Genetic and molecular basis of the microbial degradation of herbicides and pesticides

Abstract

The genes for the degradation of chlorinated compounds normally used as herbicides or pesticides, such as various chlorinated benzoic acids, 2,4-dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid, etc., are usually found to be clustered on various plasmids such as pJP4 and pAC27. The clustering of the degradative genes are allowed their cloning in broad host-range vectors, as well as the sequencing of parts of the gene cluster such as that involved in the degradation of 3-chlorobenzoic acid. The promoters involved in the efficient expression of these genes appear to be substantially different from those in Escherichia coil. Plasmid gene sequences of the 2,4,5-trichlorophenoxyacetate-degrading Pseudomonas cepacia AC1100 strain demonstrate substantial homology to a cluster of genes present on the 2,4-dichlorophenoxyacetate degradative plasmid pJP4. There is little AC1100 DNA homology to the chlorocatechol genes present on the 3-chlorobenzoate degradative plasmid pAC27. The homology to the 2,4-dichlorophenoxyacetate degradative plasmid pJP4 may therefore be attributed to other genes involved in the degradation of chlorophenoxyacetates. Transposon mutagenesis has shown that at least some of the 2,4,5-trichlorophenoxyacetate genes are chromosomal. A short chromosomal DNA sequence associated with a putative catabolic gene is repeated many times on both the chromosome and plasmid of strain AC1100. Such a repeated sequence may be important in the evolution of the degradative plasmids as well as in genetic rearrangements that often allow efficient degradation of novel substrates.