EVOLUTIONARY RELATIONSHIP OF UPTAKE SYSTEMS FOR BIOPOLYMERS IN ESCHERICHIA-COLI - CROSS-COMPLEMENTATION BETWEEN THE TONB EXBB EXBD AND THE TOLA TOLQ TOLR PROTEINS

Escherichia coli possesses two energy-coupled import systems through which substances of low concentration and of a size too large to permit diffusion through the porins are translocated across the outer membrane. Group B colicins, ferric siderophores and vitamin B-12 are taken up via the TonB-ExbB-ExbD, group A colicins via the TolA-TolQ-TolR system. Cross-complementation between the two systems was demonstrated in that tolQ tolR mutants transformed with plasmids carrying exbB exbD became sensitive to group A colicins, and exbB exbD mutants transformed with plasmid-encoded tolQ tolR became sensitive to group B colicins. TolQ-TolR interacted through TonB, and ExbB-ExbD interacted through TolA with the outer membrane receptors and colicins. Activity of ExbB ExbD via TolA was higher in cells lacking TonB, and activity of TolQ TolR via TonB was increased when TolA was missing. The very distinct TolA and TonB proteins mediate exclusive interaction with group A and group B receptors, respectively. ExbB-TolR and ExbD-TolQ mixtures showed little if any complementation of exbB exbD and tolQ tolR mutants indicating co-evolution of ExbB with ExbD and TolQ with TolR. Sequence homology and mutual functional substitution of ExbB-ExbD and TolQ-TolR suggest the evolution of the two import systems from a single import system.