Isolation of PDX2, a second novel gene in the pyridoxine biosynthesis pathway of eukaryotes, archaebacteria, and a subset of eubacteria.

In this paper we describe the isolation of a second gene in the newly identified pyridoxine biosynthesis pathway of archaebacteria, some eubacteria, fungi, and plants. Although pyridoxine biosynthesis has been thoroughly examined in Escherichia coli, recent characterization of the Cercospora nicotianae biosynthesis gene PDX1 led to the discovery that most organisms contain a pyridoxine synthesis gene not found in E. coli. PDX2 was isolated by a degenerate primer strategy based on conserved sequences of a gene specific to PDX1-containing organisms. The role of PDX2 in pyridoxine biosynthesis was confirmed by complementation of two C. nicotianae pyridoxine auxotrophs not mutant in PDX1. Also, targeted gene replacement of PDX2 in C. nicotianae results in pyridoxine auxotrophy. Comparable to PDX1, PDX2 homologues are not found in any of the organisms with homologues to the E. coli pyridoxine genes, but are found in the same archaebacteria, eubacteria, fungi, and plants that contain PDX1 homologues. PDX2 proteins are less well conserved than their PDX1 counterparts but contain several protein motifs that are conserved throughout all PDX2 proteins.