THE PCA-POB SUPRAOPERONIC CLUSTER OF ACINETOBACTER-CALCOACETICUS CONTAINS QUIA, THE STRUCTURAL GENE FOR QUINATE-SHIKIMATE DEHYDROGENASE

An 18-kbp Acinetobacter calcoaceticus chromosomal segment contains the pcaIJFBDKCHG operon, which is required for catabolism of protocatechu ate, and pobSRA, genes associated with conversion of p-hydroxybenzoate to protocatechuate. The genetic function of the 6.5 kbp of DNA betwee n pcaG and pobS was unknown. Deletions in this DNA mere designed by re moval of fragments between restriction sites, and the deletion mutatio ns were introduced into A. calcoaceticus by natural transformation. Th e mutations prevented growth with either quinate or shikimate, growth substrates that depend upon qui gene function for their catabolism to protocatechuate. The location of quiA, a gene encoding quinate-shikima te dehydrogenase, was indicated by its expression in one of the deleti on mutants, and the position of the gene was confirmed by determinatio n of its 2,427-bp nucleotide sequence. The deduced amino acid sequence of QuiA confirmed that it is a member of a family of membrane-associa ted, pyrrolo-quinoline quinone-dependent dehydrogenases, as had been s uggested by earlier biochemical investigations. Catabolism of quinate and shikimate is initiated by NAD(+)-dependent dehydrogenases in other microorganisms, so it is evident that different gene pools were calle d upon to provide the ancestral enzyme for this metabolic step.