Fig. 2: Experimental key structures of the early PIC tRNA-chaperoned assembly cycle.

Isosurface representations of cryo-EM structures are shown and crucial structural elements are labeled. The intermediates are labeled I3–I5 according to their occurrence in the presumed assembly cycle. The color code of the individual factors is identical to that of Figs. 1, 3 and 4. Each structure is shown in a standard view that corresponds to that used in Fig. 3 and an additional view with 40° rotation. Rpo30 is highlighted in ochre only for minimal vRNAP. Note that, in general, all bound factors occupy corresponding positions; therefore, the visualized structures document an incremental buildup, eventually yielding to complete vRNAP in the absence of major conformational reorganizations (full assembly cycle in Fig. 3).