8 On the Origin of the Ribosome: Coevolution of Subdomains

Translation is one of the most complicated of biological processes, in-volving literally hundreds of specific macromolecules. Not the least of this complexity is the structure of the ribosome itself, which even in the relatively simple Escherichia coli version, consists of over 50 different proteins and three RNA molecules, comprising more than 4500 nucleo-tides, giving an aggregate mass of around 2.5 million daltons (Hill et al. 1990; Matheson et al. 1995). The difficulty of imagining how such a structure evolved is eased somewhat by accepting the notion that the orig-inal ribosome was made solely of RNA, as tentatively suggested by Crick (1968) more than two decades ago, and asserted with increasing force (Woese 1980) and enthusiasm (Gesteland and Atkins 1993) since then. Adherents of RNA World scenarios imagine, to varying degrees, that something resembling protein synthesis was carried out by ribozyme-like proto-ribosomes prior to the advent of ribosomal proteins and translation factors (not to mention aminoacyl-tRNA synthetases). Such scenarios solve the chicken-or-the-egg problem of the molecular evolution of ribo-somes but raise some difficult new questions. The most obvious is that rRNA itself has a vast and intricate structure containing thousands of nu-cleotides (see, e.g., Fig. 1), and is not likely to have evolved by chance in a few simple evolutionary steps. Compounding this problem is that, prior to the existence of protein synthesis, it is difficult to imagine what selec-tive pressures could have driven its evolution; in other words, the RNA World could not have anticipated the invention of protein synthesis. More likely, it evolved from preexisting RNA functions. The questions are, what were these functions, and how could simpler, primitive RNAs have evolved into the vast, complex macromolecular machine that is our pres-ent-day ribosome