STUDIES ON NUCLEAR STRUCTURE AND FUNCTION IN TETRAHYMENA PYRIFORMIS: I. RNA Synthesis in Macro- and Micronuclei

Live CLEM Imaging of Tetrahymena to Analyze the Dynamic Behavior of the Nuclear Pore Complex

Tetrahymena is a fascinating organism for studying the nuclear pore complex because it has two structurally and functionally distinct nuclei (macronucleus and micronucleus) within a cell, and there are two compositionally distinct nuclear pore complexes (NPCs) with different functions in each nucleus. Therefore, it is possible to link the function of a specific constituent protein with the nuclear function of the macronucleus and micronucleus. Additionally, these NPCs undergo dynamic changes in their structures and compositions during nuclear differentiation. Live CLEM imaging, a method of correlative light and electron microscopy (CLEM) combined with live cell imaging, is a powerful tool for visualizing these dynamic changes of specific molecules/structures of interest at high resolution. Here, we describe Live CLEM that can be applied to the study of the dynamic behavior of NPCs in Tetrahymena cells undergoing nuclear differentiation.

Transcribed germline-limited coding sequences in Oxytricha trifallax

The germline-soma divide is a fundamental distinction in developmental biology, and different genes are expressed in germline and somatic cells throughout metazoan life cycles. Ciliates, a group of microbial eukaryotes, exhibit germline-somatic nuclear dimorphism within a single cell with two different genomes. The ciliate Oxytricha trifallax undergoes massive RNA-guided DNA elimination and genome rearrangement to produce a new somatic macronucleus (MAC) from a copy of the germline micronucleus (MIC). This process eliminates noncoding DNA sequences that interrupt genes and also deletes hundreds of germline-limited open reading frames (ORFs) that are transcribed during genome rearrangement. Here, we update the set of transcribed germline-limited ORFs (TGLOs) in O. trifallax. We show that TGLOs tend to be expressed during nuclear development and then are absent from the somatic MAC. We also demonstrate that exposure to synthetic RNA can reprogram TGLO retention in the somatic MAC and that TGLO retention leads to transcription outside the normal developmental program. These data suggest that TGLOs represent a group of developmentally regulated protein-coding sequences whose gene expression is terminated by DNA elimination.

Roles of Noncoding RNAs in Ciliate Genome Architecture

Ciliates are an interesting model system for investigating diverse functions of noncoding RNAs, especially in genome defence pathways. During sexual development, the ciliate somatic genome undergoes massive rearrangement and reduction through removal of transposable elements and other repetitive DNA. This is guided by a multitude of noncoding RNAs of different sizes and functions, the extent of which is only recently becoming clear. The genome rearrangement pathways evolved as a defence against parasitic DNA, but interestingly also use the transposable elements and transposases to execute their own removal. Thus, ciliates are also a good model for the coevolution of host and transposable element, and the mutual dependence between the two. In this review, we summarise the genome rearrangement pathways in three diverse species of ciliate, with focus on recent discoveries and the roles of noncoding RNAs.

•

Ciliate genomes undergo massive rearrangement and reduction during development.

•

Transposon elimination is guided by small RNAs and carried out by transposases.

•

New pathways for noncoding RNA production have recently been discovered in ciliates.

•

Diverse ciliate species have different mechanisms for RNA-guided genome remodeling.

Biased assembly of the nuclear pore complex is required for somatic and germline nuclear differentiation in Tetrahymena

Ciliates have two functionally distinct nuclei, a somatic macronucleus (MAC) and a germline micronucleus (MIC) that develop from daughter nuclei of the last postzygotic division (PZD) during the sexual process of conjugation. Understanding this nuclear dimorphism is a central issue in ciliate biology. We show, by live-cell imaging of Tetrahymena, that biased assembly of the nuclear pore complex (NPC) occurs immediately after the last PZD, which generates anterior-posterior polarized nuclei: MAC-specific NPCs assemble in anterior presumptive MACs but not in posterior presumptive MICs. MAC-specific NPC assembly in the anterior nuclei occurs much earlier than transport of Twi1p, which is required for MAC genome rearrangement. Correlative light-electron microscopy shows that addition of new nuclear envelope (NE) precursors occurs through the formation of domains of redundant NE, where the outer double membrane contains the newly assembled NPCs. Nocodazole inhibition of the second PZD results in assembly of MAC-specific NPCs in the division-failed zygotic nuclei, leading to failure of MIC differentiation. Our findings demonstrate that NPC type switching has a crucial role in the establishment of nuclear differentiation in ciliates.

