RNA polymerase III in Cajal bodies and lampbrush chromosomes of the Xenopus oocyte nucleus

Characterization of axolotl lampbrush chromosomes by fluorescence in situ hybridization and immunostaining

The lampbrush chromosomes (LBCs) in oocytes of the Mexican axolotl (Ambystoma mexicanum) were identified some time ago by their relative lengths and predicted centromeres, but they have never been associated completely with the mitotic karyotype, linkage maps or genome assembly. We identified 9 of the axolotl LBCs using RNAseq to identify actively transcribed genes and 13 BAC (bacterial artificial clone) probes containing pieces of active genes. Using read coverage analysis to find candidate centromere sequences, we developed a centromere probe that localizes to all 14 centromeres. Measurements of relative LBC arm lengths and polymerase III localization patterns enabled us to identify all LBCs. This study presents a relatively simple and reliable way to identify each axolotl LBC cytologically and to anchor chromosome-length sequences (from the axolotl genome assembly) to the physical LBCs by immunostaining and fluorescence in situ hybridization. Our data will facilitate a more detailed transcription analysis of individual LBC loops.

Immunochemical Detection of Modified Cytosine Species in Lampbrush Chromatin

The lampbrush chromosomes found in the giant nucleus or germinal vesicle (GV) of amphibian oocytes provide unique opportunities for discrete closed and open chromatin structural domains to be directly observable by simple light microscopy. Moreover, the method described here for preparing spreads of lampbrush chromatin for immunostaining enables a straightforward approach to establishing the distributions of modified nucleotides within and between structurally and functionally distinctive chromatin domains.

Cold shock induces novel nuclear bodies in Xenopus oocytes

Here we describe novel spherical structures that are induced by cold shock on the lampbrush chromosomes (LBCs) of Xenopus laevis oocytes. We call these structures cold bodies or C-bodies. C-bodies are distributed symmetrically on homologous LBCs, with a pattern similar to that of 5S rDNA. Neither active transcription nor translation is necessary for their formation. Similar protrusions occur on the edges of some nucleoli. Endogenous LBCs as well as those derived from injected sperm form C-bodies under cold shock conditions. The function of C-bodies is unknown.

Common Features of the Pericentromere and Nucleolus

Both the pericentromere and the nucleolus have unique characteristics that distinguish them amongst the rest of genome. Looping of pericentromeric DNA, due to structural maintenance of chromosome (SMC) proteins condensin and cohesin, drives its ability to maintain tension during metaphase. Similar loops are formed via condensin and cohesin in nucleolar ribosomal DNA (rDNA). Condensin and cohesin are also concentrated in transfer RNA (tRNA) genes, genes which may be located within the pericentromere as well as tethered to the nucleolus. Replication fork stalling, as well as downstream consequences such as genomic recombination, are characteristic of both the pericentromere and rDNA. Furthermore, emerging evidence suggests that the pericentromere may function as a liquid-liquid phase separated domain, similar to the nucleolus. We therefore propose that the pericentromere and nucleolus, in part due to their enrichment of SMC proteins and others, contain similar domains that drive important cellular activities such as segregation, stability, and repair.

Imaging the dynamics of transcription loops in living chromosomes

When in the lampbrush configuration, chromosomes display thousands of visible DNA loops that are transcribed at exceptionally high rates by RNA polymerase II (pol II). These transcription loops provide unique opportunities to investigate not only the detailed architecture of pol II transcription sites but also the structural dynamics of chromosome looping, which is receiving fresh attention as the organizational principle underpinning the higher-order structure of all chromosome states. The approach described here allows for extended imaging of individual transcription loops and transcription units under conditions in which loop RNA synthesis continues. In intact nuclei from lampbrush-stage Xenopus oocytes isolated under mineral oil, highly specific targeting of fluorescent fusions of the RNA-binding protein CELF1 to nascent transcripts allowed functional transcription loops to be observed and their longevity assessed over time. Some individual loops remained extended and essentially static structures over time courses of up to an hour. However, others were less stable and shrank markedly over periods of 30-60 min in a manner that suggested that loop extension requires continued dense coverage with nascent transcripts. In stable loops and loop-derived structures, the molecular dynamics of the visible nascent RNP component were addressed using photokinetic approaches. The results suggested that CELF1 exchanges freely between the accumulated nascent RNP and the surrounding nucleoplasm, and that it exits RNP with similar kinetics to its entrance. Overall, it appears that on transcription loops, nascent transcripts contribute to a dynamic self-organizing structure that exemplifies a phase-separated nuclear compartment.

