Purine biosynthesis in Escherichia coli K12: structure and DNA sequence studies of the purHD locus

Abstract

The de novo purine biosynthetic enzymes 5‐amino‐4‐imidazolecarboxamide‐ribonucleotide (AICAR) transformylase (EC 2.1.2.3), IMP cyclohydrolase (EC 3.5.4.10) and glycineamide‐ribonucleotide (GAR) synthetase (EC 2.1.2.2) are encoded by the purHD locus of Escherichia coli. The DNA sequence of this locus revealed two open reading frames encoding polypeptides of Mr57335 and 45945 (GAR synthetase), respectively, that formed an operon. The DNA sequence, maxicell and complementation analyses all supported the concept that the M_r 57335 polypeptide is the product of the purH gene and encodes a bifunctional protein containing both AICAR trans‐formylase and IMP cyclohydrolase activities. The 5′ end of the purHD mRNA was determined by primer extension mapping and contains two regions of dyad symmetry capable of forming ‘hairpin’ loops where the formation of the one would prevent the formation of the other but not vice versa. Regulation by the purR gene product was explained by the discovery of a purR binding site in the purHD control region.