A Pair of Iron-Responsive Genes Encoding Protein Kinases with a Ser/Thr Kinase Domain and a His Kinase Domain Are Regulated by NtcA in the CyanobacteriumAnabaenasp. Strain PCC 7120

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Abstract
The filamentous cyanobacteriumAnabaenasp. strain PCC 7120 can fix N2when combined nitrogen is not available in the growth medium. It has a family of 13 genes encoding proteins with both a Ser/Thr kinase domain and a His kinase domain. The function of these enzymes is unknown. Two of them are encoded bypkn41(alr0709) andpkn42(alr0710). These two genes are separated by only 72 bp on the chromosome, and our results indicate that they are cotranscribed. The expression ofpkn41andpkn42is induced by iron deprivation irrespective of the nature of the nitrogen source. Mutants inactivating eitherpkn41,pkn42, or both grow similarly to the wild type under normal conditions, but their growth is impaired either in the presence of an iron chelator or under conditions of nitrogen fixation and iron limitation, two situations where the demand for iron is particularly strong. Consistent with these results, these mutants display lower iron content than the wild type and a higher level of expression fornifJ1andnifJ2, which encode pyruvate:ferredoxin oxidoreductases. BothnifJ1andnifJ2are known to be induced by iron limitation. NtcA, a global regulatory factor for different metabolic pathways, binds to the putative promoter region ofpkn41, and the induction ofpkn41in response to iron limitation no longer occurs in anntcAmutant. Our results suggest thatntcAnot only regulates the expression of genes involved in nitrogen and carbon metabolism but also coordinates iron acquisition and nitrogen metabolism by activating the expression ofpkn41andpkn42.