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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.
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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 ml1 for 15N-labeled DNA, relative to 0.036 g ml1 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 ml1. 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.
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