ABSTRACT
Brucella abortus is a facultative, intracellular, pathogenic bacterium that replicates within macrophages and resists macrophage microbicidal mechanisms. To study gene expression and to elucidate the defense mechanisms used by B. abortus to resist destruction within macrophages, protein synthesis by B. abortus was examined by pulse-labeling techniques during intracellular growth within J774A.1, a macrophage-like cell line. Prominent changes observed include increased synthesis of Brucella proteins with estimated molecular masses of 62, 28, 24, and 17 kDa. The 62-kDa protein was identified by immunoprecipitation analysis as Hsp62, a GroEL homolog. A protein of 60 kDa was expressed during acid shock and may represent a modified form of Hsp62. The 28- and 17-kDa proteins have not been observed under any in vitro stress condition and presumably represent macrophage-specific induction. The 24-kDa protein comigrates with an unidentified protein induced by acid shock, designated Asp24. Expression of Asp24 is optimal at pH values below 4.0 and within the first 3 h following a shift from pH 7.3 to 3.8. This corresponds directly with a period of optimal bacterial survival at a reduced pH and suggests an active role for this protein in resistance to such environments. The identification of these gene products and the mechanisms controlling their expression is an important step in understanding the resistance of Brucella spp. to intracellular destruction within macrophages.
