Gonococcal transferrin-binding protein 2 facilitates but is not essential for transferrin utilization

Abstract

Pathogenic Neisseria species have been shown to scavenge iron from transferrin (Tf), although the mechanism is not yet fully understood. Two iron-repressible proteins that exhibit Tf-binding activity have been identified. This work describes the cloning and sequencing of tbpB, a 2.1-kb gene in N. gonorrhoeae that encodes Tbp2, an 85-kDa iron-repressible lipoprotein. Transcriptional interruption of tbpB had a strong polar effect on tbpA, the structural gene for Tbp1 that is located immediately downstream from tbpB. Such tbpB mutants did not express either Tbp2 or Tbp1, did not bind Tf to whole cells, did not grow on Tf plates, and did not take up iron from Tf. A mutant in which most of tbpB was deleted, presumably leaving tbpA under transcriptional control of the tbpB promoter, was constructed. This mutant did not express Tbp2 but expressed wild-type levels of Tbp1 and possessed the phenotype of reduced binding of Tf, decreased iron uptake from Tf, but normal growth on Tf plates. Mutants expressing Tbp2 and not Tbp1 bound less Tf, did not grow on Tf plates, and did not take up iron from Tf. These results suggest that tbpB and tbpA are polycistronic. Tbp2 apparently facilitates binding of Tf but is not essential for acquisition of iron from Tf under these in vitro conditions.