Galactose-1-phosphate uridyl transferase (EC 2.7.7.12) (galT) catalyzes the transfer of an uridyldiphosphate group on galactose (or glucose) 1-phosphate. During the reaction, the uridyl moiety links to a histidine residue. In the Escherichia coli enzyme, it has been shown [1] that two histidine residues separated by a single proline residue are essential for enzyme activity. The first one is a ligand to a zinc ion and the second act as a nucleophile.

On the basis of sequence similarities, two apparently unrelated families seem to exist. Class-I enzymes are found in eukaryotes as well as some bacteria such as Escherichia coli or Streptomyces lividans, while class-II enzymes have been found so far only in some Gram-positive bacteria such as Bacillus subtilis or Lactobacillus helveticus [2].

We developed signature patterns for both families. For class-I enzymes the signature is based on the active site residues. For class-II enzymes we chose a region which also includes two conserved histidines.