Fig. 1.

(A) Structures of the antibiotics used in this study. Clarithromycin is the 6-methoxy derivative of erythromycin A. (B) Comparison of the structures of ribosome-bound tylosin obtained by crystallography (thick lines; ref. 52) and modeling (thin lines; this study). The positions of the tylosin 5-mycaminose (blue) amino sugar and the 5-mycarose (green) and 23-mycinose (green) neutral sugars are color-coded as in A and can be seen to interact with A2058 (blue), U2506 (orange), and G748 (yellow), respectively. The 23-mycinose is positioned where it additionally interacts with A752 (red), protecting the base from chemical modification (34).