The secondary structure is a fundamental feature of both non-coding RNAs (ncRNAs) and messenger RNAs (mRNAs). However, our understanding of the secondary structures of mRNAs, especially those of the coding regions, remains elusive, likely due to translation and the lack of RNA-binding proteins that sustain the consensus structure like those binding to ncRNAs. Indeed, mRNAs have recently been found to adopt diverse alternative structures, but the overall functional significance remains untested. We hereby approach this problem by estimating the folding specificity, i.e., the probability that a fragment of an mRNA folds back to the same partner once refolded. We show that the folding specificity of mRNAs is lower than that of ncRNAs and exhibits moderate evolutionary conservation. Notably, we find that specific rather than alternative folding is likely evolutionarily adaptive since specific folding is frequently associated with functionally important genes or sites within a gene. Additional analysis in combination with ribosome density suggests the ability to modulate ribosome movement as one potential functional advantage provided by specific folding. Our findings reveal a novel facet of the RNA structurome with important functional and evolutionary implications and indicate a potential method for distinguishing the mRNA secondary structures maintained by natural selection from molecular noise.

Secondary structure is a fundamental feature for both noncoding and messenger RNA.However, our understandings about the secondary structure of mRNA, especially for the coding regions, remain elusive, likely due to translation and the lack of RNA binding proteins that sustain the consensus structure, such as those bind to noncoding RNA. Indeed, mRNA has recently been found to bear pervasive alternative structures, whose overall evolutionary and functional significance remained untested.We hereby approached this problem by estimating folding specificity, the probability that a fragment of RNA folds back to the same partner once re-folded. We showed that folding specificity for mRNA is lower than noncoding RNA, and displays moderate evolutionary conservation between orthologs and between paralogs. More importantly, we found that specific rather than alternative folding is more likely evolutionarily adaptive, since it is more frequently associated with functionally important genes or sites within a gene. Additional analysis in combination with ribosome density suggests the capability of modulating ribosome movement as one potential functional advantage provided by specific folding. Our findings revealed a novel facet of RNA structome with important functional and evolutionary implications, and points to a potential way of disentangling mRNA secondary structures maintained by natural selection from molecular noise.