174 , No. 15 Localization of Upstream Sequence Elements Required for Nitrate and Anaerobic Induction of fdn ( Formate Dehydrogenase-N ) Operon Expression in Escherichia coli K-12 JING LI ' AND VALLEY STEWART ' 2 * Sections of Microbiology ' and Genetics & Development , 2 Cornell University , Ithaca , New York 14853-8101 Received 25 March 1992/Accepted 19 May 1992 Two transcriptional activators , the FNR and NARL proteins , are required for induction of the fdnGHI operon , encoding Escherichia coli formate dehydrogenase-N .