FIGURE 2.

Structure and topology of SidNA3. A, ribbon diagram of the SidNA3 structure. The N-terminal domain is made up of a distorted six-stranded antiparallel β-barrel (red) together with an αβαβα structure formed by two predominantly parallel β-sheets flanked by α-helices (blue and purple). The C-terminal domain (orange) is made up of a three-stranded antiparallel β-sheet surrounded by three α-helices. A long loop in the C-terminal domain, located between the final β-strand and the final α-helix, loops back toward the N-terminal domain and is marked. B, topology diagram of SidNA3. Diagram is drawn in the same style as that of the PheA topology diagram in Conti et al. (9) and using the same numbering scheme for the β-strands. The circles represent α-helices and the arrows β-strands. The locations of the chain breaks (because of poor electron density) in the SidNA3 model are marked.