Precocious detection on amphibian oocyte lampbrush chromosomes of subtle changes in the cellular localisation of the Ro52 protein induced by in vitro culture

Subterminal lampbrush loops of one of the 12 bivalents of the oocyte karyotype of Pleurodeles waltl (Amphibian, Urodele) underwent prominent morphological changes upon in vitro culture. These loops exhibited a fine ribonucleoprotein (RNP) granular matrix, which evolved during culture into huge structures that we have named 'chaussons' (slippers). This phenomenon involved progressive accumulation of proteins in the RNP matrix without protein neosynthesis. One of these proteins, which translocated into the nucleus during the culture, was identified as a homolog of the human Ro52 E3 ubiquitin ligase. RNA polymerase III was also found to accumulate on the same loops. These results suggest that the subterminal loops of bivalent XII act as a storage site for the components of a nuclear machinery involved in the quality control of RNA synthesis and maturation in response to cellular stress. They also emphasise the considerable value of the lampbrush chromosome system for a direct visualisation of modifications in gene expression and open the question of a nuclear accumulation of Ro52 in human or animal oocytes cultured in vitro for assisted reproductive technologies (ART).

Pearls are novel Cajal body-like structures in the Xenopus germinal vesicle that are dependent on RNA pol III transcription

We have identified novel nuclear bodies, which we call pearls, in the giant oocyte nuclei of Xenopus laevis and Xenopus tropicalis. Pearls are attached to the lampbrush chromosomes at specific loci that are transcribed by RNA polymerase III, and they disappear after inhibition of polymerase III activity. Pearls are enriched for small Cajal body-specific RNAs (scaRNAs), which are guide RNAs that modify specific nucleotides on splicing snRNAs. Surprisingly, snRNAs themselves are not present in pearls, suggesting that pearls are not functionally equivalent to Cajal bodies in other systems, which contain both snRNAs and scaRNAs. We suggest that pearls may function in the processing of RNA polymerase III transcripts, such as tRNA, 5S rRNA, and other short non-coding RNAs.

Association of modified cytosines and the methylated DNA-binding protein MeCP2 with distinctive structural domains of lampbrush chromatin

We have investigated the association of DNA methylation and proteins interpreting methylation state with the distinctive closed and open chromatin structural domains that are directly observable in the lampbrush chromosomes (LBCs) of amphibian oocytes. To establish the distribution in LBCs of MeCP2, one of the key proteins binding 5-methylcytosine-modified DNA (5mC), we expressed HA-tagged MeCP2 constructs in Xenopus laevis oocytes. Full-length MeCP2 was predominantly targeted to the closed, transcriptionally inactive chromomere domains in a pattern proportional to chromomeric DNA density and consistent with a global role in determining chromatin state. A minor fraction of HA-MeCP2 was also found to associate with a distinctive structural domain, namely a short region at the bases of some of the extended lateral loops. Expression in oocytes of deleted constructs and of point mutants derived from Rett syndrome patients demonstrated that the association of MeCP2 with LBCs was determined by its 5mC-binding domain. We also examined more directly the distribution of 5mC by immunostaining Xenopus and axolotl LBCs and confirmed the pattern suggested by MeCP2 targeting of intense staining of the chromomeres and of some loop bases. In addition, we found in the longer loops of axolotl LBCs that short interstitial regions could also be clearly stained for 5mC. These 5mC regions corresponded precisely to unusual segments of active transcription units from which RNA polymerase II (pol II) and nascent transcripts were simultaneously absent. We also examined by immunostaining the distribution in lampbrush chromatin of the oxidized 5mC derivative, 5-hydroxymethylcytosine (5hmC). Although in general, the pattern resembled that obtained for 5mC, one antibody against 5hmC produced intense staining of restricted chromosomal foci. These foci corresponded to a third type of lampbrush chromatin domain, the transcriptionally active but less extended structures formed by clusters of genes transcribed by pol III. This raises the possibility that 5hmC may play a role in establishing the distinctive patterns of gene repression and activation that characterize specific pol III-transcribed gene families in amphibian genomes.

