catalysts might, at one time, have been common.

I believe that the genetic code was the greatest discovery of the twentieth century. When I saw the recent paper by Reiter et al. on the crystal structure of the P complex, my thoughts went to the origin of the genetic code and to the path from the RNA world to the protein world. As reviewed by Kazantsev and Pace, RNase P is an ancient universal ribozyme that cleaves the 5leader sequence of newly synthesized tRNAs, which harbour the nucleotide triplets of the genetic code, to generate mature tRNAs. Although the discovery of RNA catalysis made plausible the idea of an early RNA era, much of modern biology emanates from protein catalysts. However, somewhere between the RNA world and the contemporary theatre of proteins, a transition through ribonucleoprotein (RNP) catalysts, which include RNase P, can be imagined.

The aminoacylations of tRNAs establish the rules of the genetic code. Because Xiao et al., among others, have demonstrated that tRNA-specific aminoacylations can be generated by laboratory-created ribozymes, RNAs could in principle provide the origins of a code for peptide synthesis. Advantageously, peptides can coat and protect RNA structures that are vulnerable to hydrolysis and unfolding. Indeed, RNase P has long been known as a catalytic RNA that is covered, in part, by a small protein cofactor. But the identity and interactions of the RNA's active site, and the particulars of the protein's role, were not clear. The wonder of seeing the active site of this ribozyme contacting the bound tRNA in a universally conserved way, and of the protein fitting onto and protecting a conserved structural motif in the catalytic RNA, reinforces the possibility that catalysts might, at one time, have been common. My guess is that this structure is a prototype from that transitional world.