Genetic organization of multiple fep genes encoding ferric enterobactin transport functions in Escherichia coli.

Three genes were shown to provide functions specific for ferric enterobactin transport in Escherichia coli: fepA encoded the outer membrane receptor, fepB produced a periplasmic protein, and the fepC product was presumably a component of a cytoplasmic membrane permease system for this siderophore. A 10.6-kilobase-pair E. coli chromosomal EcoRI restriction fragment containing the fepB and fepC genes was isolated from a genomic library constructed in the vector pBR328. Both cistrons were localized on this clone (pITS24) by subcloning and deletion and insertion mutagenesis to positions that were separated by approximately 2.5 kilobases. Within this region, insertion mutations defining an additional ferric enterobactin transport gene (fepD) were isolated, and polarity effects from insertions into fepB suggested that fepD is encoded downstream on the same transcript. A 31,500-dalton FepC protein and a family of FepB polypeptides ranging from 34,000 to 37,000 daltons were identified in E. coli minicells, but the product of fepD was not detectable by this system. Another insertion mutation between entF and fepC was also shown to disrupt iron transport via enterobactin and thus defined the fepE locus; fepE weakly expressed a 43,000-dalton protein in minicells. It is proposed that these newly identified genes, fepD and fepE, provide functions which act in conjunction with the fepC product to form the ferric enterobactin-specific cytoplasmic membrane permease. An additional 44,000-dalton protein was identified and shown to be expressed from a gene that is situated between fepB and entE and that is transcribed in the direction opposite that of fepB. Although the function of this protein is uncharacterized, its membrane location suggests that it too may function in iron transport.