When Pseudomonas aeruginosa PA01c or P. putida PP0220 or PP0300 carry plasmid pJP4, which encodes enzymes for the degradation of 2,4-dichlorophenoxyacetic acid (TFD) or 2-chloromaleylacetate, cells do not grow on TFD and UV-absorbing material with spectral characteristics of chloromaleylacetate accumulates in the culture medium. Using plasmid pR01727, DNA fragments which contain the gene for maleylacetate reductase, were cloned from the chromosome of a nonfluorescent pseudomonad, Pseudomonas sp. strain PK01, 6- and 0.5-kilobase BamHI. A novel polypeptide with an estimated molecular weight of 18,000 was detected in cell extracts of P. aeruginosa carrying either plasmid pR01944 or plasmid pR0945. Maleylacetate reductase activity was induced in cells of P putida carrying plasmid pR01945, as well as in cells in Pseudomonas strain PK01, when grown on L-tyrosine, suggesting that the tyrosine catabolic pathway might be the source from which maleylacetate reductase is recruited for the degradation of TFD in pJP4-bearing cells of Pseudomonas sp. strain PK01. (Copyright (c) 1989 American Society for Microbiology.)