Connection between stop codon reassignment and frequent use of shifty stop frameshifting

Ciliated protozoa of the genus Euplotes have undergone genetic code reassignment, redefining the termination codon UGA to encode cysteine. In addition, Euplotes spp. genes very frequently employ shifty stop frameshifting. Both of these phenomena involve noncanonical events at a termination codon, suggesting they might have a common cause. We recently demonstrated that Euplotes octocarinatus peptide release factor eRF1 ignores UGA termination codons while continuing to recognize UAA and UAG. Here we show that both the Tetrahymena thermophila and E. octocarinatus eRF1 factors allow efficient frameshifting at all three termination codons, suggesting that UGA redefinition also impaired UAA/UAG recognition. Mutations of the Euplotes factor restoring a phylogenetically conserved motif in eRF1 (TASNIKS) reduced programmed frameshifting at all three termination codons. Mutation of another conserved residue, Cys124, strongly reduces frameshifting at UGA while actually increasing frameshifting at UAA/UAG. We will discuss these results in light of recent biochemical characterization of these mutations.

[Cloning and characterization of a novel trinucleotide repeat-containing gene GARP from Euplotes octocarinatus]

The expansion of trinucleotide repeats in genome is related to the phthogenesis of several neurodegenerative diseases. A GARP (glutamic acid-rich protein) gene was isolated from the macronuclear plasmid mini library of Euplotes octocarinatus. A micronuclear version of the GARP gene was amplified by polymerase chain reaction. The macronuclear DNA molecule carrying the GARP gene is 460 bp long and shows the characteristics of macronuclear chromosomes of hypotrichous ciliates. One of the three cysteines is encoded by the opal codon TGA(88-90). The predicted open reading frame encodes a 112-amino acid polypeptide, with a predicted molecular mass of 13 kDa and an isoelectric point of 3.82. Micronuclear version of the GARP gene contains two internal eliminated sequences (IES), IES1 and IES2. IES1 is 41 bp long and is flanked by 5'-GA-3' direct repeats. IES2 is 41 bp long and flanked by 5'-TA-3' direct repeats. Transcriptional activity of GARP gene was confirmed by reverse transcription polymerase chain reaction (RT-PCR).

[EoRab43 regulating vesicular transport around the macronucleus in Euplotes octocarinatus]

Rab family proteins play a crucial role in regulating vesicular traffic in eukaryotic cells. EoRab43 is an atypical Rab gene identified in Euplotes octocarinatus. In order to understand the function of EoRab43, the 153bp fragment of the 3'-end of EoRab43 gene was subcloned into expression vector pGEX-6P-1, and the recombinant plasmid pGEX-EoRab43(153bp) was transfered into E.coli BL21 (DE3) to express the fusion protein. The fusion protein GST-EoRab43C was expressed and purified by affinity chromatography. BALB/c mice were immunolized by purified GST-EoRab43C. The titer of anti-EoRab43C polyclonal antibody was detected by indirect ELISA assay and the specificity of the antibody was detected by Western blot. Immunofluorescence experiments were performed using anti-EoRab43C antibody in the cells of Euplotes. The results showed that EoRab43 displayed a punctuate pattern in the cytoplasm around the macronuclear chromosome of Euplotes.

Genetic code supports targeted insertion of two amino acids by one codon

Strict one-to-one correspondence between codons and amino acids is thought to be an essential feature of the genetic code. However, we report that one codon can code for two different amino acids with the choice of the inserted amino acid determined by a specific 3' untranslated region structure and location of the dual-function codon within the messenger RNA (mRNA). We found that the codon UGA specifies insertion of selenocysteine and cysteine in the ciliate Euplotes crassus, that the dual use of this codon can occur even within the same gene, and that the structural arrangements of Euplotes mRNA preserve location-dependent dual function of UGA when expressed in mammalian cells. Thus, the genetic code supports the use of one codon to code for multiple amino acids.

