Cryo- EM structure of ribosome from pathogenic protozoaEntamoeba histolytica,reveals unique features of its architecture

Entamoeba histolytica , an anaerobic parasite protozoan, is the causative agent of amoebiasis, the bloody diarrhea, and liver abscesses in humans. Amoebiasis is more predominant in tropical areas with poor sanitation conditions, and it remains the fourth leading cause of death due to a protozoan infection. E. histolytica life cycle spans between an infective ‘cyst stage’ and an active disease-causing ‘trophozoite stage’. We have determined cryo-EM structures of E. histolytica ribosomes, large subunit (LSU), 53S ribosome at 2.8 Å resolution and associated, 75S ribosome at 3.3 Å resolution, isolated from its trophozoite stage. The overall core of the ribosome is conserved. However, the periphery has evolved with Entamoeba specific unique features. The most notable features are, the presence of the rRNA triple helix near the peptide exit tunnel on LSU and the co-evolution of rRNA expansion segments and extensions in r-proteins. To the best of our knowledge, this structure reports the presence of RNA triple helix in the ribosome for the first time.