The purB gene of Escherichia coli K-12 is located in an operon

The structural gene (purB) for succinyl-AMP (S-AMP) lyase and three additional ORFs are on the same DNA strand of the chromosome of Escherichia coli. Cassette mutagenesis and primer extension mapping demonstrated that purB is co-transcribed with an upstream gene (ORF23. or ycfC) encoding a 22.9 kDa membrane-associated protein of non-essential. but unknown, function unrelated to purine biosynthesis. The purB operon lies between phoP and an ORF expressing an essential function which may correspond to asuE (trmU). S-AMP lyase was purified to near homogeneity. The purified enzyme is a homotetramer of 50 kDa subunits, has a K-m for S-AMP of 3.7 mu M and a pH optimum of 7.4-7.6.