Phenotypic traits and regulatory role of RNA folding in molecular selection.
We concentrate on instances in which the phenotypic expression of information encoded in an RNA primary sequence might be revealed by the folding of the RNA itself. We have discovered that this situation finds concrete realization in the design of RNA molecules capable of maximizing the rate of autocatalytic synthesis when incubated with viral Q beta-replicase. This requires that we introduce the notion of phenotypic traits at the molecular level. Thus, the problem of finding RNA sequences whose phenotype favorably influences propagation amounts go finding RNA sequences which fold so as to optimize enzymatic performance and are in addition endowed with the proper recognition sites. The proof that these two problems are indeed equivalent has two steps: First we predict the metastable folded structures formed as a template RNA chain grows by sequential incorporation of nucleotides. The transient folded states appear to be involved in the regulation of the enzyme activity and they occur in a manner which is "oblivious" of thermodynamic time scales. Secondly, we compute the time-dependent activation energy for relaxation of each intermediate structure. This is done to establish constraints necessary for optimization of the regulatory role of RNA folding. The search for prospective template sequences is subject to such constraints. Our results aim at elucidating an optimization process realized by molecular selection in de novo (template-free) RNA synthesis by Q beta-replicase. We argue that the phenotype which mediates selection is given by metastable folding which emerges together with the printing of the genotype, that is, within the time span of a replication turnover.