Cajal bodies: where form meets function

The cell nucleus contains dozens of subcompartments that separate biochemical processes into confined spaces. Cajal bodies (CBs) were discovered more than 100 years ago, but only extensive research in the past decades revealed the surprising complexity of molecular and cellular functions taking place in these structures. Many protein and RNA species are modified and assembled within CBs, which have emerged as a meeting place and factory for ribonucleoprotein (RNP) particles involved in splicing, ribosome biogenesis and telomere maintenance. Recently, a distinct structure near histone gene clusters--the Histone locus body (HLB)--was discovered. Involved in histone mRNA 3'-end formation, HLBs can share several components with CBs. Whether the appearance of distinct HLBs is simply a matter of altered affinity between these structures or of an alternate mode of CB assembly is unknown. However, both structures share basic assembly properties, in which transcription plays a decisive role in initiation. After this seeding event, additional components associate in random order. This appears to be a widespread mechanism for body assembly. CB assembly encompasses an additional layer of complexity, whereby a set of pre-existing substructures can be integrated into mature CBs. We propose this as a multi-seeding model of CB assembly.

Novel domains in the hnRNP G/RBMX protein with distinct roles in RNA binding and targeting nascent transcripts

The heterogenous nuclear ribonucleoprotein G (hnRNP G) controls the alternative splicing of several pre-mRNas. While hnRNP G displays an amino terminal RNA recognition motif (RRM), we find that this motif is paradoxically not implicated in the recruitment of hnRNP G to nascent transcripts in amphibian oocytes. In fact, a deletion analysis revealed that targeting of hnRNP G to active transcription units depends on another domain, centrally positioned, and consisting of residues 186-236. We show that this domain acts autonomously and thus is named NTD for nascent transcripts targeting domain. Furthermore, using an RNA probe previously characterized in vitro as an RNA that interacts specifically with hnRNP G, we demonstrate a new auxiliary RNA binding domain (RBD). It corresponds to a short region of 58 residues positioned at the carboxyl terminal end of the protein, which recognizes an RNA motif predicted to adopt an hairpin structure. The fact that the NTD acts independently from both the RRM and the RBD strongly suggests that the initial recruitment of hnRNP G to nascent pre-mRNAs is independent of its sequence-specific RNA binding properties. Together, these findings highlight the modular organization of hnRNP G and offer new insights into its multifunctional roles.

Tips and tricks for preparing lampbrush chromosome spreads from Xenopus tropicalis oocytes

Due to their large size and fine organization, lampbrush chromosomes (LBCs) of amphibian oocytes have been for decades one of the favorite tools of biologists for the analysis of transcriptional and post-transcriptional processes at the cytological level. The emergence of the diploid Xenopus tropicalis amphibian as a model organism for vertebrate developmental genetics and the accumulation of sequence data made available by its recent genomic sequencing, strongly revive the interest of LBCs as a powerful tool to study genes expressed during oogenesis. We describe here a detailed protocol for preparing LBCs from X. tropicalis oocyte and give practical advice to encourage a large number of researchers to become familiar with these chromosomes.

Working map of the lampbrush chromosomes of Xenopus tropicalis: a new tool for cytogenetic analyses

The amphibian Xenopus tropicalis, whose genome has been recently sequenced, has become an important model organism for vertebrate developmental genetics. The development of cytogenetic tools in this new model organism should contribute to an understanding of the organization of the amphibian genome and the mapping of a variety of loci of interest. In this respect, oocyte lampbrush chromosomes are particularly useful for the localization of genomic sequences expressed during oogenesis. We have constructed a working map of X. tropicalis lampbrush chromosomes, which allows the 10 bivalents of the oocyte karyotype to be readily identified by distinctive combinations of specific landmark structures composed of lateral loops, spheres, and granules. We have also established the patterns of RNA Pol III sites over the chromosomes by immunofluorescence using antibodies directed against two Pol III subunits. Specific staining patterns were found for each chromosome, which constitute a supplementary tool for their identification. Developmental Dynamics 238:1492-1501, 2009. (c) 2009 Wiley-Liss, Inc.