[How translation termination factor eRF1 Euplotes does not recognise UGA stop codon]

In universal-code eukaryotes, a single class-1 translation termination factor eRF1 decodes all three stop codons, UAA, UAG, and UGA. In some ciliates with variant genetic codes one or two stop codons are used to encode amino acid(s) and are not recognized by eRF1. In Stylonychia, UAG and UAA codons are reassigned as glutamine codons, and in Euplotes, UGA is reassigned as cysteine codon. In omnipotent eRF1s, stop codon recognition is associated with the N-terminal domain of eRF1. Because variant-code ciliates most likely evolved from universal code ancestor(s), structural features should exist in ciliate eRF1s that restrict their stop codon recognition. To find out amino acid residues which confer UAR-only specificity to Euplotes aediculatus eRF1, eRFI chimeras were constructed by swapping eRF1 E. aediculatus N-terminal domain sequences with the matching ones from the human protein. In these chimeras the MC-domain was from human eRF1. Functional analysis of these chimeric eRFI highlighted the crucial role of the two regions (positions 38-50 and 123-145) in the N-terminal domain of E. aediculatus eRF1 that restrict E. aediculatus eRF1 specificity toward UAR codons. Possibly, restriction of eRF1 specificity to UAR codons might have been an early event occurring in independent instances in ciliate evolutionary history, possibly facilitating the reassignment of UGA to sense codons.

Morphology, Ultrastructure, Molecular Phylogeny, and Autecology of Euplotes elegans Kahl, 1932 (Hypotrichida; Euplotidae) Isolated from the Anoxic Mariager Fjord, Denmark

The morphology, autecology, and molecular phylogeny of an euryhaline Euplotes isolate collected from the anoxic water column of the Mariager Fjord in Denmark were investigated. The isolate matches the original description of Euplotes elegans Kahl, 1932 very well. However, its dorsal silverline system is clearly distinct from the redescription of this species by Tuffrau. Thus, a neotypification is proposed for E. elegans Kahl, 1932. The oval-shaped cell has a mean size of 107 x 51 microm and is characterized by 9.4 dorsolateral kineties, seven prominent dorsal ridges, large elongated ampullae, which encircle the dorsal kinetids, 18 kinetids in the middorsal row, nine frontoventral cirri, five transversal cirri, and three caudal cirri (two right caudal cirri and one left marginal cirrus). The dorsal silverline system is of the double type with the narrow polygons located on the right side of the dorsal kinetids. The ecological tolerances of this species to pH, salinity, temperature, and oxygen match the ambient environmental conditions of the sampling site. Molecular phylogeny was studied using small subunit rRNA (SSU rRNA) gene sequences. The molecular data cluster E. elegans with Euplotes raikovi, a member of the Euplotopsis group. The data suggest that the E. elegans-E. raikovi clade represents an isolated and deep branch at the base of the Euplotes tree.

Identification of translational release factor eRF1a binding sites on eRF3 in Euplotes octocarinatus

Translation termination in eukaryotes is mediated by two polypeptide chain-release factors, eRF1 and eRF3. eRF1 recognizes stop signals, while eRF3 is a ribosome-dependent and eRF1-dependent GTPase. eRF1 forms a stable complex with eRF3 in vivo and in vitro. In the present study, a variety of truncated forms of Euplotes octocarinatus eRF3 were created, and systematic analysis of the interaction between E. octocarinatus eRF1a and these eRF3 mutants was performed by employing both in vivo a yeast two-hybrid assay and in vitro a pull-down assay. The results demonstrated that a short portion of the C-terminal domain of eRF3 is sufficient for eRF1a binding in E. octocarinatus. Specifically, the eRF1a-binding sites on eRF3 are located at a region containing amino acid residues 640-723 in E. octocarinatus eRF3. Amino acid sequence analysis of eRF3 from E. octocarinatus, humans and yeast showed that the eRF1a binding domain on E. octocarinatus eRF3 was similar to that of yeast eRF3 but different from that of human eRF3.

[Cloning and sequence analysis of a novel member of the rab gene family from Euplotes octocarinatus]

Rab proteins belong to a subfamily of small GTP-binding proteins of the Ras superfamily, which play an important role in intracellular vesicular traffic. In this study, a rab gene was obtained from Euplotes octocarinatus by polymerase chain reaction (PCR) and RT-PCR. The rab gene from macronucleic DNA was 884 bp in length, including non-coding regions and telomeric sequences at both ends. The rab gene from micronuclear DNA (723 bp), lacking of internal eliminated sequences, was identical to rab gene from macronuclear DNA. RT-PCR showed that the opening reading frame of the rab gene was 663 bp long. The rab gene from macronuclear DNA contained an intron of 60 bp at the position from 153 bp to 212 bp of macronuclear DNA. The rab gene had two in-frame TGAs encoding for cysteine in Euplotes octocarinatus. The rab gene used TAG as stop codon, which was the first report in Euplotes octocarinatus. The result of BLAST in NCBI demonstrates that the Rab shares a homology of 49-52% at the amino acid level with Rab1 proteins from a number of other eukaryote, which suggesting that the Rab is a Rab1 homolog. The rab gene was therefore designated Eo-rab-1N (GenBank accession number: DQ105562). The evolution of Eo-rab-1N was analyzed using phylogenetic tree of amino acids sequences of Rab1 obtained from GenBank.

