Studies on the Conformation of DNA‐Dependent RNA Polymerase in Solution by Small‐Angle X‐Ray Measurements

The size and shape of DNA‐dependent RNA polymerase from Escherichia coli K 12 were studied by small‐angle X‐ray measurements. The equilibrium between the monomeric and dimeric form of the polymerase was investigated in dependence on the ionic strength of the solution. Homodisperse solutions of the holo‐enzyme monomer and dimer and of the core enzyme monomer were investigated. For each particle the radius of gyration R , the molecular weight M r and the volume V were determined; in detail the following values were obtained: RNA polymerase holo‐enzyme monomer ( R = 6.2 nm, M r = 4.9 × 10 5 , V = 9.8 × 10 2 nm 3 ), holo‐enzyme dimer ( R = 9.5 nm, M r = 9.95 × 10 5 , V = 35 × 10 2 nm 3 ), core enzyme ( R = 6.0 nm, M r = 3.8 × 10 5 , V = 9.3 × 10 2 nm 3 ). The hole‐enzyme dimer is equivalent in scattering to a flat hollow cylinder of the following dimensions: length 10 nm, outer diameter 25 nm, inner diameter 5 nm. The shape of the holo‐enzyme monomer is not equivalent in scattering to any body which can be obtained by dividing this cylinder into two halves. The shape of the monomer is similar to that of the core enzyme; both are elongated bodies, which show a cross‐section factor.