r- spacer, is ribose counterpart of the d-spacer. R-spacer is also known as abasic site. In nature, RNA abasic sites occur less commonly than DNA abasic sites but they are considerably more stable. This feature might have an impact on the half-life tRNAs and rRNAs, though this point is still debated. Experimentally, introduction of an r-spacer in the hammerhead ribozyme was shown to compromise ribozyme activity.
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P.A. Kupfer, and C.J. Leumann. The chemical stability of abasic RNA compared to abasic DNA. Nucleic Acids Res. (2007), 35: 58-68.
A. Peracchi, L. Beigelman, N. Usman, D. Herschlag. Rescue of abasic hammerhead ribozymes by exogenous addition of specific bases. Proc. Natl. Acad. Sci. USA. (1996), 93: 11522-11527.