Normal precursors of periplasmic proteins accumulated in the cytoplasm are not exported post‐translationally in Escherichia coli
Résumé
Hyperproduction of phosphate‐binding protein, PhoS, in strains carrying a multicopy plasmic containing the phoS gene, resulted in saturation of export sites. As a consequence, pre‐PhoS was accumulated both in the inner membrane and in the cytoplasm. This was evidenced both in electron‐microscopy and after cell fractionation. Only the membrane‐associated precursor could be matured and exported. The signal sequence of the cytoplasmic pre‐PhoS was slowly degraded. It was first cleaved about in its middle and then completely removed. Electron microscope studies demonstrated that the cytoplasmic pre‐PhoS cannot be exported post‐translationally.