Speculations on the early course of evolution.

Abstract

The proposal that RNA preceded DNA in evolution is more than 15 years old. In light of recent studies on RNA processing (including protein-free reactions), present knowledge about eukaryotic gene structure, and studies comparing ribosomal RNA sequences, we propose a train of events for precellular and early cellular evolution.

Multiple self-splicing introns in the 16S rRNA genes of giant sulfur bacteria

“…This hypothesis is based on the assumption that introns might persist longer in organisms that spend only a small fraction of their energy on replication (60). This energy surplus could be produced either by mitochondria in eukaryotes (60) or by the storage of tremendous amounts of energy reserves as in the group of large sulfur bacteria (16,18,19,61). The replication, transcription, and excision of introns may thus not represent a major bioenergentic expense for large sulfur bacteria.…”

The Persistent Contributions of RNA to Eukaryotic Gen(om)e Architecture and Cellular Function

“…Processing signals might have been placed, in part, between the fragments corresponding to mature RNAs, presumably the origin of intergenic or even intronic space (Brosius 2003c). With the advent of templated protein biosynthesis, a major evolutionary transition included co-option of existing functional RNAs as well as longer "chromosomal RNAs" by RNA cutting and pasting to stitch together messenger RNAs (mRNAs) with open and increasingly longer reading frames, which might suggest a very early origin of RNA splicing in a ribonucleoprotein world (RNP world) (Reanney 1974;Darnell and Doolittle 1986). Alternative splicing and other rearrangements would be one of the mechanisms to enhance variation for generating translation products out of a limited repertoire of functional RNAs also doubling as templates for translation (Brosius 2001).…”

“…Linear arrangement of RNA genes, as well as the transition to the RNP world evolving an ancestor of the reverse transcriptase enzyme, happened to constitute a useful precondition for a next major evolutionary transition, namely, the conversion of RNA to DNA, the latter merely serving as bookkeeper (Darnell and Doolittle 1986;Gould 2002). For this and other reasons, the central dogma of biology could be revisited or supplemented by a different graphic account, both in appreciation of the major significance of RNA in the cell and the chronological order of the major transitions ( Fig.…”

The Unusual Phylogenetic Distribution of Retrotransposons: A Hypothesis

“…The first is theoretical and is based on the now widely accepted proposal that an RNA world preceded the form of biology with which we are familiar, the DNA world. Darnell first articulated that RT must have been present during the time of the transition between these two worlds, and therefore, must be considered ancient (Darnell and Doolittle 1986;Fig. 1).…”

“…The resulting DNA, together with the integrase protein (processed previously by an element-encoded protease from the RT precursor protein Gag-Pol) Figure 1 An ancient origin for reverse transcriptase. An early origin of a cellular reverse transcriptase is posited by the RNA world hypothesis (Darnell and Doolittle 1986) (left). The widespread existence of RT genes in prokaryotes, in eukaryotes, and their great molecular diversity (represented by the size of the boxes at the tip of each branch) also suggests an early origin for the RT gene.…”