Life clues: X-rays go deeper into origins

Vital signs

New hints at how life emerged from the "primordial soup" have been recovered by US chemists investigating the intramolecular communication within a large RNA-protein enzyme. X-ray data on the system, which is responsible for expressing the genetic code for the amino acid glutamine, point to a deeper origin for this aspect of biochemistry.

Annia Rodriguez and John Perona of the University of California Santa Barbara in attempting to decipher the way chemical communications take place within a large RNA-protein enzyme may have inadvertently glimpsed how the genetic coding of life may have emerged from its prebiotic origins.

Abiogenesis, or biopoesis, is the process by which life emerged through natural processes on the early earth from non-biological reaction systems. Many of the components are thought to have been present in the so-called primordial soup, but exactly how they organised and pieced themselves together to form the most primitive, self-replicating entities that ultimately evolved into unicellular and multicellular organisms remains a mystery. At some stage in the process, a genetic code that carried the necessary information to replicate those most primitive systems emerged. Today, as always it seems, life relies on chemicals "reading" this coded information held within living cells in the form of DNA to string together specific sequences of amino acids to form the proteins from which organisms are built. One of the key steps in this decoding process is the attachment of a particular amino acid to one end of the small messenger molecule transfer RNA (tRNA). This process is moderated by the enzyme aminoacyl-tRNA synthetase.

Surprising enzymic insights

Rodriguez found that separately removing seven different components, her car's "gears", from a distant part of the molecule each caused the amino acid to bond more tightly to the enzyme. The work provides the first systematic analysis to demonstrate that long-range communication occurs in an enzyme that depends on RNA for its function. "What we think is going on is that these enzyme-RNA interactions far from the amino acid binding site evolved together with the needs of the cell to respond to subtle cues from its environment - especially in terms of how much amino acid is available," Perona adds. "It [does] make sense in terms of evolution."

The diagram (top right) shows the crystal structure of glutaminyl tRNA synthetase (GlnRS, green) in complex with its substrate tRNA^Gln (yellow). The residues coloured blue depict favourable effects on the free energy of glutamine binding from mutation at this position. Red residues are disfavoured. The effects of mutation on the ability of GlnRS to catalyze amino acid attachment to the tRNA is shown in the right-hand panel, where all effects are unfavourable.

It might be argued that this research is quite far removed from the transition from pre-biotic chemistry to molecular biology that took place several billions years ago. It does, however, nudge our understanding back another small step towards that point as well as perhaps offering hope of new avenues of investigation in life's origin.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.