ermC is an inducible antibiotic resistance gene from Staphylococcus aureus, one of several whose expression is regulated at the level of mRNA secondary structure. During induction of ermC, the inhibition of a ribosome active in translation of a short leader peptide by low levels of antibiotic belonging to the macrolide-lincosamide-streptogramin b family is believed to cause a rearrangement in mRNA secondary structure. The resultant conformational isomerization unmasks the methylase ribosome binding site and initiator Met codon, causing increased translation of the ermC transcript. Expression of ermC can also be demonstrated in Bacillus subtilis carrying plasmid pE194. To probe the ermC transcript in vivo during induction, ermC was transferred to B. subtilis by transformation and the resultant transformants were treated with dimethyl sulfate which reacts with N-1 of adenine and N-3 of cytosine residues in a manner that is sensitive to secondary structure. The bases modified in vivo were detected by primer extension with reverse transcriptase using total cellular RNA as template and a complementary ermC-specific oligonucleotide as primer. Physical evidence was obtained for the secondary structural rearrangements predicted by the ermC regulatory model. Additionally, physical evidence was obtained demonstrating that during induction, the stalled ribosome protects codons 9 and 10 of the leader peptide from modification by dimethyl sulfate, in agreement with genetic data obtained previously that identified the integrity of codons 5-9 as critical for induction of ermC by erythromycin.