Contrasting patterns of evolutionary divergence within the Acinetobacter calcoaceticus pca operon.

The six enzymes required for catabolism of protocatechuate to succinate and acetylCoA are encoded by the pca genes in the Gram-bacterium, Acinetobacter calcoaceticus. The clustered A. calcoaceticus cat genes encode an analogous set of enzymes associated with the metabolic dissimilation of catechol. The nucleotide (nt) sequences of pcaIJFB and pcaK, reported here, complete evidence showing that all of the pca structural genes are tightly grouped in the order pcaIJFBDKCHG within a single operon. The pcaIJF region is nearly identical in nt sequence to the A. calcoaceticus catIDJF region which exhibits a G+C content and a codon usage pattern exceptional for A. calcoaceticus. In contrast, pcaD, pcaC, pcaH and pcaG have diverged substantially from their evolutionary counterparts in the cat region; all of these divergent genes exhibit G+C contents and codon usage patterns that are typical for A. calcoaceticus. The pcaIJF and catIJF regions are known to exchange DNA sequence information, and this property may have contributed to their nt sequence conservation. The pcaK gene has no counterpart among known cat genes. The deduced amino-acid sequence of PcaK indicates that it may be a transmembrane protein associated with transport.

Acquisition of apparent DNA slippage structures during extensive evolutionary divergence of pcaD and catD genes encoding identical catalytic activities in Acinetobacter calcoaceticus.

The pca operon from the Gram- bacterium Acinetobacter calcoaceticus encodes all of the enzymes required for catabolism of protocatechuate to common intermediary metabolites. This report presents the 2754-nucleotide (nt) sequence of a HindIII restriction fragment containing pcaD, the 801-bp gene encoding beta-ketoadipate enol-lactone hydrolase I. The deduced primary structure of A. calcoaceticus PcaD shares 44% amino acid (aa) sequence identity with the aligned primary structure of CatD (beta-ketoadipate enol-lactone hydrolase II) from the same organism, and the overall nt sequence identity of the two genes is 51.8%. In the 56% of the genes where selection for identical aa residues was not imposed, pcaD and catD have diverged so extensively that nt sequence identity of the aligned segments is only 28.2%; the G+C contents of these segments from the respective genes differ by 8%. Conserved within the aligned PcaD and CatD aa sequences is a Ser residue corresponding to the nucleophile within the alpha/beta-fold of many hydrolytic enzymes. In this region of primary structure, PcaD and CatD appear to have maintained some different aa sequences derived from a common ancestor. Conservation of the different aa sequences during extreme evolutionary divergence suggests that separate segments of primary structure, conserved within either PcaD or CatD, may be functionally incompatible within recombinant enzymes. Consequently, selection for avoidance of genetic exchange between pcaD and catD could account for the thorough nt substitution in regions where identical aa were not selected. Sequence repetitions within pcaD suggest that the multiple mutations required for its extensive divergence from catD were achieved in part by acquisition of a complex DNA slippage structure.

Selection of Acinetobacter calcoaceticus mutants deficient in the p-hydroxybenzoate hydroxylase gene (pobA), a member of a supraoperonic cluster.

p-Hydroxybenzoate hydroxylase, the product of the pobA gene, gives rise to protocatechuate, which is metabolized by enzymes encoded by the pca operon in Acinetobacter calcoaceticus. Mutations in pcaD prevented growth of A. calcoaceticus with succinate in the presence of p-hydroxybenzoate. Mutants selected on this medium contained the original mutation in pcaD and also carried spontaneous mutations in pobA. These independently expressed genes were cotransformed with a frequency of 15% and thus are components of a supraoperonic cluster.