ENZYMATIC RNA-SYNTHESIS AND RNASE-P - EVOLUTIONARY ASPECTS

Transfer RNA recognition was used as leit-motiv in the illustration of possible links between a hypothetical primordial RNA world and the co ntemporary DNA world. In an RNA world, 'proto- tRNA' could have functi oned as replication origin and as primitive telomere. Possibly, this p rimitive structure is preserved in a 'universal substrate' for modern tRNA-specific enzymes. The combination of acceptor stem and T arm (plu s 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 r eplication origin, and furthermore, the recognition of similar structu res as cryptic promoters is universally conserved for template-depende nt RNA polymerases. Another common property of modern polymerases is t heir high, but clearly limited and condition-dependent substrate speci ficity. Very likely, also substrate recognition by primitive polymeras es was not more stringent, and this lead to the ocurrence of mixed nuc leic acids as intermediates in the transition from genomic RNA to cont emporary genomic DNA.