Fig. 5.

A, B De novo initiation and elongation complexes. A De novo initiation of RNA synthesis involves binding of the initiating nucleotide (GTPi; red) at the priming or initiation site (P-site;green box) and binding of the first NTP substrate (GTPi+1; blue) to the nucleotide binding site (N-site; white box). Specific binding sites for divalent cations (pink circles A and B) are shown in close proximity to the α-, β-, and γ-phosphates of the first nucleotide substrate. B Elongation complex. Nucleotide addition during elongation involves binding of the nascent RNA primer strand, positioning of the 3′-terminal nucleotide in the P-site, and binding of the first NTP substrate (i+1, blue) to the nucleotide binding site (N-site; white box)