Colchicine-induced degeneration of the micronucleus during conjugation in Tetrahymena

One of the most dramatic examples of nuclear morphogenesis occurs during conjugation in Tetrahymena when the micronucleus elongates to a size longer than the cell itself. After contraction to a spherical shape, the nucleus moves directly to chromosome separation in the first meiotic division. Here we investigate the consequences of interrupting the elongation process. Colchicine, a microtubule inhibitor, caused retraction of elongated structures. With time, cells began to lose their micronuclei, and by five hours more than half of the paired cells had at least one cell missing a micronucleus. After reversing the colchicine block, existing micronuclei did not undergo elongation again, nor did meiosis occur. These observations indicate that micronuclear elongation is critical to subsequent meiotic division. Further, nuclear elimination occurs, which could be due to meiotic failure or possibly a problem downstream from meiosis. An analysis of the process of colchicine-induced micronuclear degeneration indicated that it was regulated by a caspase-dependent mechanism, characteristic of apoptosis, and then resorbed by a lysosome-dependent autophagic mechanism. Amicronucleate cells failed to grow when returned to nutrient medium, likely because of a lesion in the post-conjugation reconstruction of a functioning oral apparatus. The ease by which a large number of nuclei are induced to "self-destruct" may make this system useful in investigating the link between colchicine treatment and nuclear death in Tetrahymena, and in investigating how nuclear death could be regulated in living cells more generally. Finally, we note that this phenomenon might relate to the evolution of amicronucleate species of Tetrahymena.

Tetrahymena in the laboratory: strain resources, methods for culture, maintenance, and storage

The ciliated protozoan Tetrahymena thermophila has been an important model system for biological research for many years. During that time, a variety of useful strains, including highly inbred stocks, a collection of diverse mutant strains, and wild cultivars from a variety of geographical locations have been identified. In addition, thanks to the efforts of many different laboratories, optimal conditions for growth, maintenance, and storage of Tetrahymena have been worked out. To facilitate the efficient use of Tetrahymena, especially by those new to the system, this chapter presents a brief description of many available Tetrahymena strains and lists possible resources for obtaining viable cultures of T. thermophila and other Tetrahymena species. Descriptions of commonly used media, methods for cell culture and maintenance, and protocols for short- and long-term storage are also presented.

Nuclear architecture in developmental biology and cell specialisation

Epigenetic changes, including DNA methylation patterns, histone modifications and histone variants, as well as chromatin remodelling play a fundamental role in the regulation of pre‐ and postimplantation mammalian development. Recent studies have indicated that nuclear architecture provides an additional level of regulation, which needs to be explored in order to understand how a fertilised egg is able to develop into a full organism. Studies of 3D preserved nuclei of IVF preimplantation embryos from different mammalian species, such as mouse, rabbit and cow, have demonstrated that nuclear architecture undergoes major changes during early development. Both similarities and species‐specific differences were observed. Nuclear transfer experiments demonstrated changes of nuclear phenotypes, which to some extent reflect changes seen in IVF preimplantation embryos albeit with a different timing compared with IVF embryos. The dynamics of nuclear architecture is further substantiated by major changes during postmitotic terminal cell differentiation. Recent breakthroughs of 3D fluorescence microscopy with resolution beyond the conventional Abbe limit in combination with 3D electron microscopy provide the potential to explore the topography of nuclear structure with unprecedented resolution and detail.

Refined annotation and assembly of the Tetrahymena thermophila genome sequence through EST analysis, comparative genomic hybridization, and targeted gap closure

BACKGROUND

Tetrahymena thermophila, a widely studied model for cellular and molecular biology, is a binucleated single-celled organism with a germline micronucleus (MIC) and somatic macronucleus (MAC). The recent draft MAC genome assembly revealed low sequence repetitiveness, a result of the epigenetic removal of invasive DNA elements found only in the MIC genome. Such low repetitiveness makes complete closure of the MAC genome a feasible goal, which to achieve would require standard closure methods as well as removal of minor MIC contamination of the MAC genome assembly. Highly accurate preliminary annotation of Tetrahymena's coding potential was hindered by the lack of both comparative genomic sequence information from close relatives and significant amounts of cDNA evidence, thus limiting the value of the genomic information and also leaving unanswered certain questions, such as the frequency of alternative splicing.

