Competitive gel retardation experiment. (A) Model of eight overlapping 50 bp DNA fragments covering the 220 bp upstream region of amtB. (B). When each of these fragments was added in 1000-fold surplus to gel retardation samples consisting of 220 bp digoxigenin-labeled amtB promoter fragment and purified MBP-GlnR, an inhibition of the shift was spotted for fragments 4, 5, 6 and weakly 2. (C) Model of three overlapping 25 bp DNA fragments (6.1-6.3) and four 15 bp fragments (6a-d) covering the 50 bp fragment 6 in the upstream region of amtB. (D) Addition of these fragments in 1,000-fold surplus to gel retardation samples containing 50 bp digoxigenin-labeled fragment 6 and 600 ng MBP-GlnR each. An inhibition of the shift was detected for fragment 6.1. -: free DNA as negative control. +: DNA plus 600 ng MBP-GlnR as positive control.