The ribosomal exit tunnel functions as a discriminating gate.

Translation of SecM stalls unless its N-terminal part is "pulled" by the protein export machinery. Here we show that the sequence motif FXXXXWIXXXXGIRAGP that includes a specific arrest point (Pro) causes elongation arrest within the ribosome. Mutations that bypass the elongation arrest were isolated in 23S rRNA and L22 r protein. Such suppressor mutations occurred at a few specific residues of these components, which all face the narrowest constriction of the ribosomal exit tunnel. Thus, we suggest that this region of the exit tunnel interacts with nascent translation products and functions as a discriminating gate.