Circular Noncoding Ribosomal Transcripts and Recombination of Retrotransposons: Two Charming Secrets Revealed by a Less-Explored Human Parasite

Abstract

Parasites, through their diverse cellular metabolic pathways, help us to appreciate the dif ferent directions in which evolution works, while maintaining a unified theme. To illustrate this I describe our recent observations with two basic processes, namely regulation of ribosomal RNA transcription, and the biology of retrotranspos ition in Entamoeba histolytica, a highly prevalent protozoan parasite that causes amebiasis. Ribosomal RNA synthesis is generally tightly regulated in response to growth rate such that cells subjected to growth stress shut down their rRNA transcription. In E. histolytica we observed that upon growth stress, rRNA synthesis did not shut down. Instead, unprocessed pre-rRNA accumulated to high levels along with a novel class of circular RNAs derived from the 5’-external transcribed spacer (etsRNA). The etsRNA can self circularize in vitro, a property not previously known in spacer RNAs. We hypothesize that circular etsRNAs would escape exonucleolytic decay and inhibit pre-rRNA processing, possibly by titrating away the processing factors which normally bind to them. In the study on retrotransposition of non-long terminal repeat retrotransposons (EhLINEs/SINEs), we successfully mobilized EhSINE in a cell-line made retrotransposition-competent by transfection with multiple constructs to express the polypeptides required for retrotransposition, a first for any protozoan parasite. While tracking retrotransposition of a marked SINE copy we found that the newly retrotransposed SINEs had undergone high-frequency recombination, presumably due to the known ability of reverse transcriptase to perform template jumping. Such recombination has not been reported for retrotransposons, and may be important in generating sequence polymorphism.