The ermC mRNA leader segment, which encodes a 19 amino acid leader peptide, MGIFSIFVISTVHYQPNKK, plays a key role in regulating expression of the ErmC methylase. The contribution of specific leader peptide amino acid residues to induction of ermC was studied using a model system in which the ErmC methylase was translationally fused to Escherichia coli beta-galactosidase as indicator gene. Codons of the ermC leader peptide were altered systematically by replacement of leader DNA segments with double-stranded DNA constructed from chemically synthesized oligonucleotides. Missense mutations that resulted in reduced efficiency of induction involved codons for amino acid residues 5 to 9 (-SIFVI-). Nonsense mutations causing termination of the leader peptide at codons 10 (-S-) or 12 (-V-) remained inducible. These findings suggest that the codons for residues 5 to 9 of the leader peptide comprise the critical region in which ribosomes stall in the presence of erythromycin.