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 Å resolution and belong to space group P1, with unit‐cell parameters a = 79.91, b = 84.97, c = 202 Å, α = 90.36, β = 92.97, γ = 109.94°. 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.