Relationship between the transport of iron and the amount of specific colicin Ia membrane receptors in Escherichia coli

Abstract

Strains of Escherichia coli K-12 defective in their ability to utilize exogenously supplied iron due to genetic defects in the entF, tonB, fes, or fep gene exhibited elevated levels of the specific outer-membrane receptor for colicin Ia when compared with parental strains. Although entF, fes, and fep strains showed a higher degree of Ia sensitivity than did the parental strains, tonB strains were resistant to colicin action. The colicin insensitivity of tonB strains was not due to hyperproduction of enterochelin. Growth in medium containing 101.8 muM Fe2+ led to a lowering of receptor levels in all the above strains and resulted in decreased colicin Ia sensitivity in all strains except tonB, which was already at maximal resistance. Growth in citrate plus iron (1.8 muM) or in ferrichrome resulted in a substantial reduction in both receptor levels and Ia sensitivity in ent, fes, and fep strains but had no effect on receptor levels in tonB strains. Growth in citrate did not lead to an alteration in receptor levels in a mutant specifically defective in citrate-mediated iron transport. The presence of enterochelin during growth led to a reduction in the number of receptors in the parental and ent strains but not in tonB, fes, or fep strains. Thus, in all cases examined, there was an inverse relationship between the number of colicin receptors per cell and the ability of the strain to take up iron from the growth medium. This suggests that under conditions of iron limitation there is a derepression of colicin Ia receptor biosynthesis. These results may point to a role of the colicin I receptor in iron uptake.

Use of cir-lac operon fusions to study transcriptional regulation of the colicin Ia receptor in Escherichia coli K-12

“…Both isolates were found to utilize ferric enterochelin as a source of iron and to be sensitive to colicin B and phage 480 vir and are, therefore, tonB+. Furthermore, in contrast to tonB mutants, which show enhanced levels of colicin Ia receptor due to iron stress (11), strains PW400 and PW402 bound little or no '"I-colicin Ia (data not shown). Indeed, colicin Ia resistance due to a defect in colicin adsorption is the classic phenotype defining cir mutants (3).…”

“…It is concluded that the observed regulation by iron of the cir gene is under transcriptional control.The biosynthesis of enterochelin and the production of the outer membrane receptor specific for this phenolate siderophore are coordinately regulated by the amount of intracellular iron (2,9,10,12,15,16). In addition, the levels of two other outer membrane polypeptides seem to be similarly regulated: an 83,000-dalton protein with no known function and the 74,000-dalton cir gene product, which functions as the receptor for colicins Ia and Ib (2,(9)(10)(11)(12)15). It is known that the colicin I receptor is not required for ferric enterochelin transport (19).…”

“…Portions of the culture were then plated on LB agar, and survivors were picked on the following day for further study.As spontaneous mutation to colicin resistance is not a rare event, survivors of the colicin treatment were screened to determine whether ,Bgalactosidase activity was affected by factors that are known to regulate the amount of the cir gene-determined product. The number of colicin Ia receptors is low in cells grown in medium containing high levels of iron, yet elevated in media containing ZnCl2 (11). Therefore, mutants carrying the presumptive operon fusions were streaked on parallel 5-bromo-4-chloro-3-indoyl-,8-D-galactoside mininal plates containing 100 uM FeCl3 and 100 t,M ZnCl2, respectively.After incubation at 30°C for 48 h, isolates displaying high levels of 8-galactosidase activity on plates containing zinc but low activity on plates containing high iron were picked and retested for resistance to colicin Ia.…”

“…As spontaneous mutation to colicin resistance is not a rare event, survivors of the colicin treatment were screened to determine whether ,Bgalactosidase activity was affected by factors that are known to regulate the amount of the cir gene-determined product. The number of colicin Ia receptors is low in cells grown in medium containing high levels of iron, yet elevated in media containing ZnCl2 (11). Therefore, mutants carrying the presumptive operon fusions were streaked on parallel 5-bromo-4-chloro-3-indoyl-,8-D-galactoside mininal plates containing 100 uM FeCl3 and 100 t,M ZnCl2, respectively.…”

“…Addition of 10 mM citrate to growth medium leads to the induction of a citrate-specific highaffinity iron uptake system in Escherichia coli (7). Cells grown in citrate have decreased levels of the cir gene product (11). When either cir-lac fusion strain was grown in iron-depleted medium, elevated levels of ,B-galactosidase were found compared with the strains grown in normal medium.…”