SufR coordinates two [4Fe-4S]^2+,1+ clusters and functions as a transcriptional repressor of the sufBCDS operon and an autoregulator of sufR in cyanobacteria

The sufR gene encodes a protein that functions as a transcriptional repressor of the suf regulon in cyanobacteria. It is predicted to contain an N-terminal helix loop helix DNA binding motif and a C-terminal Fe/S binding domain. Through immunoblotting assays of cell extracts, the sufR product in Synechocystis sp. PCC 6803 was shown to have a mass of ∼25 kDa. This indicates that the second ATG in the open reading frame is the correct start codon and that sufR encodes a protein of 216 amino acids (SufR₂₁₆) rather than the originally predicted 240 amino acids. Recombinant SufR ₂₁₆ harbored [4Fe-4S]^2+,1+ clusters, which were present in a mixture of S = 1/2 and 3/2 ground spin states, and the holoprotein was a homodimer, containing 3.7 of non-heme irons and 3.5 labile sulfides per monomer. Thus, two [4Fe-4S]^2+,1+ clusters are coordinated by each SufR ₂₁₆ homodimer. SufR₂₁₆ bound to two DNA sequences in the regulatory region between the divergently transcribed sufR gene and the sufBCDS operon, and its binding affinity depended on the presence and redox state of the [4Fe-4S]^2+,1+ clusters. A high affinity binding site, which controls sufBCDS expression, and a low affinity binding site, which controls sufR expression, were identified. The SufR binding sites, which are separated by 26 base pairs, each contain a perfect inverted repeat, CAAC-N₆-GTTG, and are highly conserved in cyanobacteria. The Fe/S protein SufR thus functions both as a transcriptional repressor of the sufBCDS operon and as an autoregulator of sufR.