BIRTH OF THE SNORNPS - THE EVOLUTION OF THE MODIFICATION-GUIDE SNORNAS

Bacteria and eukaryotes adopt very different strategies to modify thei r rRNAs. Most sites of eukaryotic rRNA modification are selected by gu ide small nucleolar RNAs (snoRNAs), while bacteria rely on numerous si te-specific modification enzymes. This raises a 'chicken and egg' dile mma: how could a system of modification that requires a large number o f snoRNA cofactors have developed? Did it arise in a de novo fashion, or evolve from a pre-existing protein-based system? The rRNA sequences are well conserved in evolution, but the pattern of modification is o nly moderately conserved, and many more sites are modified in eukaryot es than in bacteria; why is this so? We propose a model for the origin s of the modification-guide snoRNAs that attempts to answer these ques tions.