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 different 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 retrotransposition 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.Key Words: Entamoeba histolytica; Circular Non Coding RNA; Ribosomal RNA Spacer; Ehsine; Non LTR Retrotransposon; SINE Recombination IntroductionEntamoeba histolytica, a protozoan parasite, is classically defined as the causative agent of the disease Amoebiasis. It resides in the human colon along with a number of nonpathogenic species of Entamoeba. The estimates available world-wide state that 500 million persons are infected with Entamoeba spp., with about 50 million cases of invasive amoebiasis per year; resulting in about 100,000 deaths (WHO/PAHO/UNESCO report, 1997). The parasite is directly transmitted through the faecal-oral route without the need of a vector system. It therefore flourishes where humans live in crowded conditions with poor sanitation. It continues to be a major public 432 Sudha Bhattacharya health problem in India due to recurrent infections caused by the high parasite load in our environment, and lack of effective immunity in infected individuals. The morbidity caused by the intestinal form of the disease results in great economic loss, although mortality is reduced, thanks to the freely-available and over-used anti-amoebic drug, metronidazole (Upcroft and Upcroft, 2001). The parasite also invades the liver and other organs, causing abscesses ...