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Pr77 and L1tcrz a Dual System Within the 5'-End of L1tc Retrotransposon, Internal Promoter and HDV-Like Ribozyme

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Pr77 and L1tcrz a Dual System Within the 5'-End of L1tc Retrotransposon, Internal Promoter and HDV-Like Ribozyme LETTER TO THE EDITOR Mobile Genetic Elements 2:1, 1–7; January/February 2012; G 2012 Landes Bioscience Pr77 and L1TcRz A dual system within the 5'-end of L1Tc retrotransposon, internal promoter and HDV-like ribozyme Francisco J. Sánchez-Luque,1 Manuel C. López,1,* Francisco Macias,1 Carlos Alonso2 and M. Carmen Thomas1,* 1Departamento de Biología Molecular; Instituto de Parasitología y Biomedicina “López Neyra”; Consejo Superior de Investigaciones Científicas; Parque Tecnológico de Ciencias de la Salud; Granada, Spain; 2Centro de Biología Molecular “Severo Ochoa”; Universidad Autónoma de Madrid; Consejo Superior de Investigaciones Científicas; Madrid, Spain Keywords: retrotransposons, LINE, SINE, HDV-like ribozyme, promoter, retrotransposition machinery, Trypanosoma cruzi, L1Tc, trypanosomatid, genetic regulation The sequence corresponding to the first 77 nucleotides of the L1Tc and NARTc non-LTR retrotransposons from Trypanosoma cruzi is an internal promoter (Pr77) that generates abundant, although poorly translatable, un-spliced transcripts. It has been recently described that L1TcRz, an HDV-like ribozyme, resides within the 5’-end of the RNA from the L1Tc and NARTc retrotransposons. Remarkably, the same first 77 nucleotides of L1Tc/NARTc elements comprise both the Pr77 internal promoter and the HDV-like L1TcRz. The L1TcRz cleaves on the 5’-side of the +1 nucleotide of the L1Tc element insuring that the promoter and the ribozyme functions travel with the transposon during retrotransposition. The ribozyme activity would prevent the mobilization of upstream sequences and insure the individuality of the L1Tc/NARTc copies transcribed from © associated2012 tandems. The Pr77/L1TcRz Landes sequence is also found in other trypanosomatidBioscience.’s non-LTR retrotransposons and degenerated retroposons. The possible conservation of the ribozyme activity in a widely degenerated retrotransposon, as the Leishmania SIDERs, could indicate that the presence of this element and the catalytic activity could play some favorable genetic regulation. The functional implications of the Pr77/L1TcRz dual system in the regulation of the L1Tc/NARTc retrotransposonsDo and in the genenot expression of trypanosomatids distribute. are also discussed in this paper. Retrotransposons are DNA sequences able inserted into a new position in the genome (short interspersed nuclear elements) and to mobilize their own copies within a host (Fig. S1B). Thus, the intermediate RNA LINE truncated elements. A high copy genome by transcription and reverse of the non-LTR retrotransposons is likely number of these elements is present in the transcription of an intermediate RNA. to bear within its sequence the informa- genome of the etiological agent of Chagas The generation of this intermediate RNA tion required for its own transcription. disease, the protozoan parasite Trypanosoma is a crucial step for the mobilization Although internal promoters that satisfy cruzi.8 mechanism. The LTR retrotransposons these requirements have been described The L1Tc (5 kb in length) is a non- (with long terminal repeats at both ends) in a few elements,3-5 the way to generate LTR retrotransposon widely distributed have an external RNA polymerase II the mRNAs by many others elements is along the T. cruzi genome. L1Tc is promoter located upstream of the trans- still unknown. Eickbush, Lupták and a LINE element that codes for the cription initiation site. A complex and coworkers recently described the system mobilization machinery including AP- discontinuous reverse transcription process of the Drosophila R2 retrotransposon endonuclease,9,10 reverse transcriptase,11 that requires two DNA strand transfers for the generation of mRNAs involving RNase H12 and nucleic acids chaperone in addition to several enzymatic activities an HDV-like ribozyme coupled to an activities.13,14 In addition, an active is necessary to regenerate the promoter1 upstream host-promoter.6,7 picornavirus-like 2A autoproteolytic motif (Fig. S1A). On the contrary, the retro- Based on their coding capacity, the non- resides at the N-terminal end of the transposition mechanism described for LTR retrotransposons are classified in encoded L1Tc polypeptide. The 2A auto- non-LTR retrotransposons, called Target two major groups: those that code for proteolytic activity is expected to regulate Primed Reverse Transcription (TPRT),2 their own retrotransposition machinery the composition and abundance of the consists of a single-step reverse transcrip- or LINEs (long interspersed nuclear ele- enzymatic machinery required for auto- tion of the mRNA element and the ments) and those that lack translation nomous mobilization of L1Tc.15 NARTc synthesis of a cDNA strand to generate a capability and are thought to mobilize using (263 nt in length) is a non-autonomous new double-stranded DNA copy able to be LINEs’ machinery in trans, like