Inactivation of iron-sulfur cluster biogenesis regulator SufR in Synechocystis sp. PCC 6803 induces unique iron-dependent protein-level responses

Abstract

<h4>Background</h4><p>Iron-sulfur (Fe-S) clusters are protein-bound cofactors associated with cellular electron transport and redox sensing, with multiple specific functions in oxygen-evolving photosynthetic cyanobacteria. The aim here was to elucidate protein-level effects of the transcriptional repressor SufR involved in the regulation of Fe-S cluster biogenesis in the cyanobacterium <em>Synechocystis</em> sp. PCC 6803.</p><h4>Methods</h4><p>The approach was to quantitate 94 pre-selected target proteins associated with various metabolic functions using SRM in <em>Synechocystis</em>. The evaluation was conducted in response to <em>sufR</em> deletion under different iron conditions, and complemented with EPR analysis on the functionality of the photosystems I and II as well as with RT-qPCR to verify the effects of SufR also on transcript level.</p><h4>Results</h4><p>The results on both protein and transcript levels show that SufR acts not only as a repressor of the <em>suf</em> operon when iron is available but also has other direct and indirect functions in the cell, including maintenance of the expression of pyruvate:ferredoxin oxidoreductase NifJ and other Fe-S cluster proteins under iron sufficient conditions. Furthermore, the results imply that in the absence of iron the <em>suf</em> operon is repressed by some additional regulatory mechanism independent of SufR.</p><h4>Conclusions</h4><p>The study demonstrates that Fe-S cluster metabolism in <em>Synechocystis</em> is stringently regulated, and has complex interactions with multiple primary functions in the cell, including photosynthesis and central carbon metabolism.</p><h4>General significance</h4><p>The study provides new insight into the regulation of Fe-S cluster biogenesis via <em>suf</em> operon, and the associated wide-ranging protein-level changes in photosynthetic cyanobacteria.</p>