FIGURE 1. X-ray crystal structure of the mutant β^148–152 clamp protein.

(A) Face view (C-side) of the structure of the mutant β^148–152 clamp protein depicted in ribbon form. The three subdomains comprising each protomer (A1–3, and B1–3) are indicated, as are the positions of the loops containing the 148-HQDVR-152 poly-Ala substitutions (150; depicted as space filled atoms), and residues L21–L27 (24; depicted in ribbon form). Protomer A is in green, and protomer B is in blue. (B) Face view (C-side) of the wild type β clamp protein (purple) encircling primed DNA, the backbone of which is shown in gold (PDB: 3BEP)¹¹. Protomer A of β^148–152 from panel A (green) is superimposed onto protomer A of the wild type clamp (purple). Positions of the 148-HQDVR-152 poly-Ala substitutions, G66 and G174, which are substituted with E and G, respectively, in β159, as well as P20, which is substituted with L in dnaN783, and acts to suppress the temperature sensitive growth phenotype of the dnaN159 allele,²⁵ are indicated. Enlarged views of the loop containing residues 148-HQDVR-152 in protomer A (C) or B (D) are shown. Wild type β (PDB: 1OK7)²⁰ is depicted in green, while β^148–152 is in yellow (C) or purple (D). (E) Enlarged view of the loop containing residues L21–L27. Protomer A of wild type β (PDB: 1OK7) is depicted in yellow, while protomer A of β^148–152 is in green. (F) Enlarged view of the water network near the β^148–152 dimer interface. Positions R269 of protomer A (yellow), and S101 and S104 of protomer B (purple) are indicated. Water is depicted as space filled atoms. (G) Enlarged view of the interaction of protomer A of β^148–152 with calcium present in the crystallization buffer. Positions of resides E52, N118, and L119 are indicated. Calcium is depicted as space filled atoms.