Nucleotide sequence of the fhuC and fhuD genes involved in iron (III) hydroxamate transport: domains in FhuC homologous to ATP-binding proteins.

The transport of Fe3+ into cells of Escherichia coli occurs via siderophores and the uptake through the outer membrane of three Fe3+-siderophore compounds containing hydroxamate residues requires three specific receptor proteins. In contrast, transport through the cytoplasmic membrane is catalysed by three common proteins encoded by the fhuB, fhuC and fhuD genes. The nucleotide sequence of a DNA fragment containing the fhuC and fhuD genes has been determined: the open reading frame of fhuC contains 795 nucleotides which encode a polypeptide with a molecular weight of 29,255 and the largest open reading frame of the fhuD region comprises 888 nucleotides. However, we propose that translation of fhuD initiates at the fourth potential start codon resulting in a polypeptide with a molecular weight of 28,282. Both proteins are moderately nonpolar and membrane-bound. They lack obvious signal sequences. Segments of the FhuC protein display strong homology to ATP-binding proteins, suggesting a function in Fe3+ uptake similar to the ATP-binding proteins of transport systems that depend on periplasmic proteins. This study completes the nucleotide sequence of the fhu operon which consists of the four genes fhuA fhuC fhuD fhuB arranged in this order on the E. coli chromosome and transcribed from fhuA to fhuB.