John Golbeck

Professor of Biochemistry and Biophysics
Professor of Chemistry

310 South Frear Laboratory, University Park, PA 16802
Phone: (814) 865-1163
Fax: (814) 863-7024
E-mail: jhg5@psu.edu

B.S. in Chemistry from Valparaiso University
Ph.D. in Chemistry from Indiana University

Golbeck Lab Web Site

Our approach to study the assembly of the Fe/S clusters is to use genetic techniques to uncover the genes, proteins and factors that are involved in this complicated process. We identified a gene, termed sll0088 in Synechocystis sp. PCC 6803, that regulates the amount of Photosystem I in cyanobacterial cells. We uncovered this gene as a second-site suppressor to mutations involving the Cys ligands to the FA/FB clusters in PsaC. We found that the sll0088 gene, which we have renamed ‘sufR’ functions as a transcriptional repressor of the adjacent suf operon, which encodes proteins that are involved in the assembly of Fe/S clusters. The protein encoded by sufR has a high sequence similarity to transcription regulatory proteins with a conserved DNA binding domain and can be classified in the DeoR family of helix-loop-helix proteins. The protein falls into a further subclass that contains a C-(X)12-C-(X)13-C-(X)14-C motif near the C-terminus, which binds an Fe/S cluster, as shown by optical and EPR spectroscopy. We propose a feedback mechanism in which SufR senses the metabolic requirement for Fe/S clusters and responds by functioning as a repressor of the suf operon (Figure 2.2.2.). When demand for Fe/S clusters is low, SufR binds the Fe/S cluster, and the suf regulon is down-regulated. When demand is high, SufR does not bind the Fe/S cluster, and the suf regulon is up-regulated. To our knowledge, this is the first reported instance of a specific transcriptional regulator of the suf operon in any organism.The suf system functions to maintain Fe/S cluster homeostasis in the cyanobacterial cell, particularly in response to conditions of oxidative stress.

Figure 2.2.2. Model depicting genes (boxes), promoters (diamonds) and proteins (ovals) involved in the regulation of Fe/S cluster biogenesis in cyanobacteria. The dashed lines represent mRNA; the solid lines represent the flow of atoms; and the arrows with boxes represent the flow of electrons. The lighter color arrows represent variable factors that control whether or not transcription of the suf genes occurs. A simple competition for Fe/S clusters from SufA governs whether SufR has an Fe/S cluster, and therefore whether the transcription of the suf regulon occurs. Note that Photosystem I generates superoxide, which can inactivate Fe/S clusters in proteins, including FX, FB and FA as well as the Fe/S cluster in SufR. Note also the role of ferredoxin, FTR and thioredoxin in providing a secondary control mechanism that actives SufA via scission of disulfide bonds. Fd is ferredoxin, FTR is ferredoxin: thioredoxin reductase, and Tr is thioredoxin.