Inducible expression of several antibiotic resistance genes relies on recognition of an inducing antibiotic and a characteristic nascent peptide in the ribosome exit tunnel. Specific rRNA residues play the role of sensors that monitor the structure of the nascent peptide. Specific ribosomal sensors recognize the presence of the antibiotic. The signal from the peptide- and antibiotic-specific sensors is integrated and relayed from the exit tunnel to the ribosomal peptidyl transferase center via an allosteric change which likely also involves rRNA. Regulatory nascent peptides that control inducible expression of antibiotic resistance genes exhibit considerable variation in their structure. The requirement for the antibiotic cofactor depends on the structure of the peptide ' different peptides cooperate with different inducing antibiotics. This observation suggests that the sensory capacity of the tunnel can be optimized for recognition of different small molecules.