THE BATTLE FOR IRON - CRYSTALLOGRAPHIC STUDIES OF THE IRON DEPENDENT REPRESSOR PROTEINS FROM C. DIPHTHERIA AND M. TUBERCULOSIS. Ehmke Pohl^a, Xiayang Qiu^a, Randall K. Holmes^c & Wim G.J. Hol ^a,b. ^aBiomolecular Structure Center and ^bHoward Hughes Medical Institute, University of Washington, Box 357742, Seattle, WA 98195, USA; ^cDepartment of Microbiology, University of Colorado, Health Sciences Center, Denver, CO 80262, USA.

Iron is an essential nutrient for most virulent bacteria, however the availability of free iron in the mammalian hosts is extremely limited. A number of pathogens utilize the low-iron environment as a signal for the expression of virulence factor and proteins involved in obtaining iron from the host organism.

In Corynebacterium diphtheria the Diphtheria Toxin Repressor (DtxR) is activated at about 2 mM Fe²⁺. The active dimeric protein binds specifically the tox and irp operators which encode the tox gene and the irp genes. The crystal structure of DtxR has recently been solved at 2.8 Å resolution [1]. High resolution data of DtxR in complex with different divalent metals have been collected at low temperature using conventional X-ray sources and synchrotron radiation. The metal binding sites have been unraveled in greater detail than previously reported.

A functionally homologous repressor from Mycobacterium tuberculosis has been cloned, expressed in E. coli and purified. This iron dependent repressor (IdeR) shares 60% sequence identity with DtxR [2]. Crystallization and preliminary analysis by X-ray crystallography will be presented.

[1] X. Qiu, C.L.M.J. Verlinde, Z. Zhang, M.P. Schmitt, R.K. Holmes & W.G.J. Hol (1995) Structure 3 87-100.

[2] M.P. Schmitt, M. Predich, L. Doukhan, I. Smith & R.K. Holmes (1995) Infect. Immun. 63 4284-4289.