EVIDENCE FOR LOCAL-32 SYMMETRY IN HOMOTRIMERIC RIBOFLAVIN SYNTHASE OFESCHERICHIA-COLI

Riboflavin synthase is a trimer of identical 23-kDa subunits. The prim ary structure is characterized by considerable similarity of the C-ter minal and N-terminal parts. Recombinant riboflavin synthase of Escheri chia coli and Bacillus subtilis was crystallized by the vapor diffusio n method. Crystals of E. coli riboflavin synthase belong to the orthor hombic system, space group P2(1)2(1)2(1), with unit cell dimensions a = 53.2 Angstrom, b = 117.6 Angstrom, c = 150.9 Angstrom, alpha = beta = gamma = 90 degrees. They diffract to better than 3.3 Angstrom resolu tion and have presumably one trimer in the asymmetric unit. The self r otation function indicates local 32 symmetry. Twofold local symmetry i s an unexpected result in a trimeric protein. In conjunction with prim ary structure arguments and mechanistic considerations, we propose tha t the protein is a pseudohexamer where each of the peptide subunits fo ld into two topologically similar domains. (C) 1998 Academic Press.