Transport of vitamin B12 in tonB mutants of Escherichia coli

Abstract

It is known that the tonB mutation in Escherichia coli is responsible for a defect in the transport of iron chelates. These are transported by systems that involve outer membrane components. We found that tonB mutants were also deficient in the secondary, energy-dependent phase of vitamin B12 transport, although the mutants have normal levels ofB12 receptors on their cell surface. In addition, tonB mutants derived from vitamin B12 auxotrophs required elevated levels of B12 for normal growth. Maltose uptake, mediated by another transport system involving an outer membrane component, was unaffected by the tonB mutation.Receptor proteins in the outer membrane of cells ofEscherichia coli and Salmonella typhimurium are intimately involved in the uptake of vitamin B12 and iron chelates. The bfe gene product ofE. coli is an outer membrane protein of 60,000 molecular weight which is obligatory for the cellular adsorption of phage BF23, the three E colicins, and B12 (2,6,14,15). The energy-independent binding of B12 to the bfe product has been shown to be the first step in the transport ofthis vitamin byE. coli. Numerous mutations that affect the uptake of iron chelates are known. According to the available evidence, the tonA product is an 85,000-molecular-weight protein that binds phages Ti, T5, and 480, colicin M, and iron-ferrichrome (1,11,12,23). A separate protein, which may be the product of the feu locus, appears to function as the initial receptor for colicins B and D and for the ferric-enterochelin complex (13, 24). Finally, the uptake of maltose and maltodextrins is facilitated by the function of the product of the lamB gene, the phage lambda receptor (18).Mutations in the tonB locus ofE. coli confer resistance to bacteriophages Ti and 480 and all of the group B colicins (3). In addition, tonB mutants are chromium sensitive (22), resistant to the siderophore antibiotic albomycin (12), hyperexcrete enterochelin (9), and are totally deficient in the uptake of all iron chelates. However, these mutants are still capable of reversibly binding phages Ti and 480, and they retain receptor activity for the colicins and iron chelates (3,11,16). These observations, together with the finding that no outer membrane proteins appear to be missing in tonB mutants (3), suggest that the tonB product is involved in some step of colicin or ferric-entero-chelin uptake subsequent to binding to the receptor.The activities of the uptake systems for a number of amino acids and inorganic phosphate were normal in tonB mutants (8). However, none of these transport systems appears to employ components in the outer membrane. Hence, the effect of alterations in tonB on the function of other transport systems utilizing outer membrane components was investigated. This paper will demonstrate that tonB mutants are totally deficient in the energy-dependent uptake phase of vitamin B12 transport but are unaffected in maltose transport, even at low external concentrations of the substrate. MATERIALS AND METHODSBacterial strains and media. The strains...

Reduced activity of TonB-dependent functions in strains of Escherichia coli

“…The inhibition by vitamin B~2 of phage BF23 adsorption was found to depend on tonB function, as had previously been found for the competition of phage T5 and ferrichrome [14]. The btuB451 mutation alters the vitamin B12 receptor in such a way that its binding activities remain normal but the energy-dependent transport of vitamin B~2 is completely lost [2,5]. The finding that the competition between vitamin Bt: and phage BF23 was the same in the strain with this mutation as in a tonB mutant supports the view that this receptor mutation affects the interaction with the tonB protein.…”

“…Vitsmin Bt2 uptake activity was assayed as previously described [2]. Cells were continuously kept in growth medium.…”

The importance of the exbB gene for vitamin B12 and ferric iron transport

“…The function of the tonB gene is also required for the transport of vitamin B12 [11]. The similarity in the phenotypes of tonB and exbB mutants led us to study vitamin B12 transport in the mutants.…”

Vibrio cholerae iron transport: haem transport genes are linked to one of two sets of tonB, exbB, exbD genes

“…TonB appears to interact with multiple receptors. Studies of E. coli tonB mutants indicate that TonB is required for all the high-affinity iron transport systems (Frost and Rosenberg, 1975;Williams, 1979), for vitamin B 12 transport (Bassford et al, 1976) and for sensitivity to certain phages and colicins (Braun and Herrmann, 1993). Comparison of the amino acid sequences of the TonB-dependent receptors, such as FepA, FhuA, ChuA and BtuB, the E. coli receptors for enterobactin, ferrichrome, haem and B 12 respectively, indicate significant homology (Heller and Kadner, 1985;Coulton et al, 1986;Lundrigan and Kadner, 1986;Torres and Payne, 1997).…”