As it was proposed in several reports, there are a number of potential roles for RNA helicases in RNAi [66]. Our findings in the qPCR experiments during antigenic variation suggest that RNA helicases may participate in RNAi. This could be the case of the G. lamblia putative

DEAD-box helicase GL50803_15048, which was found to present high homology with the DmBel helicase and also with the DEAD-box HDAC inhibitor RNA helicases p68 and p72. Taking into account that some studies pointed out extensive overlapping and interplay among small RNA directed silencing machineries [64] and different RNA helicases operate either at different steps or playing different roles in the RNAi pathway, the involvement of this G. lamblia RNA helicase (GL50803_15048) in post-transcriptional gene silencing deserve further analysis. Although we did not find a putative

find more helicase in Giardia with high similarity to the HCD of higher eukaryotes Dicer enzyme, it has been proposed that Dicer helicase domain is required for siRNA, but not miRNA, processing [79]. Point mutations within the helicase domain or Dicer lacking a functional HCD showed that pre-miRNA processing does not require helicase participation, but that it is necessary for long dsRNA (siRNA processing) [79]. In Giardia, we have demonstrated that purified RdRP generates high-molecular-weight VSP RNAs in vitro only when more than one VSP transcript is present in the reaction mixture [22] and proposed a mechanism where variations in either the general or local concentrations of different VSP transcripts may determine which transcript will circumvent the silencing system, as was suggested to occur in higher eukaryotes [53]. In addition, it has been proposed by others groups the presence in Giardia

of a miRNA biogenesis pathway reminiscent of the canonical Tacrolimus (FK506) miRNA biogenesis pathway found in higher organisms [25, 80], and they have ABT888 identified conserved putative microRNA target site of several variant surface protein (VSP) mRNAs. Here Giardia Dicer apparently would assume the functions of both a Drosha and a Dicer, although no RNA-binding protein DAWDLE (DDL) homolog has yet been identified in this parasite. Furthermore Giardia Dicer must shuttle between the cytoplasm and the nucleus to process pri- and pre-miRNAs, although we determined its cellular localization by expressing a hemagglutinin-tagged version of the protein. Similar to that observed in other cells, Giardia Dicer localizes to the cytoplasm [22]. On one hand, the lack of the RNA helicase domain in Giardia Dicer is in agreement with the occurrence of a miRNA pathway. But, on the other hand, it was also proposed that a deletion or mutation of the helicase domain of human Dicer leads to a more active enzyme in vitro for cleavage of a perfectly matched 37-nt linear duplex RNA [51], allowing the enzyme to rapidly reinitiate cleavage on the long substrates.