The expansion segments on mammalian and bird ribosomes confer increased complexity and mass compared to prokaryotic ribosomes due to the extraordinarily long tentacles. These tentacles appear to be dynamic, switching between G-quadruplex and duplex conformations in response to association with proteins, ions, or other RNAs. Although their conformations remain to be fully resolved, in Homo sapiens they contain multiple sequences that form G-quadruplexes of unusually high stability in vitro. In metazoans, the integrated population of RNA G-quadruplexes may be dominated by rRNA, which makes up more than 80% of the cellular RNA and presents more expansive arrays of G-tracts. The phylogenetic conservation of G-quadruplex sequences in warm-blooded animals and their surface localization suggests conserved function of protein recruitment and polysome assembly. Here, using a more recent motif definition, we report additional G-quadruplex sequences on both the large (LSU) and small (SSU) subunits of the human ribosome. We identified G-quadruplex forming sequences in rRNA using the canonical 3 x 4 sequence motif and also using a relaxed 2 x 4 criterion which allowed the first evidence of G-quadruplex formation on the SSU. The G-quadruplexes in the LSU rRNA appear to be considerably more stable and more extensive than the G-quadruplexes of the SSU rRNA that are located mainly near the termini of expansion segments. Since the surfaces of both the subunits of the human ribosome contain a sufficiently large number of G-quadruplex forming regions, it seems possible in some environments G-quadruplexes might dominate interactions of ribosomes with other cellular components.
Mestre-Fos, S., Penev, P. I., Richards, J. C., Dean, W. L., Gray, R. D., Chaires, J. B., & Williams, L. D. (2019). Profusion of G-quadruplexes on both subunits of metazoan ribosomes. PloS one, 14(12), e0226177. https://doi.org/10.1371/journal.pone.0226177