T7 RNA polymerase studied by force measurements varying cofactor concentration.

RNA polymerases carry out the synthesis of an RNA copy from a DNA template. They move along DNA, incorporate nucleotide triphosphate (NTP) at the end of the growing RNA chain, and consume chemical energy. In a single-molecule assay using the T7 RNA polymerase, we study how a mechanical force opposing the forward motion of the enzyme along DNA affects the translocation rate. We also study the influence of nucleotide and magnesium concentration on this process. The experiment shows that the opposing mechanical force is a competitive inhibitor of nucleotide binding. Also, the single-molecule data suggest that magnesium ions are involved in a step that does not depend on the external load force. These kinetic results associated with known biochemical and mutagenic data, along with the static information obtained from crystallographic structures, shape a very coherent view of the catalytic cycle of the enzyme: translocation does not take place upon NTP binding nor upon NTP cleavage, but rather occurs after PPi release and before the next nucleotide binding event. Furthermore, the energetic bias associated with the forward motion of the enzyme is close to kT and represents only a small fraction of the free energy of nucleotide incorporation and pyrophosphate hydrolysis.