Fig. 2.

Tertiary structure representation of an S-subunit. (a) Stereogram of the overall structure. The monomeric structure is displayed by ribbon representation and hydrogen-bonding residues at domain interfaces are indicated by stick models. The highly conserved PLPP sequences and DNA binding clefts are marked. A monomer consists of four successive structural domains: the globular TRD1, a long helical CCR domain, the globular TRD2, and a C-terminal DCR helix. Two almost identical TRDs are attached to both ends of complementary CR helices. The N and C termini form a β-strand and a β-sheet to close both termini. The figure was prepared by using molscript (35). (b) Topology diagram of the S-subunit of a type I R-M system from M. jannaschii. Two TRDs are symmetrically located around the complementary CR helices, except for the two β-strands (βN and βC) at the N and C terminus of TRD1.