Lampbrush chromosomes enable study of cohesin dynamics

The lampbrush chromosomes present in the nuclei of amphibian oocytes offer unique biological approaches for study of the mechanisms that regulate chromatin structure with high spatial resolution. We discuss fundamental aspects of the remarkable organization and plasticity exhibited by lampbrush chromosomes. We then utilize lampbrush chromosomes to characterize the chromosomal distribution and dynamics of cohesin, the four-protein complex (RAD21/MCD1/SCC1, SMC1, SMC3, SCC3/SA2) responsible for sister chromatid cohesion. We find that endogenous SMC3 and newly expressed hRAD21 co-localize on chromosomal axes, sites where sister chromatids are tightly paired. We present evidence suggesting that hRAD21 recruitment to lampbrush chromosomes is modulated by chromosomal SMC1 and SMC3. Notably, using a technique for de novo chromosome assembly, we demonstrate that both SMC3 and hRAD21 are recruited to single, unreplicated lampbrush chromatids. Finally, we used our novel method of analyzing the oocyte nucleus under oil combined with fluorescence recovery after photobleaching, to provide direct evidence that cohesin is highly dynamic at discrete, condensed chromosomal regions. Collectively, these data demonstrate that lampbrush chromosomes provide a unique and powerful tool for combining biochemical and cytological analyses for dissection of complex chromosomal processes.

Localized co-transcriptional recruitment of the multifunctional RNA-binding protein CELF1 by lampbrush chromosome transcription units

The highly-extended transcription units of lampbrush chromosomes (LBCs) offer unique opportunities to study the co-transcriptional events occurring on nascent transcripts. Using LBCs from amphibian oocytes, I investigated whether CELF1, an RNA binding protein involved in the regulation of alternative splicing, mRNA stability and translation, is localized to active transcription units. Antibodies raised against mammalian (CUG-BP1) and amphibian (EDEN-BP) CELF1 were used to immunostain LBC spreads prepared from several species, including Xenopus laevis and the axolotl Ambystoma mexicanum. Up to about 50 separate LBC loci were convincingly immunostained and it was clear that CELF1 was present in the nascent RNPs of lateral loops. Furthermore, myc-tagged CUG-BP1 expressed in microinjected axolotl oocytes was specifically targeted to nascent transcripts of loops that recruit endogenous CELF1. In many active transcription units CELF1 was distinctly localized, being first recruited by nascent transcripts only far downstream of the transcription start site and remaining associated until the end of transcription. Overall it appears possible that the multiple functions of CELF1 in regulating posttranscriptional gene expression could all be predetermined during transcription by virtue of a region-specific binding to the nascent transcripts of target genes.

Splicing-independent recruitment of spliceosomal small nuclear RNPs to nascent RNA polymerase II transcripts

In amphibian oocytes, most lateral loops of the lampbrush chromosomes correspond to active transcriptional sites for RNA polymerase II. We show that newly assembled small nuclear ribonucleoprotein (RNP [snRNP]) particles, which are formed upon cytoplasmic injection of fluorescently labeled spliceosomal small nuclear RNAs (snRNAs), target the nascent transcripts of the chromosomal loops. With this new targeting assay, we demonstrate that nonfunctional forms of U1 and U2 snRNAs still associate with the active transcriptional units. In particular, we find that their association with nascent RNP fibrils is independent of their base pairing with pre–messenger RNAs. Additionally, stem loop I of the U1 snRNA is identified as a discrete domain that is both necessary and sufficient for association with nascent transcripts. Finally, in oocytes deficient in splicing, the recruitment of U1, U4, and U5 snRNPs to transcriptional units is not affected. Collectively, these data indicate that the recruitment of snRNPs to nascent transcripts and the assembly of the spliceosome are uncoupled events.

Cell biology of the Arabidopsis nuclear siRNA pathway for RNA-directed chromatin modification

In Arabidopsis thaliana, the pathway for transcriptional silencing via RNA-directed DNA methylation and chromatin modification involves two forms of nuclear RNA polymerase IV (pol IVa and pol IVb), RNA-DEPENDENT RNA POLYMERASE2 (RDR2), DICER-LIKE3 (DCL3), ARGONAUTE4 (AGO4), the chromatin remodeler, DRD1, and the de novo cytosine methyltransferase, DRM2. New insight into the order of events, as well as the spatial organization of this pathway within the nucleus, has come from the combined use of protein immunolocalization, RNA fluorescence in situ hybridization (RNA-FISH), DNA-FISH, and genetic analysis. New findings show that pol IVa, pol IVb, and DRD1 colocalize with DNA loci that are both the sources and targets of small interfering RNAs (siRNAs). However, RDR2-dependent doublestranded RNA production, dicing by DCL3, and loading of siRNAs into AGO4-containing RNA-induced silencing complexes (RISCs) appear to take place at a distant site, in an siRNA processing center located in the nucleolus. This siRNA processing center shares features of Cajal bodies, which are nucleolus-associated entities involved in the processing and trafficking of RNAs found in ribonucleoprotein (RNP) complexes that splice or modify mRNA, rRNA, or telomeres. Therefore, assembly and trafficking of chromatin-modifying RISCs may share similarities with other nuclear RNPs.

