The depolymerization product of the action of intracellular PHB depolymerase on PHB is 3-hydroxybutyrate, which is further oxidized to acetoacetate by the enzyme 3-hydroxybutyrate dehydrogenase. This enzyme is encoded by the gene bdhA, and sequence analysis indicated that it is a member of the short chain alcohol dehydrogenase superfamily (Aneja, Charles 1999). Although the enzyme had been purified and biochemically characterized from a number of bacteria, this was the first description of a bacterial gene encoding 3-hydroxybutyrate dehydrogenase.
The bdhA gene is the first gene in an operon also containing xdhA2 and xdhB2, which encodes the two subunits of xanthine dehydrogenase/oxidase. Examination of cell extracts for xanthine oxidase activity on native gels confirmed that Tn5 insertion in bdhA abolishes downstream-encoded xanthine oxidase activity (Aneja, Charles 1999). This analysis also revealed the presence of a second xanthine oxidase activity that remains in the mutant strain. The recently completed genome sequence (Galibert et al. 2001) made it clear that this second activity is also encoded on megaplasmid pRmeSU47b, but in a different region of this replicón. The genes encoding this second activity have been designated xdhAl and xdhBl. These genes are found in an operon along with the downstream gene xdhC. The gene product of xdhC is probably involved in insertion of the molybdenum co-factor (Leimkiihler, Kern et al. 1998).
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