Streptavidin-bound oligonucleotides ease the determination of helicase directionality. Helicase directionality assays were performed as described in "Methods" using the substrates shown at the top (I-IV). In each substrate the longer oligonucleotide is 32P-labeled and streptavidin is marked with a closed circle. Panels A and D, spMCM; panels B and E, taMCM; panels C and F, ecDnaB. Representative results are shown (A-C) together with a summary and standard deviation of three independent experiments (D-F). The amount of protein (as hexamer, panel A, and monomer, panels B and C) used and whether ATP was present or not is indicated at the top of each panel.