Nucleotide sequences of the sfuA, sfuB, and sfuC genes of Serratia marcescens suggest a periplasmic-binding-protein-dependent iron transport mechanism

Abstract

The cloned sfu region of the Serrafia marcescens chromosome confers the ability to grow on iron-limited media to an Escherichia coli K-12 strain that is unable to synthesize a siderophore. This DNA fragment was sequenced and found to contain three genes termed sfuA, sfuB, and sfuC, arranged and transcribed in that order. The sfuA gene encoded a periplasmic polypeptide with calculated molecular weights of 36,154 for the precursor and 33,490 for the mature protein. The sfu8 gene product was a very hydrophobic protein with a molecular weight of 56,589. The sfuC gene was found to encode a rather polar but membrane-bound protein with a molecular weight of 36,671 which exhibited strong homology to consensus sequences of nucleotidebinding proteins. The number, structural characteristics, and locations of the SfuABC proteins were typical of a periplasmic-binding-protein-dependent transport mechanism. How Fe3+ is solubilized and taken up across the outer membrane remains an enigma.The insolubility of Fe3" at neutral pH requires the formation of complex compounds with siderophores for this ion to be taken up by bacteria and fungi. The Fe3+ siderophores are first bound to highly specific receptor proteins in the outer membranes of Escherichia coli and other gram-negative bacteria. Subsequently, they are taken up in an energyand TonB-dependent step across the outer membrane into the periplasmic space. Mutations in the exbB and exbD genes (6) result in a strongly reduced rate of iron uptake (8), implicating the products of these genes in the uptake process.

Identification of a locus involved in the utilization of iron by Actinobacillus pleuropneumoniae

“…1). The sequences of both clones showed three open reading frames with high similarities to the s&ABC gene cluster of S. marcescens [7]. In analogy to sfuABC, the three reading frames were designated afuA, afuB, and afuC, respectively (Fig.…”

“…A comparison of AfuA, AfuB and AfuC with homologous proteins are presented in Table 3. A. pleuropneumoniae together with other pathogenic bacteria possess a siderophore-independent mechanisms for iron sequestration ( [7,10,12], this study). In N. gonorrhoeae and N. meningitidis, two proteins (Tbpl and Tbp2) are responsible for binding transferrin to the cell surface [15].…”

“…The mechansim of iron transport from porcine transferrin into A. pleuropneumoniae is unknown. However, the presence of afu operon homologs in H. influenzae [9,10], S. marcescens [7], and N. gonorrhoeae [ 11,121 suggests that the function of this operon may be involved in high-affinity iron acquisition from the host environment.…”

“…The three polypeptides deduced from the DNA sequence were similar to that of SfuABC [7], HitABC [9,10], FbpABC [11,12] and YfuABC. Based on these data, we hypothesized that the AfuA, AfuB and AfuC polypeptides are involved the transport of ferric iron across the cytoplasmic membrane.…”

“…Table I The conserved EAA motifs from the periplasmic permeases of A. "The sequence were taken from the following sources: afuB (this study), sfua [7], fbpb [12], hitb [9] and yfuB (unpublished data). …”