Cloning and functional characterization of Neisseria gonorrhoeae tonB, exbB and exbD genes

Neisseria gonorrhoeae is able to utilize iron (Fe) from a variety of sources including transferrin (TF) and lactoferrin (LF). To gain insight into the molecular mechanisms used by gonococci to scavenge Fe from TF and LF, we cloned a 3.5 kb segment of wild-type DNA that repaired the defect in tlu mutants, which are unable to take up Fe from either TF or LF despite exhibiting apparently normal ligand binding to the receptor. Nucleotide sequence determination identified three open reading frames (ORFs), designated ORF1, ORF2, and ORF3, which were arranged in tandem. The deduced amino acid sequence of the 852 bp ORF1 encoded a 28 kDa protein that exhibited 26-32% identity with TonB proteins of nine other bacteria. The 663 bp ORF2 predicted a 24 kDa protein and the 435 bp long ORF3 predicted a 15 kDa protein. These predicted protein sequences exhibited 32-38% and 24-36% identity, respectively, with ExbB and ExbD proteins of three other bacteria. Thus, the sequence comparison identified the ORF1, ORF2 and ORF3 as gonococcal homologues of the E. coli tonB, exbB and exbD genes. An insertional mutation in the tonB homologue resulted in the failure of gonococci to grow with TF, LF or human haemoglobin (HB) as sole Fe sources and in the inability to take up 55Fe from TF and LF. The tonB mutation did not prevent the utilization of Fe from citrate (CT) or haemin (HM). Binding of TF, LF and HB to whole cells in a solid-phase binding assay was largely unaffected by the tonB mutation. We conclude that the pathways for utilization of Fe bound to TF, LF and HB but not to HM or CT were dependent on the TonB system.