Expansion segments (ES) are insertions of a few to hundreds of nucleotides at discrete locations on eukaryotic ribosomal RNA (rRNA) chains. Some cluster around 'hot spots' involved in translation regulation and some may participate in biogenesis. Whether ES play the same roles in different organisms is currently unclear, especially since their size may vary dramatically from one species to another and very little is known about their functions. Most likely, ES variation is linked to adaptation to a particular environment. In this chapter, we compare the interaction networks of ES from four kinetoplastid parasites, which have evolved in diverse insect vectors and mammalian hosts: Trypanosoma cruzi, Trypanosoma brucei, Leishmania donovani and Leishmania major. Here, we comparatively analyze ribosome structures from these representative kinetoplastids and ascertain meaningful structural differences from mammalian ribosomes. We base our analysis on sequence alignments and three-dimensional structures of 80S ribosomes solved by cryo-electron microscopy (cryo-EM). Striking differences in size are observed between ribosomes of different parasites, indicating that not all ES are expanded equally. Larger ES are not always matched by large surrounding ES or proteins extensions in their vicinity, a particularity that may lead to clues about their biological function. ES display different species-specific patterns of conservation, which underscore the density of their interaction network at the surface of the ribosome. Making sense of the conservation patterns of ES is part of a global effort to lay the basis for functional studies aimed at discovering unique kinetoplastid-specific sites suitable for therapeutic applications against these human and often animal pathogens.
Keywords: Eukaryotic translation; Expansion segment; Kinetoplastid parasite; Ribosomal RNA; Ribosome structure.