Glucose-responsive regulators of gene expression in Saccharomyces cerevisiae function at the nuclear periphery via a reverse recruitment mechanism

Regulation of gene transcription is a key feature of developmental, homeostatic, and oncogenic processes. The reverse recruitment model of transcriptional control postulates that eukaryotic genes become active by moving to contact transcription factories at nuclear substructures; our previous work showed that at least some of these factories are tethered to nuclear pores. We demonstrate here that the nuclear periphery is the site of key events in the regulation of glucose-repressed genes, which together compose one-sixth of the Saccharomyces cerevisiae genome. We also show that the canonical glucose-repressed gene SUC2 associates tightly with the nuclear periphery when transcriptionally active but is highly mobile when repressed. Strikingly, SUC2 is both derepressed and confined to the nuclear rim in mutant cells where the Mig1 repressor is nuclear but not perinuclear. Upon derepression all three subunits (alpha, beta, and gamma) of the positively acting Snf1 kinase complex localize to the nuclear periphery, resulting in phosphorylation of Mig1 and its export to the cytoplasm. Reverse recruitment therefore appears to explain a fundamental pathway of eukaryotic gene regulation.

Spatial organization of transcription by RNA polymerase III

RNA polymerase III (pol III) transcribes many essential, small, noncoding RNAs, including the 5S rRNAs and tRNAs. While most pol III-transcribed genes are found scattered throughout the linear chromosome maps or in multiple linear clusters, there is increasing evidence that many of these genes prefer to be spatially clustered, often at or near the nucleolus. This association could create an environment that fosters the coregulation of transcription by pol III with transcription of the large ribosomal RNA repeats by RNA polymerase I (pol I) within the nucleolus. Given the high number of pol III-transcribed genes in all eukaryotic genomes, the spatial organization of these genes is likely to affect a large portion of the other genes in a genome. In this Survey and Summary we analyze the reports regarding the spatial organization of pol III genes and address the potential influence of this organization on transcriptional regulation.

Identification of xenopus CENP-A and an associated centromeric DNA repeat

Kinetochores are the proteinaceous complexes that assemble on centromeric DNA and direct eukaryotic chromosome segregation. The mechanisms by which higher eukaryotic cells define centromeres are poorly understood. Possible molecular contributors to centromere specification include the underlying DNA sequences and epigenetic factors such as binding of the centromeric histone centromere protein A (CENP-A). Frog egg extracts are an attractive system for studying centromere definition and kinetochore assembly. To facilitate such studies, we cloned a Xenopus laevis homologue of CENP-A (XCENP-A). We identified centromere-associated DNA sequences by cloning fragments of DNA that copurified with XCENP-A by chromatin immunoprecipitation. XCENP-A associates with frog centromeric repeat 1 (Fcr1), a 174-base pair repeat containing a possible CENP-B box. Southern blots of partially digested genomic DNA revealed large ordered arrays of Fcr1 in the genome. Fluorescent in situ hybridization with Fcr1 probes stained most centromeres in cultured cells. By staining lampbrush chromosomes, we specifically identified the 11 (of 18) chromosomes that stain consistently with Fcr1 probes.

Nuclear bodies and compartments: functional roles and cellular signalling in health and disease

There is much interest in recent years in the possible role of different nuclear compartments and subnuclear domains in the regulation of gene expression, signalling, and cellular functions. The nucleus contains inositol phosphates, actin and actin-binding proteins and myosin isoforms, multiple protein kinases and phosphatases targeting Cdk-1 and Cdk-2, MAPK/SAPK, and Src-related kinases and their substrates, suggesting the implication of several signalling pathways in the intranuclear organization and function of nuclear bodies (NBs). NBs include the well-characterized Cajal bodies (CBs; or coiled bodies), the nucleolus, perinucleolar and perichromatin regions, additional NBs best illustrated by the promyelocytic leukemia nuclear bodies [PML-NBs, also named PML oncogenic dots (PODs), ND10, Kr-bodies] and similar intranuclear foci containing multi-molecular complexes with major role in DNA replication, surveillance, and repair, as well as messenger RNA and ribosomal RNA synthesis and assembly. Chromatin modifying proteins, such as the CBP acetyltransferase and type I histone deacetylase, accumulate at PML-NBs. PML-NBs and Cajal bodies are very dynamic and mobile within the nuclear space and are regulated by cellular stress (heat shock, apoptosis, senescence, heavy metal exposure, viral infection, and DNA damage responses). NBs strongly interact, using signalling mechanisms for the directional and ordered traffic of essential molecular components. NBs organize the delivery and storage of essential RNAs and proteins that play a role in transcription, pre-mRNA biosynthesis and splicing, and the sequestration and/or degradation of regulatory proteins, such as heterogenous nuclear ribonuclear proteins (hnRNPs), p53, Rb1, CBP, STAT3, and others. The objective of this review is to summarize some aspects of these nuclear structures/bodies/domains, including their proposed roles in cellular signalling and in human diseases, mainly neurodegenerative disorders and cancer.

