SIGNIFICANCE OF STRAND CONFIGURATION IN SELF-REPLICATING RNA MOLECULES

The kinetic theory of replication has been extended to include dual me chanisms for conversion of self-annealed single-strand RNA to double-s trand molecules, which do not replicate. An analysis of experimental r esults established that the replicate-template annealing reaction duri ng transcription significantly retarded replication in vitro among thr ee RNA variants copied by Q beta replicase. Annealing between compleme ntary RNA strands free in solution had far less significance. The find ing that an RNA variant can be replicated in a multiple hairpin config uration, but not as its single, long hairpin conformer, the correlatio n between stability of strand secondary structure and replicative fitn ess, and a lack of homology in the internal sequence of RNA variants c opied by Q beta replicase support the conclusion that template compete nce depends on strand configuration, independent of most of the underl ying base sequence. Occurrence of self-annealed strands in the Q beta replicase system was attributed to its reliance on RNA-driven strand s eparation, in the absence of enzyme catalysed strand unwinding. A 'con figuration before sequence' path to self-replication exhibited a subst antially lower combinatorial barrier than standard sequence-dependent evolution. RNA-dependent RNA synthesis in the Q beta system thus displ ays features of an RNA World and, interestingly, they reveal a rapid p ath for evolution of the first self-replicating molecule on Earth.