Sequencing of random Euplotes crassus macronuclear genes supports a high frequency of +1 translational frameshifting

Programmed translational frameshifts have been identified in genes from a broad range of organisms, but typically only a very few genes in a given organism require a frameshift for expression. In contrast, a recent analysis of gene sequences available in GenBank from ciliates in the genus Euplotes indicated that >5% required one or more +1 translational frameshifts to produce their predicted protein products. However, this sample of genes was nonrandom, biased, and derived from multiple Euplotes species. To test whether there truly is an abundance of frameshift genes in Euplotes, and to more accurately assess their frequency, we sequenced a random sample of 25 cloned genes/macronuclear DNA molecules from Euplotes crassus. Three new candidate +1 frameshift genes were identified in the sample that encode a membrane occupation and recognition nexus (MORN) repeat protein, a C(2)H(2)-type zinc finger protein, and a Ser/Thr protein kinase. Reverse transcription-PCR analyses indicate that all three genes are expressed in vegetatively proliferating cells and that the mRNAs retain the requirement of a frameshift. Although the sample of sequenced genes is relatively small, the results indicate that the frequency of genes requiring frameshifts in E. crassus is between 3.7% and 31.7% (at a 95% confidence interval). The current and past data also indicate that frameshift sites are found predominantly in genes that likely encode nonabundant proteins in the cell.

Distinct paths to stop codon reassignment by the variant-code organisms Tetrahymena and Euplotes

The reassignment of stop codons is common among many ciliate species. For example, Tetrahymena species recognize only UGA as a stop codon, while Euplotes species recognize only UAA and UAG as stop codons. Recent studies have shown that domain 1 of the translation termination factor eRF1 mediates stop codon recognition. While it is commonly assumed that changes in domain 1 of ciliate eRF1s are responsible for altered stop codon recognition, this has never been demonstrated in vivo. To carry out such an analysis, we made hybrid proteins that contained eRF1 domain 1 from either Tetrahymena thermophila or Euplotes octocarinatus fused to eRF1 domains 2 and 3 from Saccharomyces cerevisiae. We found that the Tetrahymena hybrid eRF1 efficiently terminated at all three stop codons when expressed in yeast cells, indicating that domain 1 is not the sole determinant of stop codon recognition in Tetrahymena species. In contrast, the Euplotes hybrid facilitated efficient translation termination at UAA and UAG codons but not at the UGA codon. Together, these results indicate that while domain 1 facilitates stop codon recognition, other factors can influence this process. Our findings also indicate that these two ciliate species used distinct approaches to diverge from the universal genetic code.

Cloning and sequence analysis of the micronuclear and macronuclear gene encoding Rab protein of Euplotes octocarinatus

The DNA in a micronucleus undergoes remarkable rearrangements when it develops into a macronucleus after cell mating in the hypotrichous ciliate. A Rab gene was isolated from the macronuclear plasmid mini-library of Euplotes octocarinatus. A micronuclear version of the Rab gene was amplified by polymerase chain reaction (PCR). The macronuclear DNA molecule carrying the Rab gene is 767 bp long and shows characteristics typical of macronuclear chromosomes of hypotrichous ciliates. Three of the five cysteines are encoded by the opal codon UGA. The deduced protein is a 207-amino acid (aa) with a molecular mass of 23 kDa. The protein shares 36% identity with Rab 1 protein of Plasmodium and yeast. Analysis of the sequences indicated that the micronuclear version of the Rab gene contains two internal eliminated sequences, internal eliminated sequence (IES)1 and IES2. IES1 is flanked by a pair of hepta-nucleotide 5'-AAATTTT-3' direct repeats, and IES2 is flanked by 5'-TA-3' direct repeats.

