How to fold and protect mitochondrial ribosomal RNA with fewer guanines

Mammalian mitochondrial ribosomes evolved from bacterial ribosomes by reduction of ribosomal RNAs, increase of ribosomal protein content, and loss of guanine nucleotides. Guanine is the base most sensitive to oxidative damage. By systematically comparing high-quality, small ribosomal subunit RNA sequence alignments and solved 3D ribosome structures from mammalian mitochondria and bacteria, we deduce rules for folding a complex RNA with the remaining guanines shielded from solvent. Almost all conserved guanines in both bacterial and mammalian mitochondrial ribosomal RNA form guanine-specific, local or long-range, RNA–RNA or RNA–protein interactions. Many solvent-exposed guanines conserved in bacteria are replaced in mammalian mitochondria by bases less sensitive to oxidation. New guanines, conserved only in the mitochondrial alignment, are strategically positioned at solvent inaccessible sites to stabilize the ribosomal RNA structure. New mitochondrial proteins substitute for truncated RNA helices, maintain mutual spatial orientations of helices, compensate for lost RNA–RNA interactions, reduce solvent accessibility of bases, and replace guanines conserved in bacteria by forming specific amino acid–RNA interactions.