RESULTS

We addressed the problem of MIC contamination using comparative genomic hybridization with purified MIC and MAC DNA probes against a whole genome oligonucleotide microarray, allowing the identification of 763 genome scaffolds likely to contain MIC-limited DNA sequences. We also employed standard genome closure methods to essentially finish over 60% of the MAC genome. For the improvement of annotation, we have sequenced and analyzed over 60,000 verified EST reads from a variety of cellular growth and development conditions. Using this EST evidence, a combination of automated and manual reannotation efforts led to updates that affect 16% of the current protein-coding gene models. By comparing EST abundance, many genes showing apparent differential expression between these conditions were identified. Rare instances of alternative splicing and uses of the non-standard amino acid selenocysteine were also identified.

CONCLUSION

We report here significant progress in genome closure and reannotation of Tetrahymena thermophila. Our experience to date suggests that complete closure of the MAC genome is attainable. Using the new EST evidence, automated and manual curation has resulted in substantial improvements to the over 24,000 gene models, which will be valuable to researchers studying this model organism as well as for comparative genomics purposes.

Lia1p, a novel protein required during nuclear differentiation for genome-wide DNA rearrangements in Tetrahymena thermophila

Extensive genome-wide rearrangements occur during somatic macronuclear development in Tetrahymena thermophila. These events are guided by RNA interference-directed chromatin modification including histone H3 lysine 9 methylation, which marks specific germ line-limited internal eliminated sequences (IESs) for excision. Several genes putatively involved in these developmental genome rearrangements were identified based on their proteins' localization to differentiating somatic nuclei, and here we demonstrate that one, LIA1, encodes a novel protein that is an essential component of the genome rearrangement machinery. A green fluorescent protein-Lia1 fusion protein exhibited dynamic nuclear localization during development that has striking similarity to that of the dual chromodomain-containing DNA rearrangement protein, Pdd1p. Coimmunoprecipitation experiments showed that Lia1p associates with Pdd1p and IES chromatin during macronuclear development. Cell lines in which we disrupted both the germ line and somatic copies of LIA1 (DeltaLIA1) grew normally but were unable to generate viable progeny, arresting late in development just prior to returning to vegetative growth. These mutant lines failed to properly form Pdd1p-containing nuclear structures and eliminate IESs despite showing normal levels of H3K9 methylation. These data indicate that Lia1p is required late in conjugation for the reorganization of the Tetrahymena genome.

RNA polymerase II localizes in Tetrahymena thermophila meiotic micronuclei when micronuclear transcription associated with genome rearrangement occurs

The germ line micronucleus in Tetrahymena thermophila is transcriptionally silent in vegetatively growing cells. However, micronuclear transcription has been observed in the early ("crescent") stages of the sexual process, conjugation. This transcription is proposed to play a central role in identifying sites for subsequent genome rearrangements that accompany development of the somatic macronucleus from the micronucleus. RPB3 (cnjC), a gene encoding a protein homologous to the third largest subunit of RNA polymerase II (RNAP II), was previously reported to be expressed specifically during conjugation, suggesting a role in micronucleus-specific transcription. Rpb3p localized in the micronucleus only during the meiotic prophase, when micronuclear transcription occurs, and its intranuclear distribution is strikingly similar to that for previously described sites of micronuclear RNA synthesis. By contrast, Rpc5p, the homologous subunit shared by RNAPs I and III, was not detectable in the micronucleus at any stage of the life cycle. However, Rpb3p is not specific to the transcribing micronucleus. Like Rpc5p, it also localizes to macronuclei in all stages of the life cycle. Rpb3p is encoded by a unique, essential gene in Tetrahymena. Thus, RNAP II is associated with both somatic transcription and crescent transcription and probably has an important role in genome rearrangement.

A Dicer-like protein in Tetrahymena has distinct functions in genome rearrangement, chromosome segregation, and meiotic prophase

Previous studies indicated that genome rearrangement involving DNA sequence elimination that occurs at late stages of conjugation in Tetrahymena is epigenetically controlled by siRNA-like scan (scn) RNAs produced from nongenic, heterogeneous, bidirectional, micronuclear transcripts synthesized at early stages of conjugation. Here, we show that Dcl1p, one of three Tetrahymena Dicer-like enzymes, is required for processing the micronuclear transcripts to scnRNAs. DCL1 is also required for methylation of histone H3 at Lys 9, which, in wild-type cells, specifically occurs on the sequences (IESs) being eliminated. These results argue that Dcl1p processes nongenic micronuclear transcripts to scnRNAs and is required for IES elimination. This is the first evidence linking nongenic micronuclear transcripts, scnRNAs, and genome rearrangement. Dcl1p also is required for proper mitotic and meiotic segregation of micronuclear chromosomes and for normal chromosome alignment in meiotic prophase, suggesting that DCL1 has multiple functions in regulating chromosome dynamics.