Cloning and analysis of 16 Rab genes from macronuclear DNA of Euplotes octocarinatus

Rab proteins belong to the largest family of the Ras superfamily of small GTPase that play an important role in intracellular vesicular traffic. So far, almost 60 members of Rab family have been identified in mammalian cells. To further study the diversity and function of Rab protein in evolution, unicellular protozoa ciliates, Euplotes octocarinatus, were used in this study, Rab genes were screened by PCR method from macronuclear DNA of E. octocarinatus. Sixteen Rab genes were obtained. They share 87.6-99.5% identities. Highly conserved GTP-binding domains were found. There are some hot regions that diverse sharply in these genes as well.

Ribosomal cold-adaptation: characterization of the genes encoding the acidic ribosomal P0 and P2 proteins from the Antarctic ciliate Euplotes focardii

Molecular adaptation at low temperature requires specificities represented mainly by modifications in the gene sequence and consequently in the protein primary structure. To characterize the molecular mechanisms responsible for ribosome cold-adaptation, we compared the ribosomal P0 and P2 genes from the Antarctic ciliate Euplotes focardii with homologous genes from mesophilic organisms, including the ciliates Tetrahymena thermophila and non cold-adapted Euplotes species. This analysis revealed the presence of non synonymous mutations unique to E. focardii. In the P0 protein the mutations produced amino acid substitutions that increased the molecular flexibility that may facilitate a conformational adjustment associated with the interaction with the GTPase center of the large subunit rRNA, and increased the hydrophobicity of the region involved in the interaction with P1/P2 heterodimer, probably to keep associated the ribosomal stalk in the cold. In the P2 protein the mutations produced amino acid substitutions that increased the N-terminus flexibility, which may facilitate interactions with P1 protein in the formation of the heterodimer, and reduced the mobility of the C-terminus, to stabilize the stalk during ribosomal activity. Finally, P proteins appeared to be valid markers for investigating the phylogenetic origin of early eukaryotes.

Telomere-end processing the terminal nucleotides of human chromosomes

Mammalian telomeres end in single-stranded, G-rich 3' overhangs resulting from both the "end-replication problem" (the inability of DNA polymerase to replicate the very end of the telomeres) and postreplication processing. Telomeric G-rich overhangs are precisely defined in ciliates; the length and the terminal nucleotides are fixed. Human telomeres have very long overhangs that are heterogeneous in size (35-600 nt), indicating that their processing must differ in some respects from model organisms. We developed telomere-end ligation protocols that allowed us to identify the terminal nucleotides of both the C-rich and the G-rich telomere strands. Up to approximately 80% of the C-rich strands terminate in CCAATC-5', suggesting that after replication a nuclease with high specificity or constrained action acts on the C strand. In contrast, the G-terminal nucleotide was less precise than Tetrahymena and Euplotes but still had a bias that changed as a function of telomerase expression.

Evolution of programmed ribosomal frameshifting in the TERT genes of Euplotes

A number of recent studies indicate that programmed + 1 ribosomal frameshifting is frequently required for the expression of genes in species of the genus Euplotes. In E. crassus, three genes encoding the telomerase reverse transcriptase (TERT) subunit have been previously found to possess one or two + 1 frameshift sites. To examine the origin of frameshift sites within the Euplotes group, we have isolated segments of the TERT gene from five Euplotes species. Coupled with phylogenetic analysis, the results indicate that one frameshift site in the TERT gene arose late in the evolution of the group. In addition, a novel frameshift site was identified in the TERT gene of E. minuta, a species where frameshifting has not been previously reported. Coupled with other studies, the results indicate that frameshift sites have arisen during the diversification of the euplotids. The results also are discussed in regard to the mutations necessary to generate frameshift sites, and the specialization of TERT protein function that has apparently occurred in E. crassus.

The Euplotes telomerase subunit p43 stimulates enzymatic activity and processivity in vitro

Telomerase is a reverse transcriptase that synthesizes telomeric DNA repeats at the ends of eukaryotic chromosomes. Although it is minimally composed of a conserved catalytic protein subunit (TERT) and an RNA component, additional accessory factors present in the holoenzyme play crucial roles in the biogenesis and function of the enzyme complex. Telomerase from the ciliate Tetrahymena can be reconstituted in active form in vitro. Using this system, we show that p43, a telomerase-specific La-motif protein from the ciliate Euplotes, stimulates activity and increases repeat addition processivity of telomerase. Activity enhancement by p43 requires its incorporation into a TERT.RNA.p43 ternary complex but is independent of other dissociable protein factors functioning in telomerase complex assembly. Stimulation is enhanced at elevated temperatures, supporting a role for p43 in structural stabilization of a critical region of the RNA subunit. To our knowledge, this represents the first demonstration that an authentic telomerase accessory protein can directly affect the enzymatic activity of the core enzyme in vitro.

