Bacterial hydrolases and aldolases in evolution.

Abstract

THE persistence of man-made chemicals in the biosphere is partly determined by the range of degradative capabilities of soil microbes, and a better understanding of the evolution of these capabilities is therefore desirable. The time of persistence might then be predictable, and suggestions.made as to how it might be reduced. Hegeman and Rosenberg1 have reviewed processes by which bacterial enzyme systems could have evolved. The step-wise retrograde scheme of Horowitz2,3 when combined with the concept of tandem gene duplication may account for the origin of biosynthetic pathways in bacteria; but a number of specific problems are encountered when the theory is applied to the origin of catabolic pathways. These include, for example, the chemical instability of key intermediates which are never encountered in nature except when they are formed by the very enzymes whose evolution is being considered. Thus, intermediates from aromatic catabolism include 2-hydroxymuconic semialdehyde4 and maleylpyruvate5 which are chemically unstable and are formed uniquely by oxygenases of the meta-fission and gentisate pathways respectively. Wu, Lin and Tanaka6 have also criticized the tandem gene duplication mechanism, as applied to catabolism, on the grounds that successive enzymes sometimes catalyse very dissimilar reactions. Thus, where a kinase follows a dehydrogenase in a sequence it is unlikely that the second gene would have arisen from the first. These authors studied the mechanism by which Klebsiella aerogenes acquired the ability to grow on xylitol, which it was not able to utilize originally. This was achieved by recruiting enzymic activities from preexisting pathways metabolizing ribitol and D-arabitol, which are substrates for the wild type K. aerogenes. When these enzymes were modified in sequentially derived mutants, xylitol became an acceptable substrate for growth.