A second tonB gene in Pseudomonas aeruginosa is linked to the exbB and exbD genes

The exbBD genes of Pseudomonas aeruginosa PAO were cloned by complementatio n of the growth defect of an Escherichia coil exbB role double mutant on ir on-restricted medium. Nucleotide sequence analysis confirmed that these gen es are contiguous and preceded by a second tonB gene in this organism. whic h we have designated tonB2, lacZ promoter fusions confirmed that expression of the tonB2-exbB-exbD genes is increased under conditions of iron limitat ion. Deletions within any of these genes, in contrast to deletions in the f irst tonB gene. tonB1, did not adversely affect growth on iron-restricted m edium. On the other hand, tonB1 tonB2 double mutants were more compromised as regards growth in an iron-restricted medium than a tonB1 deletion, indic ating that TonB2 could partially replace TonB1 in its role in iron acquisit ion. TonB1 but not TonB2 deletion strains were also compromised as regards the utilization of hemin or hemoglobin as sole iron sources. indicating tha t heme transport requires TonB1. (C) 2000 Federation of European Microbiolo gical Societies. Published by Elsevier Science B.V. All rights reserved.