BINDING OF HEME-HEMOPEXIN COMPLEXES BY SOLUBLE HXUA PROTEIN ALLOWS UTILIZATION OF THIS COMPLEXED HEME BY HAEMOPHILUS-INFLUENZAE

Utilization of heme-hemopexin as a source of heme by Haemophilus influ enzae type b is dependent on expression by this bacterium of the 100-k Da HxuA protein, which is both present on the bacterial cell surface a nd released into the culture supernatant (L. D. Cope, R. Yogev, U. Mul ler-Eberhard, and E. J. Hansen, J. Bacteriol. 177:2644-2653, 1995). Ra dioimmunoprecipitation analysis showed that the soluble HxuA protein p resent in H. influenzae type b culture supernatant bound heme-hemopexi n complexes in solution. An isogenic H. influenzae type b hxuA mutant was unable to utilize soluble heme-hemopexin complexes for growth in v itro unless soluble HxuA protein was provided exogenously. Soluble Hxu A protein secreted by a nontypeable H. influenzae strain also allowed growth of this H. influenzae type b hxuA mutant. These results indicat ed that the heme present in heme-hemopexin complexes is rendered acces sible to H. influenzae when these complexes are bound by the soluble H xuA protein.