There have been recent advances in the ribosomal synthesis of various molecules composed of nonnatural ribosomal substrates. However, the ribosome has strict limitations on substrates with elongated backbones. Here, we show an unexpected loophole in the E. coli translation system, based on a remarkable disparity in its selectivity for β-amino/hydroxy acids. We challenged β-hydroxypropionic acid (β-HPA), which is less nucleophilic than β- ...