Interaction of two classes of release factors from Euplotes octocarinatus

Translation termination on the ribosome is an essential process for cell viability. This process is maintained by two classes of peptide release factors (RF1/RF2, RF3 and eRF1, eRF3 in prokaryotes and eukaryotes, respectively). In protozoa ciliates Euplotes octocarinatus, an unicellular eukaryotes, universal stop codon UGA is reassigned for cysteine suggesting the specificity of evolution of translation termination system. We cloned two classes of release factors from Euplotes octocarinatus previously. In this paper, three in-frame stop codons UGA in Eo-eRF3 gene were mutated mediated by PCR site directed mutagenesis method. The interaction between eRF1 and eRF3 from E. octocarinatus was assayed in vivo using Yeast Two-hybrid System, which has an advantage of highly sensitivity. The results showed that the eRF1 x eRF3 complex was formed in living cells to function in the process of translation termination, differing from that in prokaryotes in which RF1/RF2 and RF3 function separately. The evolution of translation termination of life-form was analyzed using phylogenetic tree of amino acids sequences of RFs (32 (e) RF1s and 24 (e) RF3s) obtained from GenBank. Two classes of RFs are useful information in analysis of evolution of life-form and further elucidation of mechanism of translation termination of protein synthesis on ribosome.

Fingering the ends: how to make new telomeres

Telomerase-mediated healing of broken chromosomes gives rise to terminal deletions and is repressed in most organisms. In ciliated protozoa, however, chromosome fragmentation and de novo telomere addition are part of the developmental program. Work by in this issue of Cell indicates that in Euplotes crassus, this is mediated through switching between different telomerase reverse transcriptase isoforms.

Developmentally programmed gene elimination in Euplotes crassus facilitates a switch in the telomerase catalytic subunit

The primary function of telomerase is to maintain preexisting telomere tracts. In the ciliate Euplotes crassus, however, telomerase RNP structure and substrate recognition are altered during macronuclear development to facilitate de novo telomere addition. We found that E. crassus harbors three TERT genes encoding the telomerase catalytic subunit that not only vary in their nucleotide and predicted protein sequences, but also in their expression profiles. Expression of EcTERT-1 and -3 correlates with the requirement for telomere maintenance, while that of EcTERT-2 correlates with de novo telomere synthesis. All three genes appear to require ribosomal frameshifting for expression of catalytically active protein. The transcriptionally active form of EcTERT-2 exists only transiently in mated cells and is absent from the vegetative macronucleus. Thus, telomerase expression in Euplotes is controlled by unique regulatory mechanisms that culminate in a developmental switch to a different catalytic subunit with properties suited to de novo telomere addition.

The Euplotes La motif protein p43 has properties of a telomerase-specific subunit

Telomerase is a specialized reverse transcriptase synthesizing DNA repeats at telomeres. In addition to the RNA and catalytic protein components, telomerase from the ciliate Euplotes aediculatus contains the subunit p43. This protein is homologous to the La autoantigen, functioning in maturation of RNA polymerase III transcripts. Here we provide evidence that p43 is primarily associated with the telomerase ribonucleoprotein in vivo. Recombinant p43 binds telomerase RNA with low-nanomolar affinity in vitro, recognizing stem I and adjacent nucleotides or structures in the core of the RNA. Unlike authentic La proteins, p43 does not bind strongly to RNA polymerase III precursor transcripts and does not exhibit a marked binding preference for 3'-terminal oligouridylate residues. In isolated macronuclei, p43 largely colocalizes with telomerase RNA in discrete foci. These findings suggest that p43 is not the Euplotes La protein but instead plays a dedicated role in telomerase assembly and/or function. Thus, p43 joins the telomerase reverse transcriptase and the yeast proteins Est1p and Est3p as the only telomerase-specific proteins identified so far.

