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Applied and Environmental Microbiology, May 2007, p. 3189-3195, Vol. 73, No. 10
0099-2240/07/$08.00+0     doi:10.1128/AEM.02609-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Stable Isotope Probing with ¹⁵N Achieved by Disentangling the Effects of Genome G+C Content and Isotope Enrichment on DNA Density ^,

Department of Crop and Soil Sciences, Cornell University, Ithaca, New York 14853

Received 8 November 2006/ Accepted 12 March 2007

Stable isotope probing (SIP) of nucleic acids is a powerful tool that can identify the functional capabilities of noncultivated microorganisms as they occur in microbial communities. While it has been suggested previously that nucleic acid SIP can be performed with ¹⁵N, nearly all applications of this technique to date have used ¹³C. Successful application of SIP using ¹⁵N-DNA (¹⁵N-DNA-SIP) has been limited, because the maximum shift in buoyant density that can be achieved in CsCl gradients is approximately 0.016 g ml^–1 for ¹⁵N-labeled DNA, relative to 0.036 g ml^–1 for ¹³C-labeled DNA. In contrast, variation in genome G+C content between microorganisms can result in DNA samples that vary in buoyant density by as much as 0.05 g ml^–1. Thus, natural variation in genome G+C content in complex communities prevents the effective separation of ¹⁵N-labeled DNA from unlabeled DNA. We describe a method which disentangles the effects of isotope incorporation and genome G+C content on DNA buoyant density and makes it possible to isolate ¹⁵N-labeled DNA from heterogeneous mixtures of DNA. This method relies on recovery of "heavy" DNA from primary CsCl density gradients followed by purification of ¹⁵N-labeled DNA from unlabeled high-G+C-content DNA in secondary CsCl density gradients containing bis-benzimide. This technique, by providing a means to enhance separation of isotopically labeled DNA from unlabeled DNA, makes it possible to use ¹⁵N-labeled compounds effectively in DNA-SIP experiments and also will be effective for removing unlabeled DNA from isotopically labeled DNA in ¹³C-DNA-SIP applications.

Published ahead of print on 16 March 2007.

Supplemental material for this article may be found at http://aem.asm.org/.

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