Distribution of XCAP-E and XCAP-D2 in the Xenopus oocyte nucleus

Several antibodies were used to examine the distribution of two condensin members, XCAP-E and XCAP-D2, in the nucleus of Xenopus oocytes. XCAP-D2 was found to be associated with the lampbrush chromosomes. The chromosomal regions containing XCAP-D2 correspond precisely to domains of highly compacted chromatin, suggesting a direct contribution of XCAP-D2 in meiotic chromatin organization. In contrast, XCAP-E was found to be absent from chromosomes but was detected at a high concentration in the granular component of nucleoli. The subnucleolar localization of XCAP-E was further confirmed by double labeling using several nucleolar protein markers. The fate of nucleolar XCAP-E was also followed when changes in the nucleoli morphology were artificially induced. The apparent exclusion of XCAP-E from the ribosomal DNA and its tight association with the granular component in all preparations suggest that it might be sequestrated in nucleoli during early stages of meiosis. Interestingly, both XCAP-D2 and XCAP-E were also detected in Cajal bodies, which are organelles suspected to play a role in the assembly/modification of the RNA transcription and processing machinery. The presence of two condensins in CBs might extend such a role of assembly to chromatin macromolecular components as well.

Identification and characterization of transcription factor IIIA and ribosomal protein L5 from Arabidopsis thaliana

Thus far, no transcription factor IIIA (TFIIIA) from higher plants has been cloned and characterized. We have cloned and characterized TFIIIA and ribosomal protein L5 from Arabidopsis thaliana. Primary sequence comparison revealed a high divergence of AtTFIIIA and a relatively high conservation of AtL5 when compared with other organisms. The AtTFIIIA cDNA encodes a protein with nine Cys(2)-His(2)-type zinc fingers, a 23 amino acid spacer between fingers 1 and 2, a 66 amino acid spacer between fingers 4 and 5, and a 50 amino acid non-finger C-terminal tail. Aside from the amino acids required for proper zinc finger folding, AtTFIIIA is highly divergent from other known TFIIIAs. AtTFIIIA can bind 5S rDNA, as well as 5S rRNA, and efficiently stimulates the transcription of an Arabidopsis 5S rRNA gene in vitro. AtL5 identity was confirmed by demonstrating that this protein binds to 5S rRNA but not to 5S rDNA. Protoplast transient expression assays with green fluorescent protein fusion proteins revealed that AtTFIIIA is absent from the cytoplasm and concentrated at several nuclear foci including the nucleolus. AtL5 protein accumulates in the nucleus, especially in the nucleolus, and is also present in the cytoplasm.

Subnuclear localization and Cajal body targeting of transcription elongation factor TFIIS in amphibian oocytes

We have examined the localization and targeting of the RNA polymerase II (pol II) transcription elongation factor TFIIS in amphibian oocyte nuclei by immunofluorescence. Using a novel antibody against Xenopus TFIIS the major sites of immunostaining were found to be Cajal bodies, nuclear organelles that also contain pol II. Small granular structures attached to lampbrush chromosomes were also specifically stained but the transcriptionally active loops were not. Similar localization patterns were found for the newly synthesized myc-tagged TFIIS produced after injection of synthetic transcripts into the cytoplasm. The basis of the rapid and preferential targeting of TFIIS to Cajal bodies was investigated by examining the effects of deletion and site-specific mutations. Multiple regions of TFIIS contributed to efficient targeting including the domain required for its binding to pol II. The localization of TFIIS in Cajal bodies, and in particular the apparent involvement of pol II binding in achieving it, offer further support for a model in which Cajal bodies function in the preassembly of the transcriptional machinery. Although our findings are therefore consistent with TFIIS playing a role in early events of the transcription cycle, they also suggest that this elongation factor is not generally required during transcription in oocytes.