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Figure 3
Experimental phasing and density modification. (a) Binding positions of the different derivatives identified by cross-difference Fourier using available model phases. Tantalum, ytterbium and zinc are shown in green, red and yellow, respectively. (b) Pol I model fitted into the EM reconstructions of Pol III (EMD-1802) and the Pol II pre-initiation complex (EMD-2306), showing comparable positions for the A49/A34.5 heterodimer (violet/blue) and its Pol III and Pol II counterparts C37/C53 and TFIIF, respectively. (c) General view (top) and zoom (bottom) of the electron-density maps (contoured at 1σ) obtained during the Pol I structure determination. From left to right: solvent-flattened map after tantalum phasing in C2-93 (Ta), solvent-flattened map after ytterbium phasing in C2-90 (Yb), map resulting from a multi-crystal averaging in C2-93 (MXA) and final refined map in C2-100 (Ref). While the initial Ta map showed the position of the A49/A34.5 dimerization module, the Yb map showed improved density for the main chain and the MXA map showed details of side chains.

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ISSN: 2059-7983
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