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 ad ditional ORFs are on the same DNA strand of the chromosome of Escheric hia coli. Cassette mutagenesis and primer extension mapping demonstrat ed 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 li es between phoP and an ORF expressing an essential function which may correspond to asuE (trmU). S-AMP lyase was purified to near homogeneit y. 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.