Expression of the CopB outer membrane protein by Moraxella catarrhalis is regulated by iron and affects iron acquisition from transferrin and lactoferrin

The amino acid sequence of the cell-surface-exposed, 81-kDa CopB outer membrane protein of Moraxella catarrhalis was found to be similar to those of TonB-dependent outer membrane proteins of other gram-negative bacteria. Expression of CopB was affected by the availability of iron in the growth medium, and the extent of overexpression of CopB in response to iron limitation varied widely among the M. catarrhalis strains tested. Wild-type M. catarrhalis strains were found to be able to utilize ferric citrate, transferrin, lactoferrin, and heme as sources of iron for growth in vitro. However, an isogenic copB mutant was severely impaired in its ability to utilize transferrin and lactoferrin as sole sources of iron for growth, whereas this same mutant grew similarly to the wild-type parent strain when supplied with ferric citrate as the iron source. The copB mutant was not significantly different from its wild-type parent strain in its ability to bind transferrin and lactoferrin. In addition, the wild-type parent strain and the copB mutant exhibited equivalent rates of uptake of ⁵⁵Fe from ferric citrate. However, the copB mutant was markedly less able than the wild-type strain to take up ⁵⁵Fe from transferrin and lactoferrin. These results indicate that lack of expression of the CopB protein exerts a direct or indirect effect on the ability of M. catarrhalis to utilize iron bound to certain carrier proteins.