Regulation of the Enzymes of the β‐Ketoadipate Pathway in Moraxella

Abstract

1. The ability to oxidize quinate is elicited in Acinetobacter calco-aceticus not only by growth on quinate but also by growth on protocatechuate or its precursors such as shikimate and p-hydroxybenzoate. Accordingly the synthesis of a quinate dependent dehydrogenase was found to be induced by protocatechuate. Moreover, this enzyme seems to belong to the same coordinate block of enzymes responsible for the conversion of shikimate to /3-ketoadipyl-CoA.2. Analogies between quinate and shikimate oxidation in A . calco-aceticus have been substantiated by showing that both quinate dehydrogenase and shikimate dehydrogenase activities rely on a single enzyme.3. Several auxotrophs that have lost the ability to utilize both quinate and shikimate as sole source of carbon and energy have been isolated. Particularly relevant for this study are two of these mutants in which the dehydrogenase is severely impaired and correspondingly their ability t o oxidize the mentioned hydroaromatic compounds is substantially reduced. The properties of these auxotrophs and their revertants confirm the key role of the dehydrogenase in the dissimilation of both quinate and shikimate. The rationale of the control of this enzyme by protocatechuate is discussed.

Unusual ancestry of dehydratases associated with quinate catabolism in Acinetobacter calcoaceticus

“…Dehydroshikimate dehydratase was not present at detectable levels after growth of E. coli DH5␣(pZR571) in LB. (6,28,54), and growth of wild-type cells with protocatechuate increased the dehydroquinate dehydratase specific activity to 1.0 mol/min/mg of protein. The induced activity, attributable to quiB expression in wild-type strain ADP1, represented two-thirds of the total activity and was inactivated by heat treatment.…”

“…Thus, the failure of A. calcoaceticus AroD to complement dysfunctional QuiB effectively may be due to physical separation of the biosynthetic and catabolic dehydratases. A physiological advantage conferred by such an arrangement would be avoidance of catabolic removal of biosynthetic intermediates required for formation of aromatic acids (4,33,54).…”

“…As described here, the qui genes are associated with the conversion of quinate and shikimate to protocatechuate. All of the genes are expressed in response to protocatechuate (54). The number above each gene is its length in base pairs.…”

“…It therefore is surprising that the biosynthetic AroD function in the gram-negative bacterium Acinetobacter calcoaceticus is served by a constitutively expressed enzyme that appears to be type II as judged by its thermostability (53). Catabolism of dehydroquinate in these bacteria is initiated by an inducible QuiB enzyme that, as indicated by its thermolability, appears to be type I (28,53,54).…”

“…It therefore is surprising that the biosynthetic AroD function in the gram-negative bacterium Acinetobacter calcoaceticus is served by a constitutively expressed enzyme that appears to be type II as judged by its thermostability (53). Catabolism of dehydroquinate in these bacteria is initiated by an inducible QuiB enzyme that, as indicated by its thermolability, appears to be type I (28,53,54).Further exploration of relationships among the A. calcoaceticus dehydratases was made possible by localization of the qui genes in a supraoperonic cluster between pca genes (associated with the catabolism of protocatechuate) and pob genes (associated with conversion of p-hydroxybenzoate to protocatechuate) (2,20,23). Engineered deletion mutations were introduced into A. calcoaceticus by natural transformation, and the properties of the resulting mutant strains allowed identification of quiA, the gene for the dehydrogenase that catalyzes the conversion of quinate and shikimate to dehydroquinate and dehydroshikimate, respectively ( Fig.…”