Mechanisms of siderophore iron transport in enteric bacteria

Abstract

Uptake of 55Fe and 3H labeled siderophores and their chromic analogues have been studied in Salmonella typhimurium LT 2 and Escherichia coli K 12. In S. typhimurium LT 2, at least two different mechanisms for siderophore iron transport may be operative. Uptake of 55Fe and 3H labeled ferrichrome and kinetically inert Lambda cis chromic [3H]deferriferrichrome by the S. typhimurium LT 2 enb7 mutant, which is defective in the production of its native siderophore, enterobactin, appears to occur by two concurrent mechanisms. The first mechanism is postulated to involve either rapid uptake of iron released from the ferric complex by cellular reduction without penetration of the complex or ligand or dissociation of the complex and simultaneous uptake of both ligand and iron coupled with simultaneous expulsion of the ligand. The second mechanism appears to consist of slower uptake of the intact ferric complex. Uptake of ferrichrome by the E. coli K 12 RW193 mutant, which is also defective in the production of enterobactin, appears to occur by at least one mechanism, which is postulated to consist of transport of the intact ferric complex with simultaneous expulsion of the free ligand by the cell. Cellular release of iron from the complex is presumed to involve reduction. Transport of intact chromic deferriferrichrome in both organisms demonstrates that the Lambda cis coordination isomer can be accepted by the uptake system and suggests that uptake of the labile ferric complex cannot rely on rapid isomerization or dissociation. In S. typhimurium enb7, at least three possible mechanisms of transport are consistent with the uptake data for ferrioxamine B. Neither the cis nor trans geometrical coordination isomers of chromic [3H]deferriferrioxamine B are taken up. Uptake of the above siderophores with mutants resistant to the antibiotic albomycin was also studied. These findings demonstrate the existence of multiple mechanisms for assimilation of siderophore iron and implicate reduction of the metal ion as a prominent feature of the uptake process.