Divergent rRNAs as regulators of gene expression at the ribosome level

It is generally assumed that each organism has evolved to possess a unique ribosomal RNA (rRNA) species optimal for its physiological needs. However, some organisms express divergent rRNAs, the functional roles of which remain unknown. Here, we show that ribosomes containing the most variable rRNAs, encoded by the rrnl operon (herein designated as I-ribosomes), direct the preferential translation of a subset of mRNAs in Vibrio vulnificus, enabling the rapid adaptation of bacteria to temperature and nutrient shifts. In addition, genetic and functional analyses of I-ribosomes and target mRNAs suggest that both I-ribosomal subunits are required for the preferential translation of specific mRNAs, the Shine-Dalgarno sequences of which do not play a critical role in I-ribosome binding. This study identifies genome-encoded divergent rRNAs as regulators of gene expression at the ribosome level, providing an additional level of regulation of gene expression in bacteria in response to environmental changes.