In vivo and in vitro interactions of ArcS, HptA, and ArcA. (A) Analysis of in vivo protein-protein interactions in a bacterial two-hybrid system. Interactions of the indicated proteins fused to the T18 and T25 fragments, respectively, of the B. pertussis adenylate cyclase result in a red appearance of the colonies on MacConkey agar. +, positive control (T18-zip/T25-zip); −, negative control (T18/T25 empty vectors). (B) Autoradiographic analysis of phosphotransfer between ArcS(646-1188), GST-HptA, and ArcA. Phosphorylated ArcA (10 μM) was incubated for the given amount of time (top) with equimolar amounts of the indicated components and then separated by SDS-PAGE.