Induction of anti-actin drug resistance in Tetrahymena

Both cytochalasin D and latrunculin B reversibly inhibited Tetrahymena phagocytosis at concentrations similar to those effective in mammalian systems, even though ciliate actins are known to be highly divergent from mammalian actins. Overnight exposure to relatively low (0.25 microM) concentrations of latrunculin B induced resistance in Tetrahymena to the inhibitory effects of that drug, as well as cross-resistance to cytochalasin D. However, much higher (> 30 microM) concentrations of cytochalasin D were required for induction of cross-resistance to latrunculin B. Anti-actin drug resistance in Tetrahymena may involve a general multidrug resistance mechanism and/or specific feedback regulation of F-actin assembly and stability.

A novel family of mobile genetic elements is limited to the germline genome in Tetrahymena thermophila

In the ciliated protozoan Tetrahymena thermophila, extensive DNA elimination is associated with differentiation of the somatic macronucleus from the germline micronucleus. This study describes the isolation and complete characterization of Tlr elements, a family of approximately 30 micronuclear DNA sequences that are efficiently eliminated from the developing macronucleus. The data indicate that Tlr elements are comprised of an approximately 22 kb internal region flanked by complex and variable termini. The Tlr internal region is highly conserved among family members and contains 15 open reading frames, some of which resemble genes encoded by transposons and viruses. The Tlr termini appear to be long inverted repeats consisting of (i) a variable region containing multiple direct repeats which differ in number and sequence from element to element and (ii) a conserved terminal 47 bp sequence. Taken together, these results suggest that Tlr elements comprise a novel family of mobile genetic elements that are confined to the Tetrahymena germline genome. Possible mechanisms of developmentally programmed Tlr elimination are discussed.

DNA toxicity of cocaine hydrochloride and cocaine freebase by means of DNA image analysis on Tetrahymena pyriformis

An assay for computerized scoring of the DNA content of the protozoan Tetrahymena pyriformis has been used for the detection of toxic responses to cocaine, since DNA is responsible for the replication of the genetic material and also reflects closely the number of chromosomes in the nucleus. Thus, doubling of the number of chromosomes in a cell will also correspond to doubling of the DNA content and of the nuclear volume. Two chemical forms of cocaine were used, cocaine hydrochloride and cocaine freebase (crack), at two doses of 1 and 2 mg per 100 ml of protozoan culture, respectively. Image analysis of the protozoan nucleus patterns revealed a rapid stimulating effect on the DNA content for both cocaine hydrochloride and freebase after 1 h of incubation. However, after 2 h of treatment a reduction, although not statistically significant, of the DNA content of the protozoan was observed. These observations were further correlated with the phagocytic activity of the protozoan cultures. This paper provides some possible explanations of the toxic effects of cocaine on this particular cell model.

Nongenic, bidirectional transcription precedes and may promote developmental DNA deletion in Tetrahymena thermophila

A large number of DNA segments are excised from the chromosomes of the somatic nucleus during development of Tetrahymena thermophila. How these germline-limited sequences are recognized and excised is still poorly understood. We have found that many of these noncoding DNAs are transcribed during nuclear development. Transcription of the germline-limited M element occurs from both DNA strands and results in heterogeneous transcripts of < 200 b to > 1 kb. Transcripts are most abundant when developing micro- and macronuclei begin their differentiation. Transcription is normally restricted to unrearranged DNA of micronuclei and/or developing nuclei, but germline-limited DNAs can induce their own transcription when placed into somatic macronuclei. Brief actinomycin D treatment of conjugating cells blocked M-element excision, providing evidence that transcription is important for efficient DNA rearrangement. We propose that transcription targets these germline-limited sequences for elimination by altering chromatin to ensure their accessibility to the excision machinery.

