Enzymatic RNA synthesis and RNase P. Evolutionary aspects.

TransferRNA recognition was used as leit-motiv in the illustration of possible links between a hypothetical primordial RNA world and the contemporary DNA world. In an RNA world, 'proto-tRNA' could have functioned as replication origin and as primitive telomere. Possibly, this primitive structure is preserved in a 'universal substrate' for modern tRNA-specific enzymes. The combination of acceptor stem and T arm (plus a linker) was finally revealed as sufficient for the recognition by prokaryotic and eukaryotic RNase P, as well as other tRNA enzymes. In modern life forms, a tRNA-like element in viral RNAs still serves as replication origin, and furthermore, the recognition of similar structures as cryptic promoters is universally conserved for template-dependent RNA polymerases. Another common property of modern polymerases is their high, but clearly limited and condition-dependent substrate specificity. Very likely, also substrate recognition by primitive polymerases was not more stringent, and this lead to the occurrence of mixed nucleic acids as intermediates in the transition of genomic RNA to contemporary genomic DNA.