Multicopy Suppression: an Approach to Understanding Intracellular Functioning of the Protein Export System

Escherichia coli genes were cloned onto a multicopy plasmid and selected by the ability to restore growth and protein export defects caused by a temperature-sensitive secY or secA mutation. When secASI was used as the primary mutation, only clones carrying groE, which specifies the chaperonin class of heat shock protein, were obtained. Selection using secY24 yielded three major classes of genes. The first class encodes another heat shock protein, HtpG; the most frequently obtained second class encodes a neutral histonelike protein, H-NS; and the third class, msyB, encodes a 124-residue protein of which 38 residues are acidic amino acids. Possible mechanisms of suppression as well as the significance and limitations of the multicopy suppression approach are discussed. Protein translocation across the Escherichia coli cytoplas-mic membrane is facilitated by proteinaceous factors. Re-cent progress in the biochemical dissection of the process has provided the following outlines. Secretory (periplasmic and outer membrane) proteins are initially synthesized as precursor molecules with N-terminal extensions, the leader or signal sequences. The SecB protein in the cytoplas