Genetic Identification of a High-Affinity Ni Transporter and the Transcriptional Response to Ni Deprivation in Synechococcus sp. Strain WH8102

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Abstract
One biological need for Ni in marine cyanobacteria stems from the utilization of the Ni metalloenzyme urease for the assimilation of urea as a nitrogen source. In many of the same cyanobacteria, includingSynechococcussp. strain WH8102, an additional and obligate nutrient requirement for Ni results from usage of a Ni superoxide dismutase (Ni-SOD), which is encoded bysodN. To better understand the effects of Ni deprivation on WH8102, parallel microarray-based analysis of gene expression and gene knockout experiments were conducted. The global transcriptional response to Ni deprivation depends upon the nitrogen source provided for growth; fewer than 1% of differentially expressed genes for Ni deprivation on ammonium or urea were concordantly expressed. Surprisingly, genes for putative Ni transporters, including one colocalized on the genome withsodN,sodT, were not induced despite an increase in Ni transport. Knockouts of the putative Ni transporter genesodTappeared to be lethal in WH8102, so the genes forsodTandsodNin WH8102 were interrupted with the gene for Fe-SOD,sodB, and its promoter fromSynechococcussp. strain WH7803. ThesodT::sodBexconjugants were unable to grow at low Ni concentrations, confirming that SodT is a Ni transporter. ThesodN::sodBexconjugants displayed higher growth rates at low Ni concentrations than did the wild type, presumably due to a relaxed competition between urease and Ni-SOD for Ni. BothsodT::sodBandsodN::sodBlines exhibited an impaired ability to grow at low Fe concentrations. We propose a posttranslational allosteric SodT regulation involving the binding of Ni to a histidine-rich intracellular protein loop.

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