Pr77 and L1TcRz: A dual system within the 5'-end of L1Tc retrotransposon, internal promoter and HDV-like ribozyme.

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 associated tandems. The Pr77/L1TcRz sequence is also found in other trypanosomatid'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 retrotransposons and in the gene expression of trypanosomatids are also discussed in this paper.