Timothy J. D. Goodwin

DIRS-1 and the other tyrosine recombinase retrotransposons.

DIRS-1 is a retroelement from the slime mold Dictyostelium discoideum. Until recently only two related retrotransposons had been described: PAT from the nematode Panagrellus redivivus and Prt1 from the zygomycete fungus Phycomyces blakesleeanus. Analyses of the reverse transcriptase sequences encoded by these three elements suggested that they were closely related to each other and more distantly related to the Ty3/gypsy Long Terminal Repeat (LTR) retroelements. They have several unusual structural features that distinguish them from typical LTR elements. For instance, they each encode a tyrosine recombinase (YR), but not a DDE-type integrase or an aspartic protease. Although the DIRS-1-related elements are bordered by terminal repeats these differ from typical LTRs in a number of ways. In DIRS-1, for example, the terminal repeats are inverted (complementary), non-identical in sequence, and the outer edges of the terminal sequences are repeated (adjacent to each other) in the internal region. PAT has so-called “split” direct repeats in which the unrelated terminal sequences appear as direct repeats adjacent to each other in the internal region. The only repetition displayed by Prt1 is the presence of short inverted terminal repeats, but the sequenced copy of this element is believed to be a truncated version of an element with a structure resembling DIRS-1. The unusual structure of the terminal repeats of the DIRS1-like elements appears to be related to their replication via free circular intermediates. Site-specific recombination is believed to integrate the circle without creating duplications of the target sites. In recognition of these important distinctions it is proposed that the retrotransposons that encode tyrosine recombinases be called the tyrosine recombinase (or YR) retrotransposons. Recently a large number of additional YR retrotransposons have been described, including elements from fungi (zygomycetes and basidiomycetes), plants (green algae) and a wide range of animals including nematodes, insects, sea urchins, fish and amphibia, while remnants of elements related to DIRS-1 occur in the human genome. The complete set of YR retrotransposons can be divided into two major groups, the DIRS elements and the Ngaro elements, the two groups forming distinct clades on phylogenetic trees based on alignments of RT/RH and recombinase sequences, and also having some structural distinctions. A third group of transposable elements, which we call Cryptons, also carry tyrosine recombinases. These elements do not encode a reverse transcriptase and so are believed to be DNA transposons not retrotransposons. They have been detected in several pathogenic fungi, including the basidiomycete Cryptococcus neoformans, and the ascomycetes Coccidioides posadasii and Histoplasma capsulatum. Sequence comparisons suggest that the Crypton YRs are related to those of the YR retrotransposons. We suggest that the YR retrotransposons arose from the combination of a Crypton-like YR DNA transposon and the RT/RH encoding sequence of a retrotransposon.

L1-like non-LTR retrotransposons in the yeast Candida albicans.

Abstract Non-LTR retrotransposons (also known as LINEs) have had a major influence on the structure of many eukaryote genomes. They are abundant in many multicellular eukaryotes, including mammals, but appear to be absent from the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This absence has, to date, precluded the development of a yeast model system for the study of non-LTR retrotransposons. In this report we describe several families of non-LTR retrotransposons from the yeast Candida albicans. These elements, which we call Zorro elements, are all members of the L1 clade of non-LTR retrotransposons. Some are intact, transcriptionally active and have transposed recently. This finding should allow the development of a yeast model system.

The diversity of retrotransposons in the yeast Cryptococcus neoformans

We have undertaken an analysis of the retrotransposons in the medically important basidiomycetous fungus Cryptococcus neoformans. Using the data generated by a C. neoformans genome sequencing project at the Stanford Genome Technology Center, 15 distinct families of LTR retrotransposons and several families of non‐LTR retrotransposons were identified. Members of at least seven families have transposed recently and are probably still active. For several families, only partial elements could be identified and these are quite diverse in sequence, suggesting that they are ancient components of the C. neoformans genome. Most C. neoformans elements are not closely related to previously identified fungal retrotransposons, suggesting that the diversity of fungal retrotransposons has been only sparsely sampled to date. C. neoformans has fewer distinct retrotransposon families than Candida albicans (37 or more), in particular fewer families represented solely by ancient and inactive elements, but it has considerably more families than either Saccharomyces cerevisiae (five) or Schizosaccharomyces pombe (two). The findings suggest that elimination of retrotransposons is faster in C. neoformans than in C. albicans, but perhaps not as rapid as in S. cerevisiae or Sz. pombe. The identification of the retrotransposons of C. neoformans should assist in the molecular characterization of this important pathogen, and also further our understanding of the role played by retroelements in genome evolution. Copyright

Vertebrate LTR Retrotransposons of the Tf1/Sushi Group

Abstract. LTR retrotransposons of the Tf1/sushi group from a diversity of vertebrates, including fish, amphibians, and mammals (humans, mice, and others), are described as full-length or partial elements. These elements are compared, and the mechanisms involved in self-priming of reverse transcriptase and programmed phase shifting are inferred. Evidence is presented that in mammals these elements are still transcriptionally active and are represented as proteins. This suggests that members of the Tf1/sushi group are present as functional elements (or incorporated as partial elements into host genes) in diverse vertebrate lineages.

