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Applied and Environmental Microbiology, July 2001, p. 3002-3009, Vol. 67, No. 7
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.7.3002-3009.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Ecological Physiology of Synechococcus sp. Strain SH-94-5, a Naturally Occurring Cyanobacterium Deficient in Nitrate Assimilation

Scott R. Miller* and Richard W. Castenholz

Department of Biology, University of Oregon, Eugene, Oregon 97403

Received 11 January 2001/Accepted 11 April 2001

Synechococcus sp. strain SH-94-5 is a nitrate assimilation-deficient cyanobacterium which was isolated from an ammonium-replete hot spring in central Oregon. While this clone could grow on ammonium and some forms of organic nitrogen as sole nitrogen sources, it could not grow on either nitrate or nitrite, even under conditions favoring passive diffusion. It was determined that this clone does not express functional nitrate reductase or nitrite reductase and that the lack of activity of either enzyme is not due to inactivation of the cyanobacterial nitrogen control protein NtcA. A few other naturally occurring cyanobacterial strains are also nitrate assimilation deficient, and phylogenetic analyses indicated that the ability to utilize nitrate has been independently lost at least four times during the evolutionary history of the cyanobacteria. This phenotype is associated with the presence of environmental ammonium, a negative regulator of nitrate assimilation gene expression, which may indicate that natural selection to maintain functional copies of nitrate assimilation genes has been relaxed in these habitats. These results suggest how the evolutionary fates of conditionally expressed genes might differ between environments and thereby effect ecological divergence and biogeographical structure in the microbial world.

* Corresponding author. Present address: Mailstop 239-4, NASA Ames Research Center, Moffett Field, CA 94035. Phone: (650) 604-6052. Fax: (650) 604-1088. E-mail: srmiller{at}mail.arc.nasa.gov.

Applied and Environmental Microbiology, July 2001, p. 3002-3009, Vol. 67, No. 7
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.7.3002-3009.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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