UNUSUAL ANCESTRY OF DEHYDRATASES ASSOCIATED WITH QUINATE CATABOLISM IN ACINETOBACTER-CALCOACETICUS

Catabolism of quinate to protocatechuate requires the consecutive acti on of quinate dehydrogenase (QuiA), dehydroquinate dehydratase (QuiB), and dehydroshikimate dehydratase (QuiC). Genes for catabolism of prot ocatechuate are encoded by the pea operon in the Acinetobacter calcoac eticus chromosome. Observations reported here demonstrate that A. calc oaceticus qui genes are clustered in the order quiBCXA directly downst ream from the pca operon. Sequence comparisons indicate that quiX enco des a porin, but the specific function of this protein has not been cl early established. Properties of mutants created by insertion of Omega elements show that quiBC is expressed as part of a single transcript, but there is also an independent transcriptional initiation site dire ctly upstream of quiA. The deduced amino acid sequence of QuiC does no t resemble any other known sequence. A. calcoaceticus QuiB is most dir ectly 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 lea der sequence for transport of the encoded protein into the inner membr ane, 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.