Complete nucleotide sequence of an unusual mobile element from trypanosoma brucei

Abstract

The complete nucleotide sequence of a mobile element from Trypanosoma brucei is presented along with the sequence of its target site, which shows that the insertion has generated a 7 base pair direct repeat. The cloned copy of the element is a dimeric structure, one end of each monomer consisting of a stretch of 14 A residues preceded by a putative trypanosome polyadenylation signal. Six base pairs of DNA of unknown origin are found in the dimer between the two copies of the element. Evidence suggests that the element is present in the genome mainly as a monomer whose sequence is conserved across several species of trypanosome. The element contains an open reading frame encoding the same 160 amino acid protein in both sequenced copies and is extensively transcribed from both strands.

The trypanosome spliced leader small RNA gene family: stage-specific modification of one of several similar dispersed genes

“…Given the fact that most, if not all, trypanosome mRNAs contain a 5' terminal SL, as does the small SL RNA, and the probable presence of a reverse transcription-cDNA insertion process in trypanosomes (27) it is perhaps surprising that we did not detect more dispersed SL sequences placed in the genome by RNA-mediated mechanisms, as is common in other small RNA gene families. In fact, we detected only one dispersed SL which might have resulted from such a process, on the 3.5 kb Eco RV fragment which does not hybridize to the orphon flanking region subclones nor to a probe containing most of the reiteration unit.…”

“…There is no open reading frame 5' to this area within the region examined, nor is the poly A tract preceded by the putative trypanosome polyadenylation signal AAAATTPyT (27).…”

Ingi, a 5.2-kb dispersed sequence element from Trypanosoma brucei that carries half of a smaller mobile element at either end and has homology with mammalian LINEs.

“…The sequences are compared in This analysis did not reveal either direct or inverted repeats at the ends of these ingi elements. Instead, it produced the completely unexpected finding that the extreme ends of the ingi elements were homologous with the two halves of the monomer of a previously described dimeric T. brucei mobile element, RIME (17). The sequence of one RIME (R) is included in Fig.…”

“…3. R is the sequence of one monomer from the tandem pair of RIMEs sequenced by Hasan et al (17). Dotted lines represent the continuation of the DNA through the ingi elements.…”

“…There is no evidence to suggest that reverse transcription is involved in these transpositions. The existence of retrotransposition in this organism is, however, implied by the discovery of a dimer of a 512-base-pair (bp) ribosomal mobile element (RIME), which was found inserted into a rRNA gene in one T. brucei stock (17). RIME has an oligo(dA) tail and is flanked by short target site duplications, indicating that it is a retroposon.…”