Post-transcriptional Gene Silencing and Translation in Giardia

The control of gene expression in Giardia lamblia includes several mechanisms already described in higher eukaryotes, but with some interesting features for this early-branching organism. Here we describe two gene expression control systems in Giardia, posttranscriptional gene silencing (PTGS) and translation, and the close interaction between them. For the first mechanism, all the components were identified as being active in this cell, their sequences were analyzed and their localization was identified. Even more important was the implication of this mechanism in the process of antigenic variation in this parasite, which reflects the involvement of the RNAi pathway in variant-specific surface protein (VSP) regulation and switching. Regarding the translational system, the principal characteristics of this parasite are the lack of ribosome scanning mechanism and a prokaryotic resemblance in the small ribosomal subunit recruitment process. Even though the presence of some, but not all, eukaryotic in itiation factors could represent a simplified “cap-dependent” process, there is also the possibility that microRNAs could be involved in translation regulation. In general, we can assume that this intestinal parasite has either simplified the gene expression control machinery due to their parasitic life style or, on the other hand, we are privileged witnesses of how the evolutionary process takes place.