The bhuTUV and bhuO genes play vital roles in the ability of Brucella abortus to use heme as an iron source and are regulated in an iron-responsive manner by RirA and Irr

Abstract

Brucella abortus is a Gram negative intracellular pathogen that causes the zoonotic disease brucellosis. Antibiotic treatment for brucellosis in humans is prolonged and sometimes followed by relapses. Currently, the United States employs prevention of the illness in humans through cattle vaccinations, eliminating the bacterium in its natural host. Unfortunately, these vaccine strains cause the disease in humans, and Brucella research ultimately aims to identify new vaccine targets as well as alternative treatment options. Brucella abortus resides in the phagosomal compartment of the host macrophage where essential nutrients such as iron are limited. Most bacteria need iron, and within the macrophage, heme is a likely source of iron due to the breakdown of red blood cells by the host macrophage. Heme transporters in Gram negative bacteria are highly conserved, and include components for outer membrane, periplasmic, and cytoplasmic membrane transport. BhuA has been previously characterized as the outer membrane heme transporter of Brucella abortus and here we report that BhuT, BhuU, and BhuV (BhuTUV) are the periplasmic and cytoplasmic heme transport components and that they are required in order for Brucella abortus to transport heme as an iron source. Utilization of heme as an iron source requires the breakdown of heme into ferrous iron, carbon monoxide, and biliverdin by a heme oxygenase. BhuO has been identified as a heme oxygenase in Brucella abortus , and although there seems to be more than one heme oxygenase in Brucella , this study shows that BhuO is needed for the use of heme as an iron source under iron starvation conditions in vitro. Further, both bhuTUV and bhuO are regulated in an iron-responsive manner. The iron responsive regulator Irr directly represses bhuO, which shares an operon with rirA . Then the rhizobial iron regulator RirA in turn represses the bhuTUV operon. Together, these regulators help to maintain iron homeostasis within the bacterial cell, protecting it from damaging hydroxyl radicals produced by Fenton chemistry.