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Appl. Environ. Microbiol. doi:10.1128/AEM.02855-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Transcription of hupSL is regulated by NtcA and not by hydrogen in Anabaena variabilis ATCC 29413

University of Missouri – St. Louis, Dept. of Biology, Research 223, St. Louis, MO 63121

^* To whom correspondence should be addressed. Email: thiel{at}umsl.edu.

	Abstract

Nitrogen-fixing cyanobacteria such as Anabaena variabilis ATCC 29413 use an uptake hydrogenase, encoded by hupSL, to recycle hydrogen gas that is produced as an obligate by-product of nitrogen fixation. Regulation of hupSL in A. variabilis is likely to differ from the closely related Anabaena sp. strain PCC 7120 because it lacks the excision element-mediated regulation that characterizes hupSL regulation in strain PCC 7120. Analysis of the hupSL transcript in a nitrogenase mutant of A. variabilis that does not produce any detectable hydrogen indicated that neither nitrogen fixation nor hydrogen gas was required for the induction of hupSL. Furthermore, exogenous addition of hydrogen gas did not stimulate hupSL transcription. Transcriptional reporter constructs indicated that accumulation of hupSL transcript after nitrogen step-down was restricted primarily to the microaerobic heterocysts. Anoxic conditions were not sufficient to induce hupSL transcription. Induction of hupSL after nitrogen step-down was reduced in a mutant in the global nitrogen regulator, NtcA, but was not reduced in a mutant unable to form heterocysts. A consensus NtcA-binding site was identified upstream of hupSL, and NtcA was found to bind to this region. Thus, while neither hydrogen gas nor anoxia controlled expression of hupSL, expression was controlled by NtcA. Heterocyst differentiation was not required for hupSL induction in response to nitrogen step-down, but heterocyst-localized cues may add an additional level of regulation to hupSL.