Environmental Engineering Reference
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of cyanuric acid to CO 2 and NH 4 . While initial studies proposed that cyanuric
acid was hydrolysed to urea, more recently we have shown that many bacteria
use cyanuric acid hydrolase, AtzD, to produce biuret. Instead of AtzD, some
bacteria have TrzD, a homolog of cyanuric acid hydrolase that is 44% different
in amino acid sequence to AtzD (Fruchey et al. 2003 ).
The biuret produced via AtzD is subsequently deaminated by biuret hydro-
lase (AtzE) to generate allophanate, which in turn is the substrate for AtzF
(allophanate hydrolase), which is hydrolysed to CO 2 and NH 4 . This lower
pathway is absent in some atrazine degrading bacteria, such as A. aurescens,
and consequently, these bacteria excrete cyanuric acid into the growth
medium (Strong et al. 2002 ; Sajjaphan et al. 2004 ). Nevertheless, these bacteria
still have the ability to rapidly grow on s-triazines using the alkyamines
released by AtzB and AtzC as carbon, nitrogen and/or energy sources (Strong
et al. 2002 ).
While some studies have shown that genetically diverse bacteria contain
nearly identical copies of atzABC (de Souza et al. 1998 ), others contain various
combinations of these genes, together with trzN. For example, Devers et al.
( 2007 ) reported that 17 atrazine-degrading bacteria isolated from soils had
the following combination of atrazine degradation genes: trzN-atzBC, atzABC-
trzD or atzABCDEF. Likewise, Martin-Laurent et al.( 2006 ) also reported great
diversity in atrazine degradation gene content among bacterial communities
isolated from French soils.
Gene diversity and distribution is driven by plasmids
Initial studies by de Souza et al.( 1998 ) indicated that the atrazine degradation
phenotype could be transferred from Pseudomonas ADP to E. coli by conjugation,
and that this was associated with the acquisition of an
108-kb self-transmissible
plasmid in transconjugants. This suggested that atrazine degradation genes in
this bacterium were localised on plasmids. Subsequent hybridization studies
indicated that many gram-negative atrazine-degrading bacteria contain large
molecular weight plasmids containing atrazine degradation genes (Topp et al.
2000b ; Wackett et al. 2002 ). However, the atrazine degradation genes were
located on different size plasmids, suggesting that the spread of atrazine degra-
dation genes among disparate bacteria was not solely due to direct plasmid
transfer. Moreover, this notion was further confirmed by the discovery of
bacteria with various combinations of atrazine degradation genes, including
trzN-atzBC, atzABC-trzD and atzABCDEF (Devers et al. 2007 ), and numerous reports
of the selective loss of some or all atrazine degradation genes following growth
under non-selective conditions.
The complete nucleotide sequencing of pADP-1, the atrazine gene-containing
plasmid from Pseudomonas ADP, provided insights into the arrangement and
composition of atrazine degradation genes in this bacterium (Martinez et al. 2001 ),
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