Crystallization and preliminary crystallographic analysis of T7 RNA polymerase elongation complex.

Stable transcription-elongation complexes consisting of T7 RNA polymerase (molecular mass 99 kDa) in association with a nucleic acid scaffold consisting of an 8 bp RNA-DNA hybrid and 10 bp of downstream DNA were assembled and crystallized by the sitting-drop vapour-diffusion technique under near-physiological conditions. The crystals diffract beyond 2.6 A resolution and belong to space group P1, with unit-cell parameters a = 79.91, b = 84.97, c = 202 A, alpha = 90.36, beta = 92.97, gamma = 109.94 degrees. An unambiguous molecular-replacement solution was found using the C-terminal portion of the T7 RNA polymerase structure from the early initiation complex as a search model. Model building and structure refinement are now in progress.