On an RNA-membrane protogenome
Selected ribonucleotide sequences bind well to zwitterionic phospholipid bilayer membranes, though randomized RNAs do not. There are no evident repeated sequences in selected membrane binding RNAs. This implies small and varied motifs responsible for membrane affinity. Such subsequences have been partially defined. Bound RNAs require divalents like Mg2+ and/or Ca2+, preferring more ordered phospholipids: gel, ripple or rafted membranes, in that order. RNAs also bind and stabilize bilayers that are bent or sharply deformed. In contrast, RNA binding without divalents extends to negatively charged membranes formed from simpler anionic phospholipids, and to plausibly prebiotic fatty acid bilayers. RNA-membranes also retain RNA function, such as base pairing, passive transport of tryptophan, specific affinity for peptide side chains, like arginine, and catalysis by ribozymic ligase. Multiple membrane-bound RNAs with biochemical functions, linked by specific base-pairing are readily constructed. Given these experimental facts, genetic effects seem plausible. RNA functions often reside in few nucleotides, and are easily joined in a small RNA. Base-paired groups of these can evolve to be purposeful, joining related RNA functions. Such RNA groups permit complex genome functions, but require only replication of short RNAs. RNA-membranes facilitate accurate RNA segregation at cell division, and quickly evolve by appending new base-paired functions. Thus, ancient RNA-membranes could act as a protogenome, supporting orderly encoded RNA expression, inheritance and evolution before DNA and the DNA genome.