BIOSYNTHESIS OF RIBOFLAVIN - LUMAZINE SYNTHASE OF ESCHERICHIA-COLI

A gene located at 443 kilobases on the Escherichia coli chromosome (su bsequently designated ribE) was expressed in a recombinant E. coli str ain and was shown to code for the enzyme 6,7-dimethyl-8-ribityllumazin e synthase. The recombinant enzyme was purified to homogeneity. The pr otein is an icosahedral capsid of 60 subunits with a mass of about 1 M Da as shown by hydrodynamic studies and by electron microscopy. In con trast to the icosahedral lumazine synthase-riboflavin synthase complex of Bacillus subtilis, the lumazine synthase of E. coli is not physica lly associated with another enzyme of the riboflavin pathway, and the core of the icosahedral capsid is empty. The RIB4 gene of Saccharomyce s cerevisiae was also expressed to a high level (about 40% of cellular protein) in E. coli. The recombinant protein is a pentamer of 90 kDa. An insertion of 4 amino acids into helix alpha(4) is likely to hinder the formation of an icosahedral capsid by the yeast protein, The kine tic properties of lumazine synthase of E. coli, B. subtilis, and S. ce revisiae are similar.