| Brucella abortus is a Gram-negative bacterial pathogen that resides within two distinct intracellular niches in the host. During pregnancy in ruminants, B. abortus exhibits extensive replication within the placental trophoblasts, leading to abortion or the birth of weakened calves. B. abortus also reside within the macrophages of a number of mammals establishing a long-term, chronic infection. As with most organisms, iron is essential to the survival of B. abortus, however, the mammalian host represents an extremely iron-limited environment. To persist, the brucellae must circumvent this restriction. Iron acquisition in the placental trophoblasts is mediated, in part, through the secretion of iron binding compounds called siderophores, which compete with host proteins for free iron. B. abortus synthesizes two catechol-type siderophores, 2, 3-dihydroxybenzoic acid (DHBA) and brucebactin. The biosynthesis of both requires the products of the dhbCEBA operon, and these genes are essential for the maintenance of infection in the ruminant reproductive tract. This is in contrast to the intracellular environment encountered in the macrophage, where siderophore production is dispensable and heme serves as an important source of iron for the bacterium. Because little was known about the regulation of the dhbCEBA operon, the studies presented in this dissertation were designed to identify and characterize gene products that contribute to the iron-responsive regulation of the dhbCEBA operon.;Members of the AraC family of transcriptional regulators have been shown to play an important role in the regulation of siderophore biosynthesis in several bacterial species. Here, we describe DhbR (DHBA biosynthesis regulator), an AraC-like regulator that is required for full transcriptional activation of the dhbCEBA operon in B. abortus. Experimental evidence shows that DhbR-mediated activation occurs via direct binding to the dhb promoter, and supports the role of DHBA/brucebactin as a co-activator when complexed with ferric iron. These data indicate that DhbR regulates expression of the genes required for siderophore biosynthesis when siderophore production presents as an efficient means of acquiring iron. This modulation allows the brucellae to adapt the expression of its iron acquisition systems in response to the different iron sources available in the macrophages and trophoblasts of the ruminant host. |
