COMPETITION BETWEEN RIBOSOME AND SECA BINDING PROMOTES ESCHERICHIA-COLI SECA TRANSLATIONAL REGULATION

SecA protein, the protein translocation ATPase of Escherichia coli, au togenously regulates its translation during normal protein secretion b y binding to a secretion-responsive element located near the 5' end of its gene on geneX-secA mRNA. In order to characterize this autoregula tion further, RNA footprinting and primer-extension inhibition (toepri nting) studies were carried out with a segment of geneX-secA RNA, 30S ribosomal subunits and tRNA(f)(Met) along with purified SecA protein. The results show that ribosome and SecA-binding sites overlap, indicat ing that a simple competition for binding of geneX-secA mRNA presumabl y governs the translation initiation step. Further analysis showed tha t SecA protein was able to specifically dissociate a preformed 30S-tRN A(f)(Met)-geneX-secA RNA ternary complex as indicated by the disappear ance of its characteristic toeprint after SecA addition. These finding s are consistent with secA autoregulation, and they suggest a novel me chanism for the autoregulatory behavior of this complex protein.