The telomerase-associated protein p43 is involved in anchoring telomerase in the nucleus

Telomere replication of eukaryotic chromosomes is achieved by a specialized enzyme, the telomerase. Although the biochemistry of end-replication is well understood, little is known about the organization of the end-replication machinery, its regulation throughout the cell cycle or the biological function of the telomerase-associated proteins. Here we investigate the function of the telomerase-associated protein p43 within the macronucleus of the ciliated protozoa Euplotes. It has been shown that p43 binds in vitro to the RNA subunit of telomerase and shares homology with the La autoantigen family. It therefore has been suggested that it is involved in the assembly and/or nuclear retention of telomerase. We show that the p43-telomerase complex is bound to a subnuclear structure in vivo and is resistant to electroelution. Upon inhibition of p43 or telomerase expression by RNAi, which in this study was used for the first time in spirotrichs, this complex is no longer retained in the nucleus. Further analysis revealed that the p43-telomerase complex is bound to the nuclear matrix in vivo and that after inhibition of p43 expression, telomerase is released from this structure, strongly suggesting that p43 is involved in anchoring of telomerase in the nucleus. This is the first in vivo demonstration of the biological function of this telomerase-associated component involved in telomere replication and allows us to propose a model for the organization of the end-replication machinery in the eukaryotic cell.

Identification of a novel "chromosome scaffold" protein that associates with Tec elements undergoing en masse elimination in Euplotes crassus

During macronuclear development in the ciliate Euplotes crassus, the highly repetitive, transposon-like Tec elements possess an unusual chromatin structure. We observed that the Tec element chromatin is highly resistant to salt extraction and behaves like a nuclear matrix/chromosome scaffold-associated structure. Standard matrix/scaffold extraction procedures identified two major proteins: 1) an ~140-kDa protein that seems to be topoisomerase II based on its reactivity with anti-topoisomerase II antibodies, and 2) an 85-kDa protein that we further purified by acid extraction and have shown to be a novel protein by sequence analysis of its gene. The 85-kDa protein (p85) is a developmental stage-specific protein and is located exclusively in the developing macronucleus. Immunolocalization studies of p85 show that it colocalizes with topoisomerase II in chromatin. In addition, in situ hybridization combined with immunofluorescence localization of the proteins indicates that 100% of the Tec elements colocalize with 70% of the p85, whereas no significant colocalization with a total macronuclear sequence-specific probe is observed. p85 is the first developmental stage-specific protein identified as being specifically associated with sequences undergoing elimination in E. crassus.

In situ identification by fluorescently labeled oligonucleotide probes of morphologically similar, closely related ciliate species

Ciliate protozoa are important members of microbial communities in which they play specific ecological roles. The determination of single species distribution is fundamental for food web analysis, but species recognition, which is mainly based on morphological characters, is often difficult between closely related species. The use of species-specific, purposely designed, fluorescently labeled probes for in situ hybridization is here presented as an easy and fast identification method for three closely related species belonging to the widespread genus Euplotes, namely E. crassus, E. vannus, and E. minuta, that in spite of their remarkable morphological similarity have significant metabolic and ecological differences. These three species can be detected simultaneously, provided the probes employed are bound to different fluorescent dyes: in this way their relative abundance and their population dynamics in the natural environment can be evaluated. As more ciliate sequences become available in databases, species-specific probes can be designed for other ciliates, thus rendering the application of the method of more general importance. The probes used in this study may also provide a tool to prevent erroneous species identification in future studies.

Selection on the genes of Euplotes crassus Tec1 and Tec2 transposons: evolutionary appearance of a programmed frameshift in a Tec2 gene encoding a tyrosine family site-specific recombinase

The Tec1 and Tec2 transposons of the ciliate Euplotes crassus carry a gene for a tyrosine-type site-specific recombinase. The expression of the Tec2 gene apparently uses a programmed +1 frameshift. To test this hypothesis, we first examined whether this gene has evolved under purifying selection in Tec1 and Tec2. Each element carries three genes, and each has evolved under purifying selection for the function of its encoded protein, as evidenced by a dearth of nonsynonymous changes. This distortion of divergence is apparent in codons both 5' and 3' of the frameshift site. Thus, Tec2 transposons have diverged from each other while using a programmed +1 frameshift to produce recombinase, the function of which is under purifying selection. What might this function be? Tyrosine-type site-specific recombinases are extremely rare in eukaryotes, and Tec elements are the first known eukaryotic type II transposons to encode a site-specific recombinase. Tec elements also encode a widespread transposase. The Tec recombinase might function in transposition, resolve products of transposition (bacterial replicative transposons use recombinase or resolvase to separate joined replicons), or provide a function that benefits the ciliate host. Transposons in ciliated protozoa are removed from the macronucleus, and it has been proposed that the transposons provide this "excisase" activity.