP complexes change uridines to pseudouridines in target non-coding RNAs in eukaryotes and archaea. H/ACA RNPs are comprised of a guide RNA and four essential proteins: Cbf5 (pseudouridine synthase), L7Ae, Gar1 and Nop10 in archaea. The guide RNA captures the target RNA via two antisense elements brought together to form a contiguous binding site within the pseudouridylation pocket (internal loop) of the guide RNA. Cbf5 and L7Ae interact independently with the guide RNA, and here we have examined the impacts of these proteins on the RNA in nucleotide protection assays. The results indicate that the interactions observed in a fully assembled H/ACA RNP are established in the sub-complexes, but also reveal a unique Cbf5-guide RNA interaction that is displaced by L7Ae. In addition, the results indicate that L7Ae binding at the kink (k)-turn of the guide RNA induces the formation of the upper stem, and thus also the pseudouridylation pocket. Our findings indicate that L7Ae is essential for formation of the substrate RNA binding site in the archaeal H/ACA RNP, and suggest that k-turn-binding proteins may remodel partner RNAs with important effects distant from the protein-binding site.

Alternative Conformations of the Archaeal Nop56/58-Fibrillarin Complex Imply Flexibility in Box C/D RNPs

The Nop56/58-fibrillarin heterocomplex is a core protein complex of the box C/D ribonucleoprotein particles that modify and process ribosomal RNAs. The previous crystal structure of the Archaeoglobus fulgidus complex revealed a symmetric dimer of two Nop56/58-fibrillarin complexes linked by the coiled-coil domains of the Nop56/68 proteins. However, because the A. fulgidus Nop56/58 protein lacks some domains found in most other species, it was thought that the bipartite architecture of the heterocomplex was not likely a general phenomenon. Here we report the crystal structure of the Nop56/58-fibrillarin complex bound with methylation cofactor, S-adenosyl-L-methionine from Pyrococcus furiosus, at 2.7 A. The new complex confirms the generality of the previously observed bipartite arrangement. In addition however, the conformation of Nop56/58 in the new structure differs substantially from that in the earlier structure. The distinct conformations of Nop56/58 suggest potential flexibility in Nop56/58. Computational normal mode analysis supports this view. Importantly, fibrillarin is repositioned within the two complexes. We propose that hinge motion within Nop56/58 has important implications for the possibility of simultaneously positioning two catalytic sites at the two target sites of a bipartite box C/D guide RNA.

Non-coding RNAs: lessons from the small nuclear and small nucleolar RNAs

Recent advances have fuelled rapid growth in our appreciation of the tremendous number, diversity and biological importance of non-coding (nc)RNAs. Because ncRNAs typically function as ribonucleoprotein (RNP) complexes and not as naked RNAs, understanding their biogenesis is crucial to comprehending their regulation and function. The small nuclear and small nucleolar RNPs are two well studied classes of ncRNPs with elaborate assembly and trafficking pathways that provide paradigms for understanding the biogenesis of other ncRNPs.

Formation of the conserved pseudouridine at position 55 in archaeal tRNA

Pseudouridine (Psi) located at position 55 in tRNA is a nearly universally conserved RNA modification found in all three domains of life. This modification is catalyzed by TruB in bacteria and by Pus4 in eukaryotes, but so far the Psi55 synthase has not been identified in archaea. In this work, we report the ability of two distinct pseudouridine synthases from the hyperthermophilic archaeon Pyrococcus furiosus to specifically modify U55 in tRNA in vitro. These enzymes are (pfu)Cbf5, a protein known to play a role in RNA-guided modification of rRNA, and (pfu)PsuX, a previously uncharacterized enzyme that is not a member of the TruB/Pus4/Cbf5 family of pseudouridine synthases. (pfu)PsuX is hereafter renamed (pfu)Pus10. Both enzymes specifically modify tRNA U55 in vitro but exhibit differences in substrate recognition. In addition, we find that in a heterologous in vivo system, (pfu)Pus10 efficiently complements an Escherichia coli strain deficient in the bacterial Psi55 synthase TruB. These results indicate that it is probable that (pfu)Cbf5 or (pfu)Pus10 (or both) is responsible for the introduction of pseudouridine at U55 in tRNAs in archaea. While we cannot unequivocally assign the function from our results, both possibilities represent unexpected functions of these proteins as discussed herein.

