Diversity and evolution of microorganisms and pathways for the degradation of environmental contaminants: a case study with the s-triazine herbicides - Ecology of Industrial Pollution - page 211

Table 10.1. (cont.)

Micro-organism Reference

Rhodococcus sp. RHA1, NI86/21 Warren et al. 2004

Rhizobium sp.PATR Bouquard et al. 1997

Arthrobacter sp. AG1 Dai et al. 2007

Bacillus licheniformis Marecik et al. 2008

Bacillus megaterium
Rahnella aquatilis strains
Umbelopsis isabellina
Volutella ciliate
Botrytis cinerea

Conservation of genes for atrazine degradation among disparate
micro-organisms
Pioneering work done with Pseudomonas sp. strain ADP, and subsequently with
other bacteria, indicated that atrazine mineralisation in many gram-negative
bacteria proceeds via a six step degradation pathway (Martinez et al. 2001 ;
Shapir et al. 2007 ). In Pseudomonas sp. strain ADP, atrazine degradation is
initiated by a hydrolytic dechlorination reaction, catalysed via the enzyme
atrazine chlorohydrolase AtzA, resulting in the formation of hydroxyatrazine
( Fig. 10.3 ). This compound serves as the substrate for two subsequent hydrolytic
deamination reactions, catalysed by AtzB (hydroxyatrazine ethylaminohydro-
lase) and AