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Iron Response Regulator (Irr)

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Background Information

Iron is potentially toxic to cells, as in the presence of oxygen, Fenton reactions can produce reactive oxygen species that can destroy essential biomolecules. Balancing the amount of iron in the cell is important and this importance is apparent from the elaborate mechanisms cells devote to iron homeostasis. Part of this iron balancing is achieved by regulation of iron import. The genes required for ferric citrate transport in Rhodobacter sphaeroides form a cluster in the order fecI-fecR-fecABCDE, encoding a specialized sigma factor and a putative anti-sigma factor that together are responsible for regulated transcription of the ferric citrate transport operon, encoding an ABC-type ferric citrate transporter. In Escherichia coli, fecI transcription is regulated by Fur in response to iron availability; in Bradyrhizobium japonicum, as well as R. sphaeroides, which both lack Fur, fecI transcription is thought to be regulated by another iron-responsive DNA binding protein, Irr, or the iron response regulator protein, which can also be considered to be a relative to the family of Fur proteins.

Irr and Other Iron-Regulating Proteins

Since there are bacteria that have to have iron level-mediating proteins present but do not have the Fur (ferric uptake regulator) protein, there must be another protein that takes its place. In the case of B. japonicum, which does not have the Fur protein, the Irr protein was found to be the regulator of iron levels within the cell.

Function of Irr

Irr behaves differently than other regulatory proteins. It functions as coordinating the heme biosynthetic pathway, which ends with the insertion of Fe²⁺ into a protoporphyrin ring to produce protoheme. It also controls the pathway by monitoring iron availability to prevent the accumulation of toxic porphyrin precursors under iron limitation, as when iron is limiting, heme cannot be produced.

Irr accumulates in cells under iron limitation, with very low levels of Irr being present in iron-replete cells. This is a distinction when compared to other Fur family proteins because it functions in the absence of the regulatory metal, whereas the other members require direct metal-binding for the protein to be activated.

Chemical and Physical Properties of Irr

• • • Make into a table***

Amino Acid Composition:

Ala (A) 15 9.2%

Arg (R) 10 6.1%

Asn (N) 6 3.7%

Asp (D) 10 6.1%

Cys (C) 1 0.6%

Gln (Q) 5 3.1%

Glu (E) 11 6.7%

Gly (G) 9 5.5%

His (H) 10 6.1%

Ile (I) 3 1.8%

Leu (L) 21 12.9%

Lys (K) 6 3.7%

Met (M) 6 3.7%

Phe (F) 2 1.2%

Pro (P) 8 4.9%

Ser (S) 7 4.3%

Thr (T) 13 8.0%

Trp (W) 2 1.2%

Tyr (Y) 6 3.7%

Val (V) 12 7.4%

Pyl (O) 0 0.0%

Sec (U) 0 0.0%

Molecular weight: 18338.8 Da

Theoretical pI: 6.03

Evolution of Irr/Fur

Go to top 5 on Rasmol, put in amino acid sequence and find domain conservations, anything related to Irr/Fur in the past (conserf)

Structure of the Proposed Irr Protein

spinqualitylabelsall models 3D Image of proposed Irr protein Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Go to polyview 3d, select "secondary structures" for coloring mode and re-upload

The amino acid sequence used to derive the structure shown is as follows:

1 msentaphhd ddvhaaalls grqpaltgcp whdvnemlqs aglrptrqrm algwllfgkg

61 arhltaemly eeatlakvpv slatvyntln qltdagllrq vsvdgtktyf dtnvtthhhy

121 ylenshelvd iedphlalsk mpevpegyei aridmvvrlr kkr

ReferencesReferences

1) Hamza I, S. Chauhan, R. Hassett, M. R. O'Brian, 1998. The bacterial irr protein is required for coordination of heme biosynthesis with iron availability.. Journal of Biological Chemistry 34:21669-74.

2) Small, S. K., S. Puri, and M. R. O’Brian. 2009. Heme-dependent metalloregulation by the iron response regulator (Irr) protein in Rhizobium and other alpha-proteobacteria. Biometals 22:89-97.