Crystal structure of a Cbf5-Nop10-Gar1 complex and implications in RNA-guided pseudouridylation and dyskeratosis congenita

H/ACA RNA-protein complexes, comprised of four proteins and an H/ACA guide RNA, modify ribosomal and small nuclear RNAs. The H/ACA proteins are also essential components of telomerase in mammals. Cbf5 is the H/ACA protein that catalyzes isomerization of uridine to pseudouridine in target RNAs. Mutations in human Cbf5 (dyskerin) lead to dyskeratosis congenita. Here, we describe the 2.1 A crystal structure of a specific complex of three archaeal H/ACA proteins, Cbf5, Nop10, and Gar1. Cbf5 displays structural properties that are unique among known pseudouridine synthases and are consistent with its distinct function in RNA-guided pseudouridylation. We also describe the previously unknown structures of both Nop10 and Gar1 and the structural basis for their essential roles in pseudouridylation. By using information from related structures, we have modeled the entire ribonucleoprotein complex including both guide and substrate RNAs. We have also identified a dyskeratosis congenita mutation cluster site within a modeled dyskerin structure.

Cell cycle-regulated trafficking of human telomerase to telomeres

Telomerase synthesizes telomeres at the ends of human chromosomes during S phase. The results presented here suggest that telomerase activity may be regulated by intranuclear trafficking of the key components of the enzyme in human cells. We examined the subcellular localization of endogenous human telomerase RNA (hTR) and telomerase reverse transcriptase (hTERT) in HeLa cervical carcinoma cells. Throughout most of the cell cycle, we found that the two essential components of telomerase accumulate at intranuclear sites separate from telomeres. However, during S phase, both hTR and hTERT are specifically recruited to subsets of telomeres. The localization of telomerase to telomeres is dynamic, peaking at mid-S phase. We also found complex associations of both hTR and hTERT with nucleoli and Cajal bodies during S phase, implicating both structures in the biogenesis and trafficking of telomerase. Our results mark the first observation of human telomerase at telomeres and provide a mechanism for the cell cycle-dependent regulation of telomere synthesis in human cells.

RNA-guided RNA modification: functional organization of the archaeal H/ACA RNP

In eukaryotes and archaea, uridines in various RNAs are converted to pseudouridines by RNA-guided RNA modification complexes termed H/ACA RNPs. Guide RNAs within the complexes base-pair with target RNAs to direct modification of specific ribonucleotides. Cbf5, a protein component of the complex, likely catalyzes the modification. However, little is known about the organization of H/ACA RNPs and the roles of the multiple proteins thought to comprise the complexes. We have reconstituted functional archaeal H/ACA RNPs from recombinant components, defined the components necessary and sufficient for function, and determined the direct RNA-protein and protein-protein interactions that occur between the components. The results provide substantial insight into the functional organization of this RNP. The functional complex requires a guide RNA and each of four proteins: Cbf5, Gar1, L7Ae, and Nop10. Two proteins interact directly with the guide RNA: L7Ae and Cbf5. L7Ae does not interact with other H/ACA RNP proteins in the absence of the RNA. We have defined two novel functions for Cbf5. Cbf5 is the protein that specifically recognizes and binds H/ACA guide RNAs. In addition, Cbf5 recruits the two other essential proteins, Gar1 and Nop10, to the pseudouridylation guide complex.

Circular box C/D RNAs in Pyrococcus furiosus

Box C/D RNAs are small, noncoding RNAs that function in RNA modification in eukaryotes and archaea. Here, we report that box C/D RNAs exist in the rare biological form of RNA circles in the hyperthermophilic archaeon Pyrococcus furiosus. Northern analysis of box C/D RNAs reveals two prominent RNA species of different electrophoretic mobilities in total P. furiosus RNA preparations. Together, the results of Northern, ribozyme, RT-PCR, and lariat debranching analyses indicate that the two species are circular and linear RNAs of similar length and abundance. It seems that most, if not all, species of box C/D RNAs exist as circles in P. furiosus. In addition, the circular RNAs are found in complexes with proteins required for box C/D RNA function. Our finding places box C/D RNAs among the extremely few circular RNAs known to exist in nature. Moreover, the unexpected discovery of circular box C/D RNAs points to the existence of a previously unrecognized biogenesis pathway for box C/D RNAs in archaea.

