Combinatorial peptide display on phage M13 protein pIII was used to discover peptide sequences that selectively bind to ErmC methyltransferase from Bacillus subtilis. One peptide, Ac-LSGVIAT-NH 2 , inhibited methylation in vitro with a 50% inhibitory concentration of 20 M. Interestingly, the set of six peptides which inhibited ErmC stimulated ErmSF, a homologous methyltransferase from Streptomyces fradiae. Thus, Ac-LSGVIAT-NH 2 may not act directly at the catalytic center of ErmC, but may modulate its activity by binding at a structurally unrelated, but functionally linked, site.The ErmC N-methyltransferase of Staphylococcus aureus and its close relative ErmCЈ from Bacillus subtilis confer resistance to erythromycin and related macrolide antibiotics. These enzymes act by specifically methylating a single adenine residue in the peptidyl transferase center of bacterial 23S rRNA (for reviews, see references 7 and 8). The same methylation confers coresistance to structurally unrelated lincosamide and streptogramin type B antibiotics. The three groups are collectively known as the macrolide-lincosamide-streptogramin B (MLS) antibiotics, and the form of resistance based on the Erm group of methyltransferases is found in a wide range of pathogens. The three-dimensional structures of the ErmAM and ErmCЈ methyltransferases have recently been solved by nuclear magnetic resonance (3, 9) and X-ray crystallography (1, 5), respectively. These developments bring us closer to rationally devising ligands that will selectively bind to Erm enzymes and possibly reduce the efficiency with which they confer MLS resistance.Macrolide antibiotics have served as a mainstay of antimicrobial therapy for approximately the last half century, especially in instances where the recipient of the antibiotic was allergic to beta-lactam antibiotics. Attempts have been made to discover Erm methyltransferase inhibitors that maintain the effectiveness of macrolide antibiotics in the face of the increasing frequency of resistant isolates (2, 3). The latter work (3) was part of a series of major structural studies of Erm methyltransferases (1, 5, 9).Nonpeptide ligands which displace an inhibitory peptide from its binding site on the enzyme might, themselves, have inhibitory activity. The use of inhibitory peptides might thus serve as a platform for the discovery of nonpeptide inhibitors of Erm enzymes. A proposed way to achieve this would be based on the discovery of test ligands to displace an inhibitory peptide from its association with its cognate Erm target. The displacement of an inhibitory ligand would also be easier to measure than methyltransferase activity.Large-scale screening of inhibitory ligands by a direct assay of methyltransferase catalytic activity is cumbersome since it requires the separation of product (methyl-labeled 23S rRNA) from substrate (unreacted S-adenosyl-L-methionine). We report the use of combinatorial phage display to discover peptides that inhibit ErmCЈ methylase activity and which might serve as displaceable ligands in ...