Using the human cDNA sequence corresponding to guanine deaminase, the Esche
richia call genome was scanned using the Basic Local Alignment Search Tool
(BLAST), and a corresponding 439-residue open reading frame of unknown func
tion was identified as having 36% identity to the human protein, The putati
ve gene was amplified, subcloned into the pMAL-c2 vector, expressed, purifi
ed, and characterized enzymatically. The 50.2-kDa protein catalyzed the con
version of guanine to xanthine, having a K-m of 15 mu M with guanine and a
k(cat) of 3.2 s(-1). The bacterial enzyme shares a nine-residue heavy metal
binding site with human guanine deaminase, PG[FL]VDTHIH, and was found to
contain approximately 1 mol of zinc per mol of subunit of protein. The E. c
oli guanine deaminase locus is 3' from an open reading frame which shows ho
mology to a bacterial purine base permease.