Fig 3.

Mechanism of inducible (A) and constitutive (B) MLS resistance. (C) Secondary structure prediction for a region upstream of ermC in S. haemolyticus strain C. (A) ermC mRNA was in an inactive conformation (upper portion of the panel) due to the structure of its 5ʹ end, which had a set of four inverted repeats paired as hairpin structure 1:2 and 3:4 that stalled the ribosomes and sequestered the initiation sequences for ermC. Thus, the methylase could not be synthesized because the initiation motifs for translation were not accessible to ribosomes. In the presence of inducer macrolides, the antibiotic (white diamond) first bound to the ribosome, dramatically slowing down the ribosome’s speed and enabling translation of the leader peptide upstream of ermC. This induced the destabilization of the two hairpin structures and favored the association between inverted repeats 2 and 3 to form a new hairpin structure 2:3, which uncovered the initiation sequences that increase the translation efficiency of ermC. (B) The variations in the leader peptide decreased the stability of the hairpin structure and render ermC available for translation, resulting in constitutive MLS resistance. (C) In S. haemolyticus strain C, the deletion of the leader peptide caused the other inverted repeats to pair as hairpin structure 2:3 or 3:4. The minimum free energy of hairpin structure 2:3 was lower than 3:4, indicating that the secondary structure upstream of ermC was more likely to form as hairpin structure 2:3, which would expose initiation sequences for translation.