Structure determination of fibrillarin from the hyperthermophilic archaeon Pyrococcus furiosus

The methyltransferase fibrillarin is the catalytic component of ribonucleoprotein complexes that direct site-specific methylation of precursor ribosomal RNA and are critical for ribosome biogenesis in eukaryotes and archaea. Here we report the crystal structure of a fibrillarin ortholog from the hyperthermophilic archaeon Pyrococcus furiosus at 1.97A resolution. Comparisons of the X-ray structures of fibrillarin orthologs from Methanococcus jannashii and Archaeoglobus fulgidus reveal nearly identical backbone configurations for the catalytic C-terminal domain with the exception of a unique loop conformation at the S-adenosyl-l-methionine (AdoMet) binding pocket in P. furiosus. In contrast, the N-terminal domains are divergent which may explain why some forms of fibrillarin apparently homodimerize (M. jannashii) while others are monomeric (P. furiosus and A. fulgidus). Three positively charged amino acids surround the AdoMet-binding site and sequence analysis indicates that this is a conserved feature of both eukaryotic and archaeal fibrillarins. We discuss the possibility that these basic residues of fibrillarin are important for RNA-guided rRNA methylation.

Telomerase RNA accumulates in Cajal bodies in human cancer cells

Telomerase synthesizes telomeric DNA repeats at the ends of eukaryotic chromosomes. The RNA component of the enzyme (hTR) provides the template for telomere synthesis, which is catalyzed by telomerase reverse transcriptase (hTERT). Little is known regarding the subcellular localization of hTR and hTERT and the pathway by which telomerase is assembled. Here we report the first glimpse of the detailed subcellular localization of endogenous hTR in human cells, which we obtained by fluorescence in situ hybridization (FISH). Our studies have revealed a distinctive hTR localization pattern in cancer cells. We have found that hTR accumulates within intranuclear foci called Cajal bodies in all typical tumor-derived cell lines examined (in which telomerase is active), but not in primary or ALT cells (where little or no hTERT is present). Accumulation of hTR in the Cajal bodies of primary cells is induced when hTERT is ectopically expressed. Moreover, we report that hTERT is also found in Cajal bodies. Our data suggest that Cajal bodies are involved in the assembly and/or function of human telomerase.

Nuclear RanGTP is not required for targeting small nucleolar RNAs to the nucleolus

The small GTPase Ran is the central regulator of macromolecular transport between the cytoplasm and the nucleus. Recent work has suggested that RanGTP also plays an important role in regulating some intra-nuclear processes. In this study, we have investigated whether RanGTP is required for the intra-nuclear transport of RNAs. Specifically, we directly analyzed the nucleolar localization of Box C/D and Box H/ACA small nucleolar RNAs (snoRNAs) in mammalian (tsBN2) cells, Saccharomyces cerevisiae and Xenopus oocytes under conditions that deplete nuclear RanGTP and prevent RNA export to the cytoplasm. Our data suggest that depletion of nuclear RanGTP does not significantly alter the nucleolar localization of U3 snoRNA in tsBN2 cells. Complementary studies in the budding yeast S. cerevisiae using conditional Ran mutants as well as mutants in Ran regulatory proteins also indicate that disruption of the Ran gradient or of Ran itself does not detectably affect the nucleolar localization of snoRNAs. Finally, microinjection into Xenopus oocytes was used to clearly demonstrate that a specific pool of snoRNAs could still be efficiently targeted to the nucleolus even when the RanGTP gradient was disrupted by microinjection of mutant Ran proteins. Taken together, our data from three phylogenetically distinct experimental systems suggest that nuclear RanGTP, which is essential for trafficking of RNAs between the nuclear and cytoplasmic compartments, is not required for nuclear retention or nucleolar localization of snoRNAs.

