Energy-coupled colicin transport through the outer membrane of Escherichia coli K-12: mutated TonB proteins alter receptor activities and colicin uptake

Abstract

The current model of TonB‐dependent colicin transport through the outer membrane of Escherichia coli proposes initial binding to receptor proteins, vectorial release from the receptors and uptake into the periplasm from where the colicins, according to their action, insert into the cytoplasmic membrane or enter the cytoplasm. The uptake is energy‐dependent and the TonB protein interacts with the receptors as well as with the colicins. In this paper we have studied the uptake of colicins B and Ia, both pore‐forming colicins, into various tonB point mutants. Colicin Ia resistance of the tonB mutant (G186D, R204H) was consistent with a defective Cir receptor‐TonB interaction while colicin Ia resistance of E. coli expressing TonB of Serratia marcescens, or TonB of E. coli carrying a C‐terminal fragment of the S. marcescens TonB, seemed to be caused by an impaired colicin Ia‐TonB interaction. In contrast, E. coli tonB (G174R, V178I) was sensitive to colicin Ia and resistant to colicin B unless TonB, ExbB and ExbD were overproduced which resulted in colicin B sensitivity. The differential effects of tonB mutations indicate differences in the interaction of TonB with receptors and colicins.

In Vivo Evidence for TonB Dimerization

“…TonB(G186S) conferred full sensitivity to phages T1 and 80, conferred a 10-fold reduction in sensitivity to colicins B and M, strongly impaired growth on ferrichrome, and resulted in no growth on the fungal siderophore coprogen. TonB(G174R,V178I) conferred resistance to colicin Ia but sensitivity to colicin B, whereas Serratia marcescens TonB conferred sensitivity to colicin B but resistance to colicin Ia in an E. coli tonB mutant (13,43). Some of the TonB mutants, when combined with certain FhuA point mutants, increased or decreased sensitivity to phage T5 up to 1,000-fold, although T5 does not require TonB to infect cells (25).…”

TonB of Escherichia coli activates FhuA through interaction with the β-barrel a aThe GenBank accession number for the sequence reported in this paper is Y15319.

“…The tonB mutants affect ferrichrome transport and sensitivity to albomycin, colicin M and phages T1 and φ80 differently (Traub et al, 1993 ;Traub & Braun, 1994). Sensitivity to the FhuA-specific ligands was tested with transformants of E. coli HK99 fhuA tonB carrying either plasmid pHK763 fhuA wild-type or pBK7 fhuA∆5-160, and carrying a mutant tonB gene on the low-copy-number vector pHSG576.…”

Phage inhibition, colicin resistance, and tellurite resistance are encoded by a single cluster of genes on the IncHI2 plasmid R478

“…Phage T5 also uses the FhuA receptor to infect host cells, but tonB mutant cells were previously shown to have no effect on T5 infection (10). However, it has recently been demonstrated that certain amino acid substitutions in TonB can alter the receptor activity of FhuA to phages T5, T1, and 80 (14,43). The involvement of these specific phages and the numerous colicins suggests that a link might exist between the TonB systems and the IncH operon.…”