A, location of the nine conserved sequence motifs in the DEAD box helicase RhlB. Color scheme indicates motifs involved in ATP binding and hydrolysis (red), RNA binding (green), and ATP-induced conformational change (yellow). Motif II is the Walker B motif, DEAD. The consensus amino acid sequence of motif V for E. coli DEAD box helicases is shown together with the sequence of RhlB highlighting the change in the final amino acid (underlined). B, homology model of RhlB, with domains 1 and 2 colored green and blue, respectively. The ATP, RNA, and locations of the residues mutated in this study are shown in stick representation. The inset is a close-up of the helix-loop-helix that forms theQ-motif in DEAD box helicases, with Phe-28 stacking with the adenine base and the invariantly conserved Gln-35 residue positioned to form hydrogen bonds with the N-6 and N-7 positions of the adenine base.