An unbiased adaptive sampling algorithm for the exploration of RNA mutational landscapes under evolutionary pressure

International audienceThe analysis of the relationship between sequences and structures (i.e. how mutations aect structures and reciprocally how structures in uence mutations) is essential to decipher the principles driving molecular evolution, to infer the origins of genetic diseases or to develop bioengineering applications such as the design of articial molecules. Because their structures can be predicted from the sequence data only, RNA molecules provide a good framework to study this sequence-structure relationship. We recently introduced a suite of algorithms called RNAmutants which allows, for the rst time, a complete exploration of RNA sequence-structure maps in polynomial time and space. Formally, RNAmutants takes an input sequence (or seed) to compute the Boltzmann weighted ensembles of mutants with exactly k mutations, and sample mutations from these ensembles. However, this approach suers from major limitations. Indeed, since the Boltzmann probabilities of the mutations depend of the free energy of the structures, RNAmutants has diculties to sample mutant sequences with low G+C-contents. In this paper we introduce a novel unbiased adaptive sampling algorithm that enables RNAmutants to sample regions of the mutational landscape poorly covered by classical algorithms. We applied these methods to sample mutations with low G+C-contents. These adaptive sampling techniques can be easily adapted to explore other regions of the sequence and structural landscapes which are dif- cult to sample. Importantly, these algorithms come at a minimal computational cost. We demonstrate the insights oered by these techniques on studies of complete RNA sequence structures maps of sizes up to 40 nucleotides. Our results indicate that the G+C-content has a strong in uence on the size and shape of the evolutionary accessible sequence and structural spaces. In particular, we show that low G+C-contents favor the apparition of internal loops and thus possibly the synthesis of tertiary structure motifs. On the other hand, high G+C-contents signicantly reduce the size of the evolutionary accessible mutational landscapes