Molecular characterization of a retrotransposon in Rhynchosciara americana genome and its relationship to maintenance of telomere
Rezende-Teixeira, P.1; Siviero, F.1; Machado-Santelli, G.M.1 & Santelli, R.V.2
1 Instituto de Ciências Biomédicas, Departamento de Biologia Celular e do Desenvolvimento, USP; 2 Instituto de Química, Departamento de Bioquímica, USP.
Retrotransposons are mobile elements widely present in eukaryotic genomes that transpose by reverse transcription of a RNA intermediate. They comprise two transposable elements categories grouped in class I of transposons: those that possess a long terminal repeat (LTR elements) and those that do not possess this element (non-LTR elements). Most of non-LTR retrotransposons, also known as LINEs (long interspersed nuclear elements), have two open reading frames (ORFs) with strong similarity to the gag and pol coding regions of retroviruses. The pol sequence encodes reverse transcriptase and other enzymatic activities necessary for transposition. The gag sequence (ORF I) encodes a protein of function less well understood, and the ORF I is less conserved than the pol coding sequence.
The analysis of sequences of a cDNA library constructed from mRNA of the salivary glands of R. americana showed the presence of putative class I elements. The cDNA clone with homology to a reverse transcriptase was starting point for the present study. Genomic phages were isolated, sequenced and the molecular structure of the element characterized as being a non-LTR retrotransposable element. Southern blot analysis indicated that this transposable element is represented by repeat sequences in the genome of R. americana. However, the cDNA and genomic phages clones were localized by in situ hybridisation mainly on chromosome ends. These results may represent the counterpart of the situation observed in Drosophila where two retrotransposons, HeT-A and TART are implicated in the extension of the telomere by successive transpositions in tandem arrays. Northern blot analysis showed that this retroelement has been expressed in different developmental stages and the transcript of high molecular mass detected represents the full-length non-LTR retrotransposon.
The mobile element identified in R. americana would correspond to TART non-LTR retrotransposon element of Drosophila and it is possibly the first transposable element described in R. americana with a defined role in chromosome structure. (Fapesp, CNPq)
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