Fig. 6. Structural comparison of DnaA and Cdc6/Orc1. (A) RIBBONS diagrams of DnaA and its closest structural homolog, Cdc6/Orc1. Domains are labeled as in Figure 2A, except that the HTH motifs are highlighted in yellow. (B) RIBBONS diagram superimposing the AAA+ regions of DnaA (light gray) and Cdc6/Orc1 (pale yellow). Elements critical for the spatial conservation of the bipartite ATP-interaction cleft are highlighted in magenta (DnaA) and blue (Cdc6/Orc1). The two proteins exhibit an r.m.s.d. of 2.3 Å across 139 of the 213 C_α residues that are present in core secondary structure elements. (C) Comparisons of DnaA and Cdc6/Orc1 OLEs. The RIBBONS diagram shows the related HTH topology (in yellow) of the C-terminal modules next to secondary structure topology diagrams that indicate how the HTH motif is anchored within each C-terminal fold. Secondary structure comparisons were performed using the sequential structure alignment program SSAP (Orengo and Taylor, 1996).