tailieunhanh - Báo cáo khoa học: Heme binding to the second, lower-affinity site of the global iron regulator Irr from Rhizobium leguminosarum promotes oligomerization

The iron responsive regulator Irr is found in a wide range ofa-proteobac-teria, where it regulates many genes in response to the essential but toxic metal iron. Unlike Fur, the transcriptional regulator that is used for iron homeostasis by almost all other bacterial lineages, Irr does not sense Fe 2+ directly, but, rather, interacts with a physiologically important form of iron, namely heme. | IFEBS Journal Heme binding to the second lower-affinity site of the global iron regulator Irr from Rhizobium leguminosarum promotes oligomerization Gaye F. White1 2 Chloe Singleton1 Jonathan D. Todd2 Myles R. Cheesman1 Andrew W. B. Johnston2 and Nick E. Le Brun1 1 Schoolof Chemistry Centre for Molecular and StructuralBiochemistry University of East Anglia Norwich Research Park Norwich UK 2 Schoolof BiologicalSciences University of East Anglia Norwich Research Park Norwich UK Keywords a-proteobacteria fur heme iron transcriptional regulation Correspondence N. E. Le Brun School of Chemistry University of East Anglia Norwich NR4 7TJ UK Fax 44 1603 592003 Tel 44 1603 592699 E-mail Received 3 February 2011 revised 17 March 2011 accepted 1 April 2011 doi The iron responsive regulator Irr is found in a wide range of a-proteobac-teria where it regulates many genes in response to the essential but toxic metal iron. Unlike Fur the transcriptional regulator that is used for iron homeostasis by almost all other bacterial lineages Irr does not sense Fe2 directly but rather interacts with a physiologically important form of iron namely heme. Recent studies of Irr from the N2-fixing symbiont Rhizobium leguminosarum IrrRl showed that it binds heme with submicromolar affinity at a His-Xxx-His HxH motif. This caused the protein to dissociate from its cognate DNA regulatory iron control element box sequences thus allowing expression of its target genes under iron-replete conditions. In the present study we report new insights into the mechanisms and consequences of heme binding to Irr. In addition to the HxH motif Irr binds heme at a second lower-affinity site. Spectroscopic studies of wild-type Irr and His variants show that His46 and probably His66 are involved in coordinating heme in a low-spin state at this second site. By contrast to the well-studied Irr from Bradyrhizobium japonicum neither heme site of IrrRl stabilizes .