degree of “foreignness” is actually variable from pesticide to pesticide. There appear
to be no natural counterparts for many pesticide structures, for example, the chlorinated hydrocarbons DDT, mirex, and dialdrin. However, some seemingly unusual
structures found in pesticide molecules are also found in nature. For example, soil
fungi can produce large amounts of various halogenated aromatic compounds
(De Jong et al. 1994).
Microorganisms growing at the expense of a xenobiotic (i.e., foreign to nature)
pesticide can frequently be isolated from soil only a few years after the introduction
of the chemical at particular field. Some soils degrade pesticide much more rapidly
after repeated applications, suggesting that some kind of adaption and change in the
properties of the degrading microflora may have taken place. This observation has
prompted speculation that the biodegradation of newly introduced pesticides
becomes possible because of the rapid evolution and selection of catabolic phenotypes (van der Meer et al. 1992). Enzyme involved in the metabolism of natural
chemicals may have sufficiently low substrate specificity, and they may also attack
xenobiotic analogs. Therefore, pesticides can also be degraded co-metabolically by
enzymes with low substrate specificities. For example, the oxidative lignindegrading system of Phanerochaete chrysosporium is remarkably nonspecific and
can also degrade a very wide variety of pollutants (Yadav and Reddy 1993). Under
suitable environmental conditions, all-natural compounds can be catabolized,
because the evolution of biosynthesis of natural chemicals was sufficiently slow to
permit the parallel evaluation of new catabolic functions required for their
degradation.
A number of genetic mechanisms may be involved in the evolution of new
pesticide-degrading capabilities in soil. Microorganisms can in principle acquire
new catabolic capabilities: (1) through the recruitment of genes encoding
pesticide-degrading enzymes; (2) by modification of substrate specificity; and
(3) regulation of preexisting enzymes which have other functions (van der Meer
et al. 1992). Genes encoding various enzymes, which are involved in the degradation
of organic pollutants, are frequently located on the plasmids and sometimes in
transposons (DNA elements able to replicate and insert new copies in the genome).
These mobile genetics elements can be exchanged between microorganisms in soil
and water (Fulthorpe and Wyndham 1992).
2.9.2 Mobilization of Genes to Enhance Catabolic Steps
in Pesticide Degradation Pathway
Genes encoding various enzymes, involved in the degradation of a large number of
pesticides, are located on the plasmids (Table 2.3). For example, the plasmids
contain the genes for degradation of phenoxyalkanoic and thiocarbamate herbicides,
methyl carbamate, and organophosphorus insecticides. Degradative plasmids
(DP) may encode a complete degradative pathway such as those for toluene or
56
A. Sehrawat et al.
to be no natural counterparts for many pesticide structures, for example, the chlorinated hydrocarbons DDT, mirex, and dialdrin. However, some seemingly unusual
structures found in pesticide molecules are also found in nature. For example, soil
fungi can produce large amounts of various halogenated aromatic compounds
(De Jong et al. 1994).
Microorganisms growing at the expense of a xenobiotic (i.e., foreign to nature)
pesticide can frequently be isolated from soil only a few years after the introduction
of the chemical at particular field. Some soils degrade pesticide much more rapidly
after repeated applications, suggesting that some kind of adaption and change in the
properties of the degrading microflora may have taken place. This observation has
prompted speculation that the biodegradation of newly introduced pesticides
becomes possible because of the rapid evolution and selection of catabolic phenotypes (van der Meer et al. 1992). Enzyme involved in the metabolism of natural
chemicals may have sufficiently low substrate specificity, and they may also attack
xenobiotic analogs. Therefore, pesticides can also be degraded co-metabolically by
enzymes with low substrate specificities. For example, the oxidative lignindegrading system of Phanerochaete chrysosporium is remarkably nonspecific and
can also degrade a very wide variety of pollutants (Yadav and Reddy 1993). Under
suitable environmental conditions, all-natural compounds can be catabolized,
because the evolution of biosynthesis of natural chemicals was sufficiently slow to
permit the parallel evaluation of new catabolic functions required for their
degradation.
A number of genetic mechanisms may be involved in the evolution of new
pesticide-degrading capabilities in soil. Microorganisms can in principle acquire
new catabolic capabilities: (1) through the recruitment of genes encoding
pesticide-degrading enzymes; (2) by modification of substrate specificity; and
(3) regulation of preexisting enzymes which have other functions (van der Meer
et al. 1992). Genes encoding various enzymes, which are involved in the degradation
of organic pollutants, are frequently located on the plasmids and sometimes in
transposons (DNA elements able to replicate and insert new copies in the genome).
These mobile genetics elements can be exchanged between microorganisms in soil
and water (Fulthorpe and Wyndham 1992).
2.9.2 Mobilization of Genes to Enhance Catabolic Steps
in Pesticide Degradation Pathway
Genes encoding various enzymes, involved in the degradation of a large number of
pesticides, are located on the plasmids (Table 2.3). For example, the plasmids
contain the genes for degradation of phenoxyalkanoic and thiocarbamate herbicides,
methyl carbamate, and organophosphorus insecticides. Degradative plasmids
(DP) may encode a complete degradative pathway such as those for toluene or
56
A. Sehrawat et al.