Affiliation:
1. Molecular Infectious Disease Group, Department of Paediatrics, Faculty of Medicine, St. Mary's Campus, Imperial College London, London W2 1PG
2. Department of Pathology and Infectious Diseases, The Royal Veterinary College, University of London, North Mymms, Hatfield, Hertfordshire AL9 7TA, United Kingdom
Abstract
ABSTRACT
Iron acquisition in vivo by
Actinobacillus pleuropneumoniae
depends upon a functional TonB system. Tonpitak et al. (W. Tonpitak, S. Thiede, W. Oswald, N. Baltes, and G.-F. Gerlach, Infect. Immun.
68:
1164-1170, 2000) have described one such system, associated with
tbpBA
encoding the transferrin receptor, and here we report a second, termed
tonB2
. This gene cluster (
exbB2
-
exbD2
-
tonB2
) is highly homologous to those in other
Pasteurellaceae
, unlike the earlier system described (now termed
tonB1
), suggesting that it is the indigenous system for this organism. Both
tonB2
and
tonB1
are upregulated upon iron restriction. TonB2, but not TonB1, was found to be essential for growth in vitro when the sole source of iron was hemin, porcine hemoglobin, or ferrichrome. In the case of iron provided as iron-loaded porcine transferrin, neither
tonB
mutant was viable. The
tonB1
phenotype could be explained by a polar effect of the mutation on transcription of downstream
tbp
genes. We propose that TonB2 is crucial for the acquisition of iron provided in this form, interacting with accessory proteins of the TonB1 system that have been demonstrated to be necessary by Tonpitak et al. TonB2 appears to play a much more important role in
A. pleuropneumoniae
virulence than TonB1. In an acute porcine infection model, the
tonB2
mutant was found to be highly attenuated, while the
tonB1
mutant was not. We hypothesize that acquisition of the
tonB1-tbp
gene cluster confers a biological advantage through its capacity to utilize transferrin-iron but that TonB1 itself plays little or no part in this process.
Publisher
American Society for Microbiology
Subject
Infectious Diseases,Immunology,Microbiology,Parasitology