A functional interaction between the Q823 and R829 positions. (a) Model of the T. thermophilus bridge helix kink (PDB 1IW7). The interacting residues (β' D1090 and R1096) are shown as space-filling models and the surrounding helix in green in ribbon representation. Note that the flipped-out D1090 residue juxtaposes its side chain opposite R1096. The resulting contact stabilizes the kinked α helix. (b) High-throughput transcription assay results of mjA' R829X substitutions. The results are shown relative to wild-type activity (100%; dashed line). Single substitution mutant results are shown in dark blue with the substituted residues shown along the x-axis positions; note that all substitutions, except K, result in a substantial drop of catalytic activity. The results of two double mutant constructs, Q823R/R829D (R-D) and Q823H/R829D (H-D), are shown on the same scale as a separate graph with green bars. Error bars indicate standard deviation (n = 4). (c) Abortive and elongation transcription assay results of the double mutants. Q823R/R829D is inactive; Q823H/R829D has 49% (abortive assay) and 52% (elongation assay) of wild-type activity.