Environmental Engineering Reference
In-Depth Information
These results indicate that atrazine-catabolizing micro-organisms can respond
to increased substrate availability in several ways, resulting in enhanced
degradation capacity.
Evolution of atrazine degradation genes
While it is relatively easy to envision how strong selection pressure for growth,
horizontal gene transfer and transposition can lead to the rapid assembly of
complete catabolic pathways in bacteria, it is often much more difficult to
determine where the degradation genes arose from. Conventional wisdom
dictates that these genes likely arose from mutational events impacting extant
genes with initially different functions. Although initial degradation studies
indicated that major atrazine metabolites detected in soils and water were due
to dealkylation reactions catalysed by a cytochrome P450 monooxygenase from
Rhodococcus strains (Nagy et al.
1995
), the reactions carried out by this enzyme
were subsequently shown to be non-specific. In contrast, the reactions carried
out by AtzA, AtzB and AtzC are very specific, and there are constraints on the
substrates catalysed by these enzymes, all must contain the s-triazine ring and
specific R groups. For example, AtzA catalysed hydrolysis of the chloride group
in atrazine requires that at least one of the two nitrogen side chains contain
an alkyl group (Seffernick & Wackettt
2001
). Since there has been no evidence
that natural compounds contain the s-triazine ring, it is likely that atrazine
degradation ability arose subsequent to the release of this compound in the envi-
ronment. Moreover, since many bacteria that have the ability to degrade
atrazine contain identical enzymes, and that atrazine degradation is initiated
via AtzA or TrzN, it suggests that atrazine dechlorination was the rate limiting
step in further catabolism of this substrate. Sequence analysis indicates that all
three upper pathway enzymes are members of the amidohydrolase superfamily
(Sadowsky et al.
1998
), all members have a conserved reaction mechanism
whereby one to two divalent metals, coordinated by the enzymes, activate
water for nucleophilic attack on their substrate (Seffernick et al.
2001
). Super-
family members include cytosine deaminase, urease and adenine deaminase,
where reactions result in the hydrolytic displacement of amino groups from
purine and pyrimidine rings, and the s-triazine ring-containing compounds in
many ways resemble the pyrimidine ring-containing compounds. This suggests
that the atrazine catabolic enzymes may have evolved from those involved in
intermediary metabolism for the displacement of amino groups from purine
and pyrimidine compounds. Further evidence for the rapid evolution of AtzA
comes from comparison to an enzyme involved in the catabolism of the related
s-triazines, 2,4,6-triamino-1,3,5-triaizine and melamine, TriA. The AtzA from
Pseudomonas sp. strain ADP is 98% identical at the amino acid level to TriA,
melamine deaminase (Seffernick et al.
2001
). It is striking to note that the
nine amino acid differences between these two enzymes originate from nine
