KEYA SEN * AND HIROSHI NIKAIDO

Deep rough mutants, which produce very defective lipopolysaccharides, are unable to export normal levels of porins into the outer membrane. In this study, we showed that lipopolysaccharides from such mutants were also unable to facilitate the trimerization, in vitro, of monomeric OmpF porin secreted by spheroplasts of Escherichia coli B/r. In contrast, lipopolysaccharides containing most or all of the core oligosaccharides were able to facilitate trimerization. Lipopolysaccharide (LPS), a unique component of the outer membrane of all gram-negative bacteria (13), is local-ized exclusively in the outer leaflet of this membrane (3, 12), forming an impermeable barrier in addition to imparting strong hydrophilicity to the surface. Earlier, it was found that deep rough strains of Salmonella typhimurium and Escherichia coli, in which LPS lacks much of the core oligosaccharide (especially one or both of the heptose resi-dues, see Fig. 1), had decreased amounts of major outer membrane proteins (1, 8), an effect that produces a disrupte