The distribution and evolutionary history of the PRP8 intein.

BackgroundWe recently described a mini-intein in the PRP8 gene of a strain of the basidiomycete Cryptococcus neoformans, an important fungal pathogen of humans. This was the second described intein in the nuclear genome of any eukaryote; the first nuclear encoded intein was found in the VMA gene of several saccharomycete yeasts. The evolution of eukaryote inteins is not well understood. In this report we describe additional PRP8 inteins (bringing the total of these to over 20). We compare and contrast the phylogenetic distribution and evolutionary history of the PRP8 intein and the saccharomycete VMA intein, in order to derive a broader understanding of eukaryote intein evolution. It has been suggested that eukaryote inteins undergo horizontal transfer and the present analysis explores this proposal.ResultsIn total, 22 PRP8 inteins have been detected in species from three different orders of euascomycetes, including Aspergillus nidulans and Aspergillus fumigatus (Eurotiales), Paracoccidiodes brasiliensis, Uncinocarpus reesii and Histoplasma capsulatum (Onygales) and Botrytis cinerea (Helotiales). These inteins are all at the same site in the PRP8 sequence as the original Cryptococcus neoformans intein. Some of the PRP8 inteins contain apparently intact homing endonuclease domains and are thus potentially mobile, while some lack the region corresponding to the homing endonuclease and are thus mini-inteins. In contrast, no mini-inteins have been reported in the VMA gene of yeast. There are several examples of pairs of closely related species where one species carries the PRP8 intein while the intein is absent from the other species. Bio-informatic and phylogenetic analyses suggest that many of the ascomycete PRP8 homing endonucleases are active. This contrasts with the VMA homing endonucleases, most of which are inactive.ConclusionPRP8 inteins are widespread in the euascomycetes (Pezizomycota) and apparently their homing endonucleases are active. There is no evidence for horizontal transfer within the euascomycetes. This suggests that the intein is of ancient origin and has been vertically transmitted amongst the euascomycetes. It is possible that horizontal transfer has occurred between the euascomycetes and members of the basidiomycete genus Cryptococcus.

A group of deuterostome Ty3/gypsy-like retrotransposons with Ty1/copia-like pol-domain orders

Abstract. Here we report the existence of an unusual group of LTR retrotransposons, termed Gmr1-like elements. The members of this group are most similar in sequence to elements of the Ty3/gypsy group, yet, unlike typical Ty3/gypsy elements, they have pol genes in which the integrase domain lies upstream, rather than downstream, of the reverse transcriptase domain. Such an arrangement was formerly believed to be a characteristic peculiar to Ty1/copia elements. The group includes the previously described retrotransposon Gmr1 from the Atlantic cod Gadus morhua, together with elements from a variety of other vertebrates, such as the zebrafish Danio rerio, the pufferfish Fugu rubripes and the clawed toad Xenopus laevis, as well as elements from non-vertebrate deuterostomes such as the sea squirt Ciona intestinalis. No Gmr1-like elements were found outside of the deuterostomes, nor were any other groups of elements with atypical pol-domain orders identified. Phylogenetic analyses show that the Gmr1-like elements form a monophyletic group within the larger group of Ty3/gypsy elements. Some of the newly identified elements appear to be structurally intact and may still be active. The identification of this group challenges some previously held beliefs concerning the structure and evolution of LTR retrotransposons.

A nuclear-encoded intein in the fungal pathogen Cryptococcus neoformans.

We have used comparative sequence analysis to identify an intein‐like sequence (protein splicing element) present in Cryptococcus neoformans, a fungal pathogen of humans. The sequence encoding this element is present in the C. neoformans PRP8 gene, as an in‐frame insertion relative to the PRP8 genes of other organisms. It contains sequences similar to those of the protein‐splicing domains of two previously described yeast inteins (in Saccharomyces cerevisiae and Candida tropicalis), although it lacks any recognizable internal endonuclease domain. The Cryptococcus neoformans intein (Cne PRP8) is only the second to be found in a eukaryote nuclear genome; the previously described yeast inteins occur at the same site in the VMA gene homologues of S. cerevisiae and C. tropicalis. The host gene of the Cryptococcus intein, PRP8, encodes a highly conserved mRNA splicing protein found as part of the spliceosome. The Cne PRP8 intein may be a useful drug target in addressing the cryptococcal infections so prevalent in AIDS patients. The Cne PRP8 coding sequence and the flanking sequences of the Cryptococcus neoformans PRP8 gene have been assigned GenBank Accession No. AF349436. Copyright

Multiple, non-allelic, intein-coding sequences in eukaryotic RNA polymerase genes

BackgroundInteins are self-splicing protein elements. They are translated as inserts within host proteins that excise themselves and ligate the flanking portions of the host protein (exteins) with a peptide bond. They are encoded as in-frame insertions within the genes for the host proteins. Inteins are found in all three domains of life and in viruses, but have a very sporadic distribution. Only a small number of intein coding sequences have been identified in eukaryotic nuclear genes, and all of these are from ascomycete or basidiomycete fungi.ResultsWe identified seven intein coding sequences within nuclear genes coding for the second largest subunits of RNA polymerase. These sequences were found in diverse eukaryotes: one is in the second largest subunit of RNA polymerase I (RPA2) from the ascomycete fungus Phaeosphaeria nodorum, one is in the RNA polymerase III (RPC2) of the slime mould Dictyostelium discoideum and four intein coding sequences are in RNA polymerase II genes (RPB2), one each from the green alga Chlamydomonas reinhardtii, the zygomycete fungus Spiromyces aspiralis and the chytrid fungi Batrachochytrium dendrobatidis and Coelomomyces stegomyiae. The remaining intein coding sequence is in a viral relic embedded within the genome of the oomycete Phytophthora ramorum. The Chlamydomonas and Dictyostelium inteins are the first nuclear-encoded inteins found outside of the fungi.These new inteins represent a unique dataset: they are found in homologous proteins that form a paralogous group. Although these paralogues diverged early in eukaryotic evolution, their sequences can be aligned over most of their length. The inteins are inserted at multiple distinct sites, each of which corresponds to a highly conserved region of RNA polymerase. This dataset supports earlier work suggesting that inteins preferentially occur in highly conserved regions of their host proteins.ConclusionThe identification of these new inteins increases the known host range of intein sequences in eukaryotes, and provides fresh insights into their origins and evolution. We conclude that inteins are ancient eukaryote elements once found widely among microbial eukaryotes. They persist as rarities in the genomes of a sporadic array of microorganisms, occupying highly conserved sites in diverse proteins.

DIRS retroelements in arthropods: identification of the recently active TcDirs1 element in the red flour beetle Tribolium castaneum

AbstractMembers of the DIRS family of retrotransposons differ from most other known retrotransposons in that they encode a tyrosine recombinase (YR), a type of enzyme frequently involved in site-specific recombination. This enzyme is believed to insert the extrachromosomal DNA intermediate of DIRS element retrotransposition into the host genome. DIRS elements have been found in plants, a slime mold, fungi, and a variety of animals including vertebrates, echinoderms and nematodes. They have a somewhat patchy distribution, however, apparently being absent from a number of model organisms such as Saccharomyces cerevisiae, Arabidopsis thaliana and Drosophila melanogaster. In this report we describe the first DIRS retroelement to be identified in an arthropod. This element, TcDirs1, was found in the red flour beetle Tribolium castaneum (Coleoptera). It is generally similar in sequence and structure to several previously described members of the DIRS group: it is bordered by inverted terminal repeats and it has a similar set of protein-coding domains (Gag, reverse transcriptase/ribonuclease H, and the YR), although these are arranged in a novel fashion. TcDirs1 elements exhibit several features indicative of recent activity, such as intact coding regions, a high level of sequence similarity between distinct elements and polymorphic insertion sites. Given their presence in an experimentally tractable host, these potentially active elements might serve as useful models for the study of DIRS element retrotransposition. An element closely related to TcDirs1 was also detected in sequences from a second arthropod, the honey bee Apis mellifera (Hymenoptera), suggesting that these retrotransposons are long-term residents of arthropod genomes.

Two new fungal inteins

Until recently the only intein known to be encoded by the nuclear genome of a eukaryote was the VMA intein in the vacuolar ATPase precursor of several species of saccharomycete yeast. This intein has been intensively studied and much information has been gained about its structure, mode of action and evolutionary history. We recently reported a second nuclear intein, Cne PRP8, encoded within the PRP8 gene of the basidiomycete Cryptococcus neoformans. Subsequent studies have found allelic PRP8 inteins in several species of yeast and filamentous ascomycetes. Here we report two further, non‐allelic, inteins from ascomycete species. The yeast Debaryomyces hansenii (which also has a VMA intein) has an intein encoded within the sequence of the glutamate synthase gene (GLT1). There are also inteins encoded in the homologous GLT1 genes of the yeast Candida (Pichia) guilliermondii and the filamentous fungus Podospora anserina. These allelic GLT1 inteins occupy exactly the same site in the glutamate synthase and all contain domains that indicate the presence of a homing endonuclease (HEG). Podospora anserina, in addition, contains a second, non‐allelic, intein encoded in the chitin synthase gene (CHS2); this intein also contains a HEG domain. We describe the phylogenetic relationships among the four eukaryote nuclear encoded inteins (VMA, PRP8, GLT1 and CHS2). We also consider this phylogeny in the broader context of eubacterial, archaeal and eukaryote viral and organelle inteins. Copyright