Regulatory Roles for IscA and SufA in Iron Homeostasis and Redox Stress Responses in the Cyanobacterium Synechococcus sp. Strain PCC 7002

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Abstract
SufA, IscA, and Nfu have been proposed to function as scaffolds in the assembly of Fe/S clusters in bacteria. To investigate the roles of these proteins further, single and double null-mutant strains of Synechococcus sp. strain PCC 7002 were constructed by insertional inactivation of genes homologous to sufA, iscA, and nfu. Demonstrating the nonessential nature of their products, the sufA, iscA, and sufA iscA mutants grew photoautotrophically with doubling times that were similar to the wild type under standard growth conditions. In contrast, attempts to inactivate the nfu gene only resulted in stable merodiploids. These results imply that Nfu, but not SufA or IscA, is the essential Fe/S scaffold protein in cyanobacteria. When cells were grown under iron-limiting conditions, the iscA and sufA mutant strains exhibited less chlorosis than the wild type. Under iron-sufficient growth conditions, isiA transcript levels, a marker for iron limitation in cyanobacteria, as well as transcript levels of genes in both the suf and isc regulons were significantly higher in the iscA mutant than in the wild type. Under photosynthesis-induced redox stress conditions, the transcript levels of the suf genes are notably higher in the sufA and the sufA iscA mutants than in the wild type. The growth phenotypes and mRNA abundance patterns of the mutant strains contradict the proposed scaffold function for the SufA and IscA proteins in generalized Fe/S cluster assembly and instead suggest that they play regulatory roles in iron homeostasis and the sensing of redox stress in cyanobacteria.