Prebiotic Iron Originates the Peptidyl Transfer Origin

The ribosome is responsible for protein synthesis in all living organisms. It is best known to exist around 3.5-3.7 Ga whereat life on Earth inhabited anoxic environment with abundant soluble irons. The RNAs and proteins are the two biopolymers that constitute the ribosome. However, both proteins and RNAs require metal cations to fold and to function. There are four Mg-microcluster (Mg2+-μc) structures conserved in core of large subunit, and the 23S ribosomal RNA (rRNA) was shown to catalyze electron transfer in an anoxic environment in the presence of Fe2+. The Mg2+-μc features two idiosyncratic Mg2+ ions that are chelated and bridged by a common phosphate group and along with that, the adjacent residues of RNA backbone together forming ten-membered chelation ring(s). Here, we utilized four rRNA fragments of the large subunit 23S rRNA of Haloarcula marismortui, that includes the residues that form the four Mg2+-μc's. These four rRNA fragments are shown competent to assemble with Mg2+. Our results show that when these rRNA fragments fold or assembly in the presence of Fe2+ under anoxic conditions, each Fe2+-microcluster can catalyze electron transfer. We propose that Fe2+-microclusters of the ribosome, which use Fe2+ as a cofactor to regulate electron transfer, are pivotal and primordial and may be an origin in evolution of the ribosome.