Unusual ancestry of dehydratases associated with quinate catabolism in Acinetobacter calcoaceticus

Abstract

Catabolism of quinate to protocatechuate requires the consecutive action of quinate dehydrogenase (QuiA), dehydroquinate dehydratase (QuiB), and dehydroshikimate dehydratase (QuiC). Genes for catabolism of protocatechuate are encoded by the pca operon in the Acinetobacter calcoaceticus chromosome. Observations reported here demonstrate that A. calcoaceticus qui genes are clustered in the order quiBCXA directly downstream from the pca operon. Sequence comparisons indicate that quiX encodes a porin, but the specific function of this protein has not been clearly established. Properties of mutants created by insertion of ⍀ elements show that quiBC is expressed as part of a single transcript, but there is also an independent transcriptional initiation site directly upstream of quiA. The deduced amino acid sequence of QuiC does not resemble any other known sequence. A. calcoaceticus QuiB is most directly related to a family of enzymes with identical catalytic activity and biosynthetic AroD function in coliform bacteria. Evolution of A. calcoaceticus quiB appears to have been accompanied by fusion of a leader sequence for transport of the encoded protein into the inner membrane, and the location of reactions catalyzed by the mature enzyme may account for the failure of A. calcoaceticus aroD to achieve effective complementation of null mutations in quiB. Analysis of a genetic site where a DNA segment encoding a leader sequence was transposed adds to evidence suggesting horizontal transfer of nucleotide sequences within genes during evolution.Dehydroquinate dehydratase (QuiB) and dehydroshikimate dehydratase (QuiC) catalyze consecutive reactions allowing growth of microorganisms with quinate by its conversion to protocatechuate ( Fig. 1) and subsequent metabolism by the ␤-ketoadipate pathway (41, 52). The enzymatic activity of QuiB is identical to that of AroD, an enzyme that catalyzes biosynthetic formation of dehydroshikimate (Fig. 1). Enzymes with similar catalytic activities often can be traced to a common ancestor (32), and the similarity of dehydratases associated with dehydroshikimate metabolism raised the possibility that a single ancestral protein was the evolutionary precursor of two or more of these enzymes.In contrast to this prediction, biosynthetic dehydroquinate dehydratases differ substantially from catabolic enzymes with this activity (18,30). With the exception of the dehydroquinate dehydratases of gram-positive bacteria (13, 15), enzymes with the biosynthetic function indicated as AroD in Fig. 1 are classified as type I and are thermolabile, a property that distinguishes them from the isofunctional type II enzymes which are heat stable and mediate the activity indicated as QuiB in Fig. 1 Some gram-negative bacteria elaborate a single dehydroquinate dehydratase associated solely with the biosynthetic function indicated as AroD in Fig. 1. Genes for this enzyme from Enterococcus faecalis (3), Salmonella typhi (51), and Escherichia coli (10) have been sequenced and shown to encode type I enzymes. These fi...

Cloning, sequencing, expression, purification and preliminary characterization of a type II dehydroquinase from Helicobacter pylori

“…Since that time, however, the characterization of DHQases from several sources has thrown new light on the distribution of these two classes of enzyme, especially in prokaryotes. Type I DHQases have been identified in the shikimate pathway of both the Gramnegative organism Salmonella typi [9] and the Gram-positive organism Enterococcus faecalis [10], as well as in the inducible quinate pathway of the Gram-negative Acinetobacter calcoaceticus [49]. An even greater number of type II enzymes have been identified, primarily in the shikimate pathway of microorganisms.…”

“…The latter type II DHQase is thought to serve a dual function in both the shikimate and quinate pathways. The enzyme has also been identified in the Gram-negative organisms Actinobacillus pleuropneumoniae [50] and Acinetobacter calcoaceticus [49].…”

Bacteria Are Not What They Eat: That Is Why They Are So Diverse

“…To one side of the pca operon are genes associated with catabolism of diverse compounds (11,12; Smith et al, Abstr. 98th Gen. Meet.…”

Purification and Characterization of Membrane-bound Quinoprotein Quinate Dehydrogenase

“…Elsemore and Ornston did a genetic analysis of protocatechuate catabolism 14) and claimed that the consecutive action of three genes, quiA encoding quinate dehydrogenase, quiB for 3-dehydroquinate dehydratase, and quiC for 3-dehydroshikimate dehydratase, are required in the catabolism of quinate to protocatechuate. 15) Quinate is oxidized to 3-dehydroquinate, which can be linked to 3-dehydroshikimate and then to shikimate in the shikimate pathway. Since shikimate is remote from D-glucose in the shikimate pathway, reports regarding microbial production of the intermediates in the pathway are scarce.…”