An H/ACA guide RNA directs U2 pseudouridylation at two different sites in the branchpoint recognition region in Xenopus oocytes

U2 is the most extensively modified of all spliceosomal snRNAs. We previously showed that at least some of the internally modified nucleotides in U2 snRNA are required for snRNP biogenesis and pre-mRNA splicing. Recent work from several laboratories suggests that nuclear guide RNAs facilitate U2 snRNA internal modification, including pseudouridylation and 2'-O-methylation. Here, we present a novel approach to identifying guide RNAs for U2 pseudouridylation. Several Xenopus oocyte nuclear RNAs were affinity selected with U2 snRNA substituted with 5-fluorouridine, a pseudouridylation inhibitor that sequesters pseudouridylases. One of these RNAs was sequenced and found to be a novel RNA of 134 nt. This small RNA contains an H/ACA motif and folds into a typical H/ACA RNA structure, and its authenticity as an H/ACA RNA was confirmed by immunoprecipitation analysis. The RNA contains two guide sequences for pseudouridylation (psi) of U2 snRNA at positions 34 and 44 in the branch-site recognition region, and we demonstrate that this RNA indeed guides the formation of psi34 and psi44 in U2 using a Xenopus oocyte reconstitution system. Therefore, this novel RNA was designated pugU2-34/44, for pseudouridylation guide for U2 snRNA U34 and U44. Intranuclear localization analyses indicate that pugU2-34/44 resides within the nucleoplasm rather than nucleoli or Cajal bodies where other guide RNAs have been localized. Our results clarify the mechanism of U2 snRNA pseudouridylation in Xenopus oocytes, and have interesting implications with regard to the intranuclear localization of U2 snRNA pseudouridylation.

Determinants of the interaction of the spinal muscular atrophy disease protein SMN with the dimethylarginine-modified box H/ACA small nucleolar ribonucleoprotein GAR1

Deletion or mutation of the SMN1 (survival of motor neurons) gene causes the common, fatal neuromuscular disease spinal muscular atrophy. The SMN protein is important in small nuclear ribonucleoprotein (snRNP) assembly and interacts with snRNP proteins via arginine/glycine-rich domains. Recently, SMN was also found to interact with core protein components of the two major families of small nucleolar RNPs, fibrillarin and GAR1, suggesting that SMN may also function in the assembly of small nucleolar RNPs. Here we present results that indicate that the interaction of SMN with GAR1 is mediated by the Tudor domain of SMN. Single point mutations within the Tudor domain, including a spinal muscular atrophy patient mutation, impair the interaction of SMN with GAR1. Furthermore, we find that either of the two arginine/glycine-rich domains of GAR1 can provide for interaction with SMN, but removal of both results in loss of the interaction. Finally, we have found that unlike the interaction of SMN with the Sm snRNP proteins, interaction with GAR1 and fibrillarin is not enhanced by arginine dimethylation. Our results argue against post-translational arginine dimethylation as a general requirement for SMN recognition of proteins bearing arginine/glycine-rich domains.

Site-specific cross-linking analyses reveal an asymmetric protein distribution for a box C/D snoRNP

Methylation of the ribose 2'-hydroxyl, the most widespread modification of ribosomal and splicesomal RNAs, is guided by the box C/D class of small nucleolar RNAs (snoRNAs). Box C/D small nucleolar ribonucleoproteins (snoRNPs) contain four core proteins: fibrillarin, Nop56, Nop58 and 15.5 kDa. We constructed U25 snoRNAs containing a single photoactivatable 4-thiouridine at each U position within the conserved box C/D and C'/D' motifs. Proteins assembled on the snoRNA after injection into Xenopus oocyte nuclei were identified by cross-linking, and reconstituted particles characterized by functional rescue and mutational analyses. Our data argue that box C/D snoRNPs are asymmetric, with the C' box contacting Nop56 and fibrillarin, the C box interacting with Nop58, and the D and D' boxes contacting fibrillarin. No cross-link to 15.5 kDa was detected; its binding is disrupted by 4-thiouridine substitution in position 1 of the C box. Repositioning the guide sequence of U25 upstream of box D instead of D' revealed that both C/D motifs have the potential to function as guide centers, but, surprisingly, there was no alteration in protein cross-linking.