Made of a large lactone ring and binds to the l15 protein and 23s rna at the peptidyltransferase cavity to prevent peptide bond formation?

This pertains to the antibiotic erythromycin from the macrolide family of antibiotics. Erythromycin has the characteristic large lactone ring in its structure and binds mainly to the 23S subunit (in the 50s subunit) in bacterial ribosome therefore blocking the peptide bond formation in bacteria and stopping protein synthesis. Once protein synthesis in bacteria is stopped, growth is halted as erythromycin is a bacteriostatic drug. 

Erythromycin covers most gram positive bacterial infection, usually in the setting of penicillin and cephalosporin allergy in humans.