The RNA world: receptors and their cognate ligands in Archaea, Bacteria, and Choanoflagellates

Abstract For life to exist it is necessary to have two essential classes of molecules in a primitive cell. One is RNA, a molecule that can catalyze its own replication. The other is a fatty acid, which can form a bilayer and also divide. Archaea, the ancestors of all species, contained receptors that carried out numerous important functions. An Archaea, Halobacterium halobium , contains a plasma membrane receptor, Bacteriorhodopsin. It uses photons to open a channel allowing H + ions to escape from the cell; thereby producing the universal energy molecule, ATP. Other roles played by receptors in protists (unicellular organisms) are to control the direction of flagella movement to escape from prey, and to form multilayered colonies called biofilms. Choanoflagellates, the ancestors of all animal species, contain collar cells, homologous to similar cells in sponges and other simple multicellular organisms. They also contain 10 G-protein coupled receptors while simple multicellular species contain over 100.