11 The RNP World

Abstract

Even at the dawn of the hypothetical RNA World, informational molecules and biocatalysts seem unlikely to have consisted of pure RNA, without proteinaceous helpers. After all, amino acids are formed much more easily than nucleotides in prebiotic simulation experiments, and amino acids are easily condensed into peptides. Thus, even long before the advent of information-directed protein synthesis, RNA would have functioned in concert with a host of random peptides present in its environment. The RNA World would in fact have been a primitive RNP (ribonucleoprotein) World. RNA would have evolved in the context of this background of peptides, some subset of which presumably stabilized the RNA and extended its catalytic versatility and potency.

The RNA–protein interactions we see in contemporary biological systems date from a more recent time—the advent of the translational apparatus, when RNA could reproducibly direct the synthesis of those proteins that best enhanced its function. Furthermore, instead of relying on short peptides, RNA could now make use of well-folded proteins. Importantly, because the proteins were now genetically encoded, they too were subject to evolution; the RNA and RNA-binding proteins began to coevolve.

The reader may have encountered protein–nucleic acid interactions primarily in the guise of proteins that bind double-stranded DNA. Relative to RNA, dsDNA has a very regular, predictable shape. Yes, it can be straight or bent, and in extreme cases can even adopt a left-handed conformation called Z-DNA. Mostly, however, it is a trustworthy right-handed double helix, with the phosphodiester backbones on the...