Affinity-purification of concanavalin A-binding ciliary glycoconjugates of starved and feeding Tetrahymena thermophila

Development of mating competency in Tetrahymena thermophila requires starvation for at least 70 min in low ionic strength buffer. Pair formation between conjugating cells is blocked at early stages by the lectin Concanavalin A (Con A). To investigate the role of Con A-binding proteins in this induced cellular change and pairing, and to confirm and extend an earlier study from our laboratory, a method was developed for preparation of Con A-binding proteins from ciliary membrane-rich fractions of T. thermophila. Con A-binding ciliary proteins were prepared from non-starved and starved cells from two wild type strains and a mating mutant, RH179E1. Comparison of these proteins by SDS-PAGE revealed on overall reduction in number of wild-type bands after starvation. In particular, a major band at 28 kDa was present in non-starved cells and absent in starved cells. However, in the mating mutant, no change in banding profile was seen after starvation: the 28 kDa band was present in both non-starved and starved cells. This, Con A-binding ciliary membrane proteins undergo a major change during starvation-induced development of mating competency in wild-type T. thermophila. In contrast, the mutant differed from wild-type in overall composition of its ciliary Con A-binding glycoproteins and in the response of these proteins to starvation, suggesting that it may be deficient in its ability to be initiated by starvation. Our results are consistent with the hypothesis that a change affecting ciliary membrane Con A-binding proteins is essential for the cellular response to mating signals.

A genetic screen for vegetative gene expression in the micronucleus of Tetrahymena thermophila

The presence of a micronucleus with at least a small portion of the micronuclear genome appears to be indispensable for vegetative viability in the ciliate Tetrahymena thermophila. A genetic screen was devised to detect evidence of expression of essential genes in the vegetative micronucleus by identification of thermosensitive-lethal mutations expressed in the absence of nuclear reorganization. Although control experiments demonstrated the efficacy of the method for induction and recovery of thermosensitive lethal mutations in micronuclear genes, no expressed mutations were recovered in the absence of nuclear reorganization. This finding complements the existing lack of convincing biochemical evidence for gene expression in the vegetative micronucleus and suggests that the essential function may involve genomic DNA sequences for which thermosensitive mutant alleles are not recoverable, or perhaps a non-genomic component of the organelle.

Ciliary polypeptides and glycoconjugates of wild-type and mutant Tetrahymena thermophila: starved versus nonstarved

To investigate the role of cilia in mating interactions of Tetrahymena thermophila, ciliary membrane-rich fractions were isolated from two wild-type strains, a non-discharge mucocyst mutant which possesses mating behavior similar to wild-type, and a mating mutant which is able to costimulate cells of complementary mating type but cannot enter into pair formation. In each case, proteins from the ciliary membrane-rich fractions of starved, mating-competent ("initiated") cells were compared with those from non-starved, mating-incompetent ("non-initiated") cells, by gel electrophoresis and lectin blotting. In stained gels, a 43 kDa polypeptide was reduced or absent in initiated cells but present in non-initiated cells, in all strains. In silver-stained gels, a 25 kDa polypeptide was present in all strains, both initiated and non-initiated. In blots probed with Con A-peroxidase, a 25 kDa glycoprotein was present in ciliary membrane fractions from non-initiated cells and absent in membranes of initiated cells of the two wild-type strains and the mucocyst mutant, but is present in initiated and non-initiated cells of the mating mutant (several hypotheses are presented to explain these findings). In addition, ciliary proteins of the mating mutant included at least two unique Con A-binding polypeptides. Our results support the idea that development of mating competence during starvation involves an extensive remodeling of ciliary membranes, and identify a 25 kDa glycoconjugate as having a potential role in control of pair formation during mating.

Cloning of Tetrahymena genomic sequences whose message abundance is increased during conjugation

A molecular and biochemical inquiry into protein regulation during Tetrahymena thermophila conjugation was carried out in two ways: a two-dimensional gel analysis of newly translated proteins and the molecular cloning of genes whose message abundance is increased. The two-dimensional gel analysis indicated that the synthesis of 32 predominantly basic proteins was stimulated in conjugating cells. The induction of these proteins could not be correlated with length of starvation or with mating type. The transcription pattern and molecular organization of three clones of T. thermophila genomic DNA, selected on the basis of differential hybridization to conjugating or control cell RNA, were investigated. Two of the clones, which were homologous to transcripts detected in conjugating cells, showed no rearrangements between micro- and macronuclear DNA. A third clone was divided into three segments. One segment was homologous to sequences limited to the micronucleus. A second segment hybridized to a large number of restriction fragments of micronuclear DNA digested with HindIII but to only two fragments of macronuclear DNA. A third segment, which was complementary to one transcript in conjugating cells and to two different transcripts in control cells, hybridized to two fragments in micronuclear DNA and one fragment in macronuclear DNA.

