Erythromycin B: conformational analysis and antibacterial activity

Erythromycin B, an acid-stable co-metabolite of the important antibiotic erythromycin A, differs from erythromycin A only in the absence of a hydroxyl group at C12, yet it has never been licensed for clinical use. We describe an NMR-based analysis of the conformation of erythromycin B, both free in aqueous solution and when weakly bound to bacterial ribosomes and show that it is conformationally similar to erythromycin A. The antibacterial activity of erythromycin B is shown to be similar to that of erythromycin A, but after acid-treatment, resembling exposure to the stomach, erythromycin B substantially retains antibacterial activity, whereas erythromycin A does not

PSRP1 Is Not a Ribosomal Protein, but a Ribosome-binding Factor That Is Recycled by the Ribosome-recycling Factor (RRF) and Elongation Factor G (EF-G)*

Plastid-specific ribosomal proteins (PSRPs) have been proposed to play roles in the light-dependent regulation of chloroplast translation. Here we demonstrate that PSRP1 is not a bona fide ribosomal protein, but rather a functional homologue of the Escherichia coli cold-shock protein pY. Three-dimensional Cryo-electron microscopic (Cryo-EM) reconstructions reveal that, like pY, PSRP1 binds within the intersubunit space of the 70S ribosome, at a site overlapping the positions of mRNA and A- and P-site tRNAs. PSRP1 induces conformational changes within ribosomal components that comprise several intersubunit bridges, including bridge B2a, thereby stabilizes the ribosome against dissociation. We find that the presence of PSRP1/pY lowers the binding of tRNA to the ribosome. Furthermore, similarly to tRNAs, PSRP1/pY is recycled from the ribosome by the concerted action of the ribosome-recycling factor (RRF) and elongation factor G (EF-G). These results suggest a novel function for EF-G and RRF in the post-stress return of PSRP1/pY-inactivated ribosomes to the actively translating pool