Estimating the contributions of selection and self-organization in RNA secondary structure

In addition to characteristic structural properties imposed by evolutionary modification, evolved, single-stranded RNAs also display characteristic st ructural properties imposed by intrinsic physical constraints on RNA polyme r folding. The balance of intrinsic and functionally selected characters in the folded conformation of evolved secondary structures was determined by comparing the predicted secondary structures of evolved and unevolved (rand om) RNA sequences. Though evolved conformations are significantly more orde red than conformations of random-sequence RNA, this analysis demonstrates t hat the majority of conformational order within evolved structures results not from evolutionary optimization but from constraints imposed by rules in trinsic to RNA polymer folding.