Characterization of the expression of the thcB gene, coding for a pesticide-degrading cytochrome P-450 in Rhodococcus strains.

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A cytochrome P-450 system in Rhodococcus strains, encoded by thcB, thcC, and thcD, participates in the degradation of thiocarbamates and several other pesticides. The regulation of the system was investigated by fusing a truncated lacZ in frame to thcB, the structural gene for the cytochrome P-450 monooxygenase. Analysis of the thcB-lacZ fusion showed that the expression of thcB was 10-fold higher in the presence of the herbicide EPTC (s-ethyl dipropylthiocarbamate). Similar enhancement of the thcB-lacZ expression was found with other thiocarbamate pesticides. Atrazine, simazine, or carbofuran, although metabolized by the system, had no effect on the thcB-lacZ expression. The presence of glucose slightly increased the expression of thcB-lacZ, indicating no catabolic repression of the thcB-lacZ expression. The expression of thcB-lacZ was decreased more than twofold in Luria-Bertani medium. This was due in part to cysteine, which repressed thcB-lacZ expression. It was confirmed that the thcR gene, which is transcribed divergently from thcB, codes for a positive regulatory protein which is essential for the thcB-lacZ expression. Studies of the thcR-lacZ protein fusion showed that the thcR gene is expressed constitutively.

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