Nascent proteins progress through an elongated tunnel until theyexit from the ribosome. Biochemical, genetic and structural stud-ies have shown that the tunnel is not just a passive path, but alsohas regulatory properties. Erythromycin is a clinically usefulantibiotic that binds to an rRNA pocket in the entrance of theribosomal exit tunnel and interferes with the progression of nas-cent chains. Commonly, resistance to erythromycin is acquiredby alterations of rRNA nucleotides that interact with the drug.Mutations in theb-hairpin of ribosomal protein uL22, which israther distal to the erythromycin binding site, also generate resis-tance to the antibiotic. Interestingly, most of these mutations donot reduce the affinity of erythromycin to the bacterial ribosome.We have determined the crystal structures of the large ribosomalsubunit ofDeinococcus radioduranswith a three residue insertionmutation in uL22 that renders resistance to erythromycin in itsapo and erythromycin bound states. These structures reveal thattheb-hairpin of L22, which is naturally positioned on the tunnelwalls is shifted toward the center of the exit tunnel, triggering acascade of structural changes among rRNA nucleotides thatpropagates to erythromycin binding pocket and increases its flexi-bility. Based on our results, we suggest a feasble mechanism thatexplains how nanscent proteins can be translated when ery-thromycin is bound to the ribosome. Furthermore, our findingssupport recent studies showing that the interactions betweenuL22 and specific sequences within nascent chains trigger confor-mational rearrangements in the exit tunnel that are essential forthe translation of specific genes