Resumen:
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 fi rst
mechanism, all the components were identifi ed as being
active in this cell, their sequences were analyzed
and their localization was identifi ed. Even more important
was the implication of this mechanism in the
process of antigenic variation in this parasite, which
refl ects the involvement of the RNAi pathway in
variant-specifi c 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 initiation factors could represent a
simplifi ed ?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 simplifi ed 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.