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Structural studies of −10 promoter element recognition by domain 2 of the RNA polymerase σ subunit [Feklistov & Darst (2011), Cell, 147, 1257–1269] reveal an unusual crystal-packing arrangement dominated by G-quartets. The 3′-terminal GGG motif of the oligonucleotide used in crystallization participates in G-quadruplex formation with GGG motifs from symmetry-related complexes. Stacking between neighboring G-quadruplexes results in the formation of pseudo-continuous four-stranded columns running throughout the length of the crystal (G-columns). Here, a new crystal form is presented with a different arrangement of G-columns and it is proposed that the fortuitous finding of G-­quartet packing could be useful in engineering crystal contacts in protein–ssDNA complexes.

Supporting information

PDB reference: σA2–3–TGTACAATGGG, 4ki2


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