Precise localization of an overproduced periplasmic protein in Escherichia coli: use of double immuno‐gold labelling

The subcellular localization of beta‐lactamase produced by a secretion‐cloning vector pIN‐III was studied by immunolabelling of frozen thin sections of Escherichia coli. Using double immuno‐gold detections and internal reference proteins, it is shown here that beta‐lactamase encoded by this vector can be exported and that its overproduction leads to aggregation within the periplasm. This aggregation induces the appearance of electron‐dense areas immunolabelled by the antiserum directed against the beta‐lactamase at the external side of the cytoplasmic membrane. The overproduced enzyme is also secreted to the medium in vesicles budding from the outer membrane of lpp strains.