ACQUISITION OF APPARENT DNA SLIPPAGE STRUCTURES DURING EXTENSIVE EVOLUTIONARY DIVERGENCE OF PCAD AND CATD GENES ENCODING IDENTICAL CATALYTIC ACTIVITIES IN ACINETOBACTER-CALCOACETICUS

The pea 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-nucl eotide (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% amin o acid (aa) sequence identity with the aligned primary structure of Ca tD (beta-ketoadipate enol-lactone hydrolase II) from the same organism , and the overall nt sequence identity of the two genes is 51.8%. In t he 56% of the genes where selection for identical aa residues was not imposed, pcaD and catD have diverged so extensively that nt sequence i dentity of the aligned segments is only 28.2%; the G+C contents of the se segments from the respective genes differ by 8%. Conserved within t he aligned PcaD and CatD aa sequences is a Ser residue corresponding t o the nucleophile within the alpha/beta-fold of many hydrolytic enzyme s. In this region of primary structure, PcaD and CatD appear to have m aintained 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, cons erved within either PcaD or CatD, may be functionally incompatible wit hin recombinant enzymes. Consequently, selection for avoidance of gene tic exchange between pcaD and catD could account for the thorough nt s ubstitution in regions where identical aa were not selected. Sequence repetitions within pcaD suggest that the multiple mutations required f or its extensive divergence from catD were achieved in part by acquisi tion of a complex DNA slippage structure.