This project is a part of a larger effort to uncover
non-coding RNAs (e.g. microRNAs). microRNAs, a subset of the
small interfering RNAs
(siRNAs) which serve as a guide for RNA interference (RNAi), are
responsible for the regulation of gene expression during plant and
animal development. In 1993, the first miRNA, lin-4, was
discovered in Caenorhabditis elegans. By cloning the gene locus,
Lee et al. discovered that lin-4 did not produce a protein-coding mRNA,
but a 22 nucleotide non-coding RNA (ncRNA). This miRNA functions
to regulate the expression of lin-14 mRNA by binding to the 3â€™
UTR. It wasnâ€™t until 2000 with the discovery of let-7, a miRNA
that targets lin-41 and hbl-1 in a functionally similar manner, that
miRNA was established as a key element of gene regulatory networks (12).
In plant and animal cells, miRNAs originate in the nucleus as
Pri-miRNA. A 20-22 nt segment is removed from the pre-miRNA hairpin
either by an RNase III, DICER, in the cytoplasm, as is seen in animals,
or by the RNase III enzyme DICER-LIKE 1 (DCL1) in the nucleus, in the
case of plants. The double-stranded miRNA is then split by a
helicase into two separate strands, one of which is preferentially
loaded into the RNA-induced silencing complex (RISC) (12).
RNAi has been discovered in several protozoan parasites, the first
being Trypanosoma brucei by Ngo et al. in 1998. It was shown that
trypanosomes possess a mechanism to degrade mRNA through direct contact
with double-stranded RNA (dsRNA), much like exists in C. elegans.
This is not generally the case in apicomplexan parasites.
Although one report has been published stating that RNAi may exist in Toxoplasma gondii, the same can not be said for other apicomplexa, such as Plasmodium
falciparum, where such machinery does not exist. Database searches for
consensus domain sequences of Dicer, Piwi, PAZ, or RdRp showed no sign
of target gene candidates. Additionally, reports demonstrating
loss of protein function as a result of small siRNA-like particles do
not show any evidence that the small RNA sequences are indeed siRNAs.
As a result, the presence of a functioning RNAi pathway in P.
falciparum is a debatable topic (15).
Table 1. Experimental status of siRNA & microRNA in protozoan parasites
(Note: Reference numbers refer to source in the References section