Redesigning metabolic routes: manipulation of TOL plasmid pathway for catabolism of alkylbenzoates.

Increasing quantities of man-made organic chemicals are released each year into the biosphere. Some of these compounds are both toxic and relatively resistant to physical, chemical, or biological degradation, and they thus constitute an environmental burden of considerable magnitude. Genetic manipulation of microbial catabolic pathways offers a powerful means by which to accelerate evolution of biodegradative routes through which such compounds might be eliminated from the environment. In the experiments described here, a catabolic pathway for alkylbenzoates specified by the TOL plasmid of Pseudomonas was restructured to produce a pathway capable of processing a new substrate, 4-ethylbenzoate. Analysis of critical steps in the TOL pathway that prevent metabolism of 4-ethylbenzoate revealed that this compound fails to induce synthesis of the catabolic enzymes and that one of its metabolic intermediates inactivates catechol 2,3-dioxygenase (C23O), the enzyme that cleaves the aromatic ring. Consequently, the pathway was sequentially modified by recruitment of genes from mutant bacteria selected for their production of either an altered pathway operon regulator that is activated by 4-ethylbenzoate or an altered C23O that is less sensitive to metabolite inactivation. The redesigned pathway was stably expressed and enabled host bacteria to degrade 4-ethylbenzoate in addition to the normal substrates of the TOL pathway.