We have recently been investigating the regulation of the expression of SecA, a cytoplasmic ATPase that plays a major role in the translocation of proteins through the SecYEG complex in E. coli. The gene that encodes SecA resides just downstream of the gene that encodes SecM, and the two genes form a single operon. When secretion is impaired, a 17 amino acid motif near the C-terminus of SecM induces a translation arrest from within the ribosome tunnel. As a consequence, the structure of the mRNA is altered and SecA is translated more efficiently. We have used a novel application of fluorescence resonance energy transfer (FRET) to gain insight into the mechanism of SecM translation arrest. We found that the SecM C-terminus adopts a compact conformation upon synthesis of the arrest motif. This conformational change does not occur spontaneously, but rather is induced by the ribosome. Translation arrest requires both compaction of the SecM C-terminus and the presence of key residues in the arrest motif. Further analysis revealed that the ribosome also undergoes a conformational change during translation arrest that leads to the stabilization of the SecM peptidyl-tRNA. Based on these results, we have proposed that translation arrest results from a series of reciprocal interactions between the ribosome and the C-terminus of the nascent SecM polypeptide.
