Birth of the snoRNPs: the evolution of RNase MRP and the eukaryotic pre-rRNA-processing system.

The ribonucleoprotein particle RNase MRP is required for the processing of yeast pre-ribosomal RNA (pre-rRNA). A structurally related particle, RNase P, is universally required for processing of pre-tRNA, but in bacteria and archaea also cleaves a site in the pre-rRNA. This suggests that RNase MRP may have arisen in eukaryotes as a form of RNase P specialized for pre-rRNA processing. Other eukaryotic small nucleolar RNAs may have arisen as trans-acting factors that functionally replace cis-acting pre-rRNA interactions in bacteria and archaea.