An unusual Dicer-like1 protein fuels the RNA interference pathway in Trypanosoma brucei

Comparative genomic analysis of three Leishmania species that cause diverse human disease

Leishmania parasites cause a broad spectrum of clinical disease. Here we report the sequencing of the genomes of two species of Leishmania: Leishmania infantum and Leishmania braziliensis. The comparison of these sequences with the published genome of Leishmania major reveals marked conservation of synteny and identifies only approximately 200 genes with a differential distribution between the three species. L. braziliensis, contrary to Leishmania species examined so far, possesses components of a putative RNA-mediated interference pathway, telomere-associated transposable elements and spliced leader-associated SLACS retrotransposons. We show that pseudogene formation and gene loss are the principal forces shaping the different genomes. Genes that are differentially distributed between the species encode proteins implicated in host-pathogen interactions and parasite survival in the macrophage.

Depletion of newly synthesized Argonaute1 impairs the RNAi response in Trypanosoma brucei

In Trypanosoma brucei, Argonaute1 (TbAGO1) is an essential component of the RNA interference (RNAi) pathway. While characterizing a TbAGO1 conditional knockout cell line, we discovered that, upon blockage of TbAGO1 transcription, the RNAi response to transfected double-stranded RNA was severely inhibited, although there was no change in the TbAGO1 protein level. This observation suggested that steady-state TbAGO1 was not sufficient to fully support the RNAi response to transfected dsRNA and implicated newly synthesized Argonaute in this phenomenon. Indeed, a translational blockade of TbAGO1 mRNA with an antisense morpholino oligonucleotide resulted in inhibition of the RNAi response, even though the steady-state level of TbAGO1 remained unchanged during the time of the assay. Thus, we concluded that in T. brucei, newly synthesized TbAGO1 is required to support an efficient RNAi response. We speculate that newly processed siRNAs may be preferentially loaded onto newly synthesized TbAGO1, and this mechanism may contribute to the homeostasis of the RNAi pathway.