FERRIC RHIZOFERRIN UPTAKE INTO MORGANELLA-MORGANII - CHARACTERIZATIONOF GENES INVOLVED IN THE UPTAKE OF A POLYHYDROXYCARBOXYLATE SIDEROPHORE

Iron uptake in Morganella morganii, mediated by the fungal siderophore rhizoferrin, was studied. A Mud1 insertion mutant devoid of growth on ferric rhizoferrin was complemented by a chromosomal DNA fragment of M. morganii that encoded an outer membrane protein and a periplasmic p rotein named RumA and RumB (for rhizoferrin uptake into Morganella spp .), respectively. rumA and rumB have the same transcription polarity a nd are probably cotranscribed from an iron-regulated promoter upstream of rumA. A predicted Fur regulatory sequence upstream of rumA was con firmed by the Fur titration assay. At the N terminus of RumA, a putati ve TonB box contains a proline residue that inactivates TonB-dependent receptors and colicins when introduced at the same position into TonB boxes of Escherichia coli. Analysis of a 10-kb sequence flanking rumA and rumB on both sides revealed seven additional open reading frames for which no role in ferric rhizoferrin uptake could be discerned. Thu s, rumA and rumB, both essential for transport of this siderophore, fo rm an isolated operon. Additional genes required for ferric rhizoferri n translocation across the cytoplasmic membrane must map at sites dist inct from rumA and rumB. Transport studies revealed that both Fe-55(3) and [H-3]ketorhizoferrin are incorporated by M. morganii, demonstrat ing that rhizoferrin serves as a true iron carrier.