Crystallization and preliminary structure determination of Escherichia coli Mfd, the transcription-repair coupling factor.

Transcription-repair coupling factors (TRCFs) are SF2 ATPases that couple transcription to DNA-damage repair by recognizing and removing RNA polymerase-elongation complexes stalled at DNA lesions and recruiting the nucleotide excision-repair machinery to the damaged sites. As a first step towards understanding the TRCF mechanism, the 130 kDa Escherichia coli TRCF (the product of the mfd gene) has been overexpressed, purified and crystallized using an unusual precipitant, pentaerythritol ethoxylate. Initial phases were obtained using single-wavelength anomalous dispersion with a highly redundant 4 A resolution data set collected from selenomethionyl-substituted crystals and dramatically improved by density modification and phase extension to 3.2 A resolution. Model building and refinement, which are in progress, will provide insight into transcription-coupled DNA-repair pathways, as this represents the first TRCF to be crystallized to date.