Trichloroethylene degradation by Ralstonia sp. KN1-10A constitutively expressing phenol hydroxylase: Transformation products, NADH limitation, and product toxicity

Ralstonia sp. KN1-10A, which was constructed by inserting the tac promoter upstream of the phenol hydroxylase (PH) gene in the chromosomal DNA of the wild-type strain, Ralstonia sp. KN1, is a useful recombinant strain for eliminating trichloroethylene (TCE) from contaminated sites because it exhibits constitutive TCE oxidation activity. During TCE degradation by Ralstonia sp. KN1-10A, noxious chlorinated compounds, such as dichloroacetic acid, trichloroacetic acid, 2,2,2-trichloroethanol, and chloral, were not detected, and more than 95% of chlorine in TCE was released as chloride ions. Among the possible TCE transformation products, only carbon monoxide was detected, and its conversion percentage was 7 mol%. The addition of formate, which Ralstonia sp. KN1-10A could use as an exogenous electron donor, did not enhance the TCE degradation performance, suggesting that NADH depletion did not limit the degradation. The phenol degradation activity of Ralstonia sp. KN1-10A that previously degraded TCE was not markedly lower than that of cells not exposed to TCE, suggesting that Ralstonia sp. KN1-10A was not susceptible to product toxicity associated with TCE degradation. Furthermore, to clarify the mechanisms underlying TCE degradation by PH from Ralstonia sp. KN1, this enzyme was compared with another enzyme, a hybrid aromatic ring dioxygenase exhibiting a high TCE degradation activity in Escherichia coli and Pseudomonas sp. The initial TCE degradation rate of Ralstonia sp. KN1 (pKTP100), which produced PH, was 1 50 lower than that of Ralstonia sp. KN1 (pKTF200), which produced the hybrid aromatic ring dioxygenase. However, because of its lower product toxicity, the strain producing PH could degrade 2.3 times more TCE than that generated by the strain producing the hybrid aromatic ring dioxygenase.