A germ line specific DNA sequence is transcribed in Tetrahymena

In the ciliated protozoan Tetrahymena 10-20% of the DNA sequences are micronucleus (germ line) specific. Six members of a family of repeated mic-specific DNA sequences are homologous to a 1.5-kb poly(A)+ RNA. The transcript is present in mature cells starved in 10 or 60 mM Tris, in starved immature cells, and in stationary cells. RNA from log-phase and heat-shocked cells does not have detectable levels of the transcript. These data indicate that at least one germ line limited DNA sequence is transcribed in the micronucleus of Tetrahymena.

Micronucleus-specific DNA sequences in an amicronucleate mutant of Tetrahymena

DNA from the amicronucleate Tetrahymena cell line BI3840 was probed for DNA sequences which are limited to the micronucleus in wild-type cells. Four micronucleus-specific DNA sequences were not detectable in DNA from the amicronucleate cell line. Two of the six micronucleus-specific DNA sequences tested hybridized to DNA from amicronucleate cells. Both the number of fragments homologous to these sequences and the intensity of hybridization were reduced in the DNA from the amicronucleate cells relative to DNA from a wild-type cell line, indicating that less than one micronucleus equivalent of the micronucleus-specific DNA sequences was retained in the amicronucleate cell line. Thus many micronucleus-specific DNA sequences were eliminated from the developing macronucleus of BI3840 as they are in wild-type cells, but in at least two cases the elimination was incomplete. In situ hybridization suggested that the DNA sequences which are limited to the micronucleus in wild-type cells are present in the macronucleus of the amicronucleate cell line. Southern blots of DNA from the amicronucleate cell line were also probed with DNA sequences which are retained in the macronucleus. At least two types of genome rearrangements occurred in the BI3840 macronucleus as they do in wild-type cells. No spurious rearrangements were observed.

Developmental rearrangements associated with a single type of expressed alpha-tubulin gene in Tetrahymena

Southern blotting analyses with diverse heterologous probes indicate that there is only a single type of alpha-tubulin gene in the ciliated protozoan Tetrahymena thermophila. Comparisons of the germ-line configuration of alpha-tubulin sequences in the transcriptionally inert micronucleus with the somatic configuration in the transcriptionally active macronucleus indicate that rearrangements involving breakage and rejoining of sequences flanking both sides of the alpha-tubulin gene accompany macronuclear differentiation.

An amicronucleate mutant of Tetrahymena thermophila

A stable amicronucleate strain of Tetrahymena thermophila was isolated following nitrosoguanidine mutagenesis. The mutant has the same growth rate and viability as the micronucleate parent strain, and has no micronucleus detectable by chromatin-specific staining in vegetative growth or during conjugation. The mutant pairs with normal efficiency with cells of complementary mating type. Matings of the mutant with aneuploid strains which lose their micronucleus during meiosis produced cell pairs yielding one viable and one inviable cell. The mutant receives a micronucleus from a normal mating partner, but this micronucleus is lost by the mutant cells within two hundred generations.

Poly(A)+ RNA from Tetrahymena: stimulation of protein synthesis in vitro

Cell-free synthesis of high molecular weight polypeptides, programmed by RNA from Tetrahymena pyriformis strain W is reported, and methods for preparation of the RNA are described. The RNA was extracted by the SDS-phenol-chloroform-isoamyl alcohol technic. The bulk of extracted RNA was ribosomal and on sucrose gradients peaked at approximately 17S and 25S. After heat denaturation all the 25S RNA was converted to 17S, indicating the presence of hidden breaks, possibly the result of nuclease activity during extraction. Nevertheless, when poly(A) +/- RNA was collected using oligo-(dT)-cellulose column chromatography, it promoted a 15-fold increase in incorporation of [35S] methionine into TCA-precipitable material. Slab-gel electrophoresis and autoradiography of the product revealed 12 different major polypeptides, varying in weight from 28,000 to 65,000 Daltons. A method for preparation of translatable RNA from Tetrahymena will make possible the comparison of messenger RNAs associated with specific cell structures and with different developmental events.