Role of ribosomal protein bS1 in orthogonal mRNA start codon selection

In many bacteria, the location of the mRNA start codon is determined by a short ribosome binding site sequence that base pairs with the 3′-end of 16S ribosomal RNA (rRNA) in the 30S subunit. Many groups have changed these short sequences, termed the Shine-Dalgarno (SD) sequence in the mRNA and the anti-Shine-Dalgarno (ASD) sequence in 16S rRNA, to create “orthogonal” ribosomes to enable the synthesis of orthogonal polymers in the presence of the endogenous translation machinery. However, orthogonal ribosomes are prone to SD-independent translation. Ribosomal protein bS1, which binds to the 30S ribosomal subunit, is thought to promote translation initiation by shuttling mRNA to the ribosome. Thus, a better understanding of how the SD and bS1 contribute to start codon selection could help efforts to improve the orthogonality of ribosomes. Here we engineered the Escherichia coli ribosome to prevent binding of bS1 to the 30S subunit, to separate the activity of bS1 binding to the ribosome from the role of the mRNA SD sequence in start codon selection. We find that ribosomes lacking bS1 are slightly less active than wild-type ribosomes in vitro . Furthermore, orthogonal 30S subunits lacking bS1 do not have improved orthogonality. Our findings suggest that mRNA features outside the SD sequence and independent of bS1 binding to the ribosome likely contribute to start codon selection and the lack of orthogonality of present orthogonal ribosomes.