In trypanosomes, mRNAs are processed by trans splicing, in which a common spliced leader sequence (SL) is acquired at the 5' end of the mRNA to yield a mature transcript (Agabian 1990, Liang et al. 2003. SL trans splicing has been mainly characterized in trypanosomes and nematodes and requires the small nuclear ribonucleoproteins (U snRNPs) U2, U4/U6 and U5, in addition to the SL RNP (Tschudi & Ullu 1990). Intervening sequences have been described in the poly A polymerase (PAP) gene of Trypanosoma brucei and Trypanosoma cruzi and in a U1 small nuclear RNA (snRNA) sequence of T. brucei, which demonstrates that both cis and trans splicing occur in these organisms. Typically, the occurrence of trans splicing is more prevalent (Schnare & Gray 1999, Mair et al. 2000.RNA splicing is carried out by the spliceosome, which consists of the U1, U2, U4, U5 and U6 U snRNPs particles and non-snRNP proteins. The ribonucleoproteins are complexes that consist of small uridine-rich RNAs (U snRNAs) and interact with common Sm proteins and proteins that are specific for each snRNP (Mayer & Floeter-Winter 2005). The detection of the trypanosomatid U5 snRNA was puzzling for many years because the snRNA was not identified in the initial screen for TMG-capped U snRNA. Initially, spliced-leader associated RNA (SLA1) was thought to be the trypanosomatid homologue of U5 because of a sequence that was somewhat related to the invariant U5 loop. The theory that SLA1 was a U5 homologue was disproved because the Leptomonas seymouri homologue did not possess the "invariant" sequence and the "true" U5 was subsequently identified. U5 has several unique properties: it is the shortest snRNA, it lacks several conserved structural domains, including the TMG cap and m7G, and it has a phosphate terminus at the 5' end (Ambrósio et al. 2007). The invariant loop contains an A instead of a C at the second nucleotide position and the loop lacks the characteristic 2'-O-methylation (Liang et al. 2003). The Leptomonas collosoma and the L. seymouri U5 snRNAs are longer than the U5 snRNAs of T. brucei and T. cruzi because they possess a second stem-loop structure (Ambrósio et al. 2007). Analyses of mutations that were introduced into the Sm site of the L. seymouri U5 suggest that U5 snRNA binds to common proteins during a transient cytoplasmic phase before U5 is translocated to the nucleus. This process commonly occurs in mammals but not in yeast (Liang et al. 2003).Although trans splicing is well characterized in humans and yeast, little is known about the specific proteins that mediate trans splicing in trypanosomatids. In human cells, eight U5 snRNP-specific proteins, 220K, 200K, 116K, 102K, 100K, 52K, 40K and 15K, have been identified. Orthologues of the human proteins, PRP8, BRR2, SNU114, PRP6, PRP28, LIN1 or SNU40 and DIB1, respectively, have also been identified in Saccha-
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