Two distinct classes of FixJ binding sites defined by in vitro selection

The phosphorylated FixJ transcriptional activator is key to nitrogen fixation in Sinorhizobium meliloti, switching both the nifA and fixK genes on. Previously no consensus picture emerged concerning the nature of FixJ binding sites. Here we used in vitro DNA selection in order to systematically characterise FixJ binding sequences. This led to the definition of two classes of sites. Class I sites share the CTAAG-TAGTTTCCC sequence found in the fixK promoter, whereas class 11 sites are defined by a GTAMGTAG consensus octamer. Our results indicate that FixJ similar to P binds DNA following two distinct binding modes. (C) 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.