In view of the widespread occurrence of phenol, microorganisms can be found in many environments that are able to use phenol as a carbon and energy source. These microorganisms include both aerobic and anaerobic microorganisms. Many aerobic phenol-degrading bacteria have been isolated since the beginning of the 1900s, and the pathways for phenol degradation are now well established (van Schie and Young, 2000).
Aerobic degradation of phenol by microorganisms is mainly based on the ortho- and meta-cleavage pathways (Muller and Babel, 1996). During the first step of the aerobic phenol degradation pathway, molecular oxygen is used by the phenol hydroxylase enzyme to add a second hydroxyl group in the ortho-position to the one already present to produce catechol, which can then be degraded by either the ortho- or the meta-cleavage pathways. In the ortho- or ^-ketoadipate pathway, the aromatic ring is cleaved between the catechol hydroxyls by intradiol fission with a catechol 1,2-dioxygenase (C12O) enzyme. The cis,cis-muconate that results is metabolized via ^-ketoadipate to produce the common tri-carboxylic acid cycle intermediates, succinate, and acetyl-CoA. In the meta-pathway, extradiol fission occurs adjacent to the two hydroxyl groups in catechol with a catechol 2,3-dioxygenase (C23O) enzyme to produce 2-hydroxymuconic semialdehyde. This compound is further metabolized to intermediates of the tricarboxylic acid cycle.
Many bacteria have been encountered that possess either one or two of the ortho- and meta-pathways. The enzymes phenol hydroxylase and C12O or C23O catalyze the first and second steps of phenol degradation, respectively. Phenol hydroxylases are generally classified as simple single-component enzymes using flavoprotein monooxygenases or multicomponent enzymes with multiple proteins. The multicompo-nent phenol hydroxylase is organizationally similar to the multicomponent mono- and dioxygenases involved in the degradation of toluene, benzene, naphthalene, and methane, and is considered to be the major form of phenol hydroxylase in the environment (Harayama et al., 1992). The genes necessary for the expression of phenol hydroxylase can be either chromosome- or plasmid-encoded. For instance, multicomponent phenol hydroxylase together with genes encoding the meta-cleavage are organized in operons located on the chromosome of Pseudomonas sp. (Ng et al., 1995) or on the TOL plasmid (Nordlund et al., 1990).
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