Polymerase and Histidinol Phosphatase domain of bacterial polymerase X
The bacterial/archaeal X-family DNA polymerases (PolXs) have a PHP domain at their C-terminus. The bacterial/archaeal PolX core domain and PHP domain interact with each other and together are involved in metal dependent 3'-5' exonuclease activity. The PHP (also called histidinol phosphatase-2/HIS2) domain is associated with several types of DNA polymerases, such as PolIIIA and family X DNA polymerases, stand alone histidinol phosphate phosphatases (HisPPases), and a number of uncharacterized protein families. The PHP domain has four conserved sequence motifs and contains an invariant histidine that is involved in metal ion coordination. PolX is found in all kingdoms, however bacterial PolXs have a completely different domain structure from eukaryotic PolXs. Bacterial PolX has an extended conformation in contrast to the common closed 'right hand' conformation for DNA polymerases. This extended conformation is stabilized by the PHP domain. The PHP domain of PolX is structurally homologous to other members of the PHP family that has a distorted (beta/alpha)7 barrel fold with a trinuclear metal site on the C-terminal side of the barrel.
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