SINEs element thought to be mobilized by the *Correspondence to: Manuel C. López and M. Carmen Thomas; Email: [email protected] and [email protected] Submitted: 11/04/11; Revised: 12/19/11; Accepted: 01/02/12 http://dx.doi.org/10.4161/mge.19233 www.landesbioscience.com Mobile Genetic Elements 1 gen products of L1Tc.16 L1Tc and the RNA sugar-phosphate backbone of its sequences indicate that these sequences NARTc share 97% and 77% identity at own molecule, leaving a 5'-hydroxyl end upstream of the L1TcRz have the potential their first 77 and last 30 nucleotides on the downstream product, and it is to modulate the L1TcRz activity. A similar (including a short poly-A tail codified in expected to leave a 2',3'-clyclic phosphate modulation phenomenon has been the DNA), respectively. The conserved end on the upstream cleavage product, described for other known HDV-like sequence located at their 3'-end is thought similar to the HDV and other known ribozymes.7,22 L1Tc sequences down- to be recognized by the reverse transcrip- HDV-like ribozymes.19 The cleavage posi- stream of the L1TcRz have also proved tase encoded by the L1Tc element. The tion of L1TcRz is located immediately to regulate the ribozyme activity.18 It has sequence corresponding to the first 77 upstream of the +1 nucleotide of both been shown that the 49 nt downstream of nucleotides (Pr77) of L1Tc and NARTc L1Tc and NARTc. L1TcRz, then, is L1TcRz can induce an RNA structural has promoter activity and acts as an capable of determining the 5'-end of these change to a conformation not capable of internal promoter that generates abundant, two elements. The 5'-end is such that L1TcRz activity (Fig. 1B).18 The sequence although poorly translatable, transcripts.17 the ribozyme and the internal promoter located downstream of L1TcRz in NARTc sequence is present in the mature element does not produce a conformational switch HDV-Like Ribozymes Associated RNA insuring their transmission to the and consequently the ribozyme activity is with Retrotransposons: new retrotransposon copy during TPRT preserved in NARTc RNA. It is possible L1TcRz and R2Rz. process. that the downregulation effect of L1Tc The Dual Promoter-Ribozyme Co-transcriptional cleavage assays were downstream sequences may be an artificial System of L1Tc used to map the L1TcRz to the first 77nt consequence of the transcriptional activity (+1 to +77) of L1Tc/NARTc elements. of the T7 RNA polymerase used in the We have recently described the L1TcRz, This 77 nt sequence is predicted to fold in vitro assays. T7 RNA polymerase may an hepatitis delta virus (HDV)-like ribo- into an HDV-like ribozyme, and can be transcribe the downstream sequence so ©zyme that2012 resides within the 5'-end Landes of fitted to the consensus HDV secondaryBioscience.quickly, prior to ribozyme folding, allow- the RNA of both the L1Tc and NARTc structure with minimal divergences.6,20,21 ing alternative folding pathways to occur. retrotransposons of T. cruzi (Fig. 1A).18 Assays where L1TcRz sequence is pre- The inhibition of the ribozyme activity The ribozyme catalyzesDo the self-cleavage not of ceded by variousdistribute. L1Tc target derived may not occur with an eukaryotic RNA Figure 1. L1Tc RNA 5’-end region structural switching. The proposed folding of the L1TcRz is shown in (A). The three helixes P1, P2 and P4 and the two pseudoknots P1.1 and P3 are colored. The arrow points the ribozyme cleavage site. The proposed switching of the folding toward a supposed-IRES after elongation is shown in (B). RNAfold software proposes for the first 110 nt of the L1Tc RNA a structure involving three stem-loops called SL1, SL2 and SL3. P1 and P4 helixes are conserved after the folding switching but the two pseudoknots and the P2 helix are not maintained. A 5’-hydroxyl end is expected for SL1 to be occluded by the GC pair-rich P1 stem. Each ten nucleotides are marked by a dot. 2 Mobile Genetic Elements Volume 2 Issue 1 polymerase, whose polymerase activity is autonomous element is used by each non- a decay of the transcription level of distal slower than the T7 phage polymerase. autonomous element.26 However, this regions, ensuring the correct expression of Other non-LTR retrotransposon has an hypothesis is only supported by bioinfor- the last genes of each cluster (Fig. 2E). HDV-like ribozyme in its 5'-end for the matic data as the function of the upstream The origin of the Pr77-dependent purpose of processing the retrotransposon sequence motifs remains unknown. The transcription has been determined by RNA.6,7 L1Tc was the second reported implications of the Pr77/L1TcRz dual primer extension using extracted RNA case of a non-LTR retrotransposon to be function in trypanosomes are explored in from epimastigote cells and found to be associated to an HDV-like ribozyme, the next section. located at the +1 nucleotide.17 Since the being the R2 the first one. R2, unlike L1TcRz cleaves at the 5'-side of the L1Tc, is a site-specific element whose Functional Implications mentioned +1 nucleotide of the element, copies are always found at the same of Pr77/L1TcRz Dual System the result of the primer extension pre- position in the redundant rDNA genes.
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