In vivo selection of functional ribosomes with variations in the rRNA-binding site of Escherichia coli ribosomal protein S8: Evolutionary implications

The highly conserved nature of rRNA sequences throughout evolution allows t hese molecules to be used to build philogenic trees of different species, I t is unknown whether the stability of specific interactions and structural features of rRNA reflects an optimal adaptation to a functional task or an evolutionary trap. In the work reported here, we have applied an in vivo se lection strategy to demonstrate that unnatural sequences do work as a funct ional replacement of the highly conserved binding site of ribosomal protein S8. However, growth competition experiments performed between Escherichia coil isolates containing natural and unnatural S8-binding sites showed that the fate of each isolate depended on the growth condition. In exponentiall y growing cells, one unnatural Variant was found to be equivalent to wild t ype in competition experiments performed in rich media. In culture conditio ns leading to slow growth, however, cells containing the wild-type sequence were the ultimate winner of the competition, emphasizing that the wild-typ e sequence is, in fact, the most fit solution for the S8-binding site.