Control of SecA and SecM translation by protein secretion.

SecA, the protein translocation ATPase of E. coli is subject to secretion-defect-response control. SecM (secretion monitor) encoded by the 5' region of the secM-secA mRNA is involved in this regulation. SecM translation is subject to transient elongation arrest at Pro166, which is prolonged when export of the nascent SecM is blocked. An "arrest sequence", FXXXXWIXXXXGIRAGP, was identified at a carboxy-terminal region of SecM that interacts with the ribosomal exit tunnel. Presumably, the stalled ribosome disrupts the secondary structure of the secM-secA mRNA such that the Shine-Dalgarno sequence for translation of secA is exposed. Mutation studies established that the SecM elongation arrest is required for the viability of E. coli as well as for constitutive (in secretion-proficient cells) and upregulated (in secretion compromised cells) expression of SecA. Furthermore, evidence suggests that elongation-arresting SecM has a role of upregulating the functionality of newly synthesized SecA molecules, presumably by bringing the mRNA to the vicinity of the membrane/Sec translocation apparatus. These results are discussed in relation to the versatile nature of SecA in its localization and structure.