Mechanisms of siderophore iron transport in enteric bacteria

Abstract

Uptake of 55Fe- and 3H-labeled siderophores and their chromic analogues were studied in Salmonella typhimurium LT-2 and Escherichia coli K-12. In S. typhimurium LT-2, at least 2 different mechanisms for siderophore Fe transport may be operative. Uptake of 55Fe- and 3H-labeled ferrichrome and kinetically inert .LAMBDA.-cis-chromic [3H]deferriferrichrome by the S. typhimurium LT-2enb7 mutant, which is defective in the production of its native siderophore, enterobactin, appears to occur by 2 concurrent mechanisms. The 1st mechanism is postulated to involve either rapid uptake of Fe 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 Fe coupled with simultaneous expulsion of the ligand. The 2nd 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 1 mechanism, which is postulated to consist of transport of the intact ferric complex with simultaneous explusion of the free ligand by the cell. Cellular release of Fe 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 3 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. The existence of multiple mechanisms for assimilation of siderophore Fe is demonstrated and reduction of the metal ion as a prominent feature of the uptake process is implicated.