The molecular machines known as ribosomes translate RNA’s sequence of nucleotides into a protein’s corresponding sequence of amino acids. Once a protein is made, the information encoded in its sequence can’t make it back to RNA or DNA. That one-way flow presents a problem: Given that ribosomes are made in part of protein, how did they ever evolve?
In 1962 Alexander Rich proposed an answer: The earliest self-replicating molecules were made of RNA; DNA and proteins came later. In 1986 Walter Gilbert developed Rich’s idea and dubbed it “RNA World.” Even the smallest RNA molecule in our bodies (from the ribosome’s 5S subunit) contains about 120 nucleotides and clocks in at about 40 kilodaltons. The spontaneous formation of such a molecule might seem implausible. Nevertheless, lab experiments under conditions thought to have prevailed when life began have shown that you can make RNA from such simple molecules as cyanoacetylene (HCCCN), cyanamide (NH2CN), glycolaldehyde (HOCH2CHO), urea (NH2CONH2), and hydroxylamine (NH2OH).
Astronomer Víctor M. Rivilla of the Astrobiology Center in Madrid, Spain, noticed that all but one of those RNA precursors, hydroxylamine, have been observed in the interstellar medium. He and his colleagues set out to find hydroxylamine using the 30-meter IRAM radio telescope (shown above). Situated in Spain’s highest mountain range, Sierra Nevada, the single-dish telescope can readily observe a molecule’s characteristic quantized rotational transitions, which occur in the millimeter bands.
As their hunting ground, Rivilla and his colleagues chose the molecular cloud G+0.693-0.027 in the constellation Sagittarius. Earlier this year, a different biologically significant molecule, propargylimine (HCCCHNH), was discovered in the same cloud. The spectra observed at IRAM were crowded with thousands of emission lines from multiple molecular species. Picking out hydroxylamine or any other species of similar size is possible because their quantized rotational transitions produce photons at known frequencies.
RNA is made up of the purine-derived nucleotides adenine and guanine and the pyrimidine-derived nucleotides cytosine and uracil. Last year, Thomas Carell of the Ludwig-Maximilians University Munich and his collaborators presented a plausible set of reaction pathways to synthesize both types of nucleotides under prebiotic conditions. Hydroxylamine was added ready-made to the mix, although how the molecule could form on early Earth is unclear. Carell and his collaborators devised and demonstrated a pathway that involves partial reduction of NO2– ions by HSO3– ions, which form when sulfur dioxide belched from volcanoes reacts with water.
The work of Rivilla and his colleagues suggests an alternative way of supplying early Earth with hydroxylamine. Several pathways exist for making the molecule on the surfaces of dust grains at low temperature—conditions that prevail in G+0.693-0.027 and other molecular clouds. Those grains could eventually become incorporated into meteorites and comets, which could carry hydroxylamine to Earth. The notion is not far-fetched. Last year, Yoshihiro Furukawa of Tohoku University in Japan and his collaborators reported the detection of ribose and other bio-essential sugars in two carbon-rich meteorites. Ribose is a component of RNA. (V. M. Rivilla et al., Astrophys. J. Lett. 899, L28, 2020.)