Crystallization and preliminary X-ray crystallographic analysis of MbtI, a protein essential for siderophore biosynthesis in Mycobacterium tuberculosis

Abstract

Mycobacterium tuberculosis, the causative agent of tuberculosis, depends on the secretion of salicylate-based siderophores called mycobactins for the acquisition of extracellular iron, which is essential for the growth and virulence of the bacterium. The protein MbtI is thought to be the isochorismate synthase enzyme responsible for the conversion of chorismate to isochorismate, the first step in the salicylate production required for mycobactin biosynthesis. MbtI has been overexpressed in Escherichia coli, purified and crystallized. The crystals diffract to a maximum resolution of 1.8 A. They belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 51.8, b = 163.4, c = 194.9 A, consistent with the presence of either two, three or four molecules in the asymmetric unit.

Figure 1

The chemical structure of mycobactin T, the major mycobactin siderophore produced by M. tuberculosis, annotated with its biosynthetic constituents. Redrawn from De Voss et al. (1999 ▶).

Figure 2

The biosynthesis of anthranilate and salicylate. The TrpE–TrpG complex is responsible for the conversion of chorismate to anthranilate via 2-amino-2-deoxyisochorismate (ADIC; the ‘amino analogue’ of isochorismate) with the concomitant deamidation of glutamine and release of pyruvate. MbtI is the putative isochorismate synthase, presumed to catalyze the analagous conversion of chorismate to isochorismate. The gene that encodes the isochorismate-pyruvate lyase in M. tuberculosis has not been identified.

Figure 3

Crystals of M. tuberculosis MbtI, showing the characteristic bundled-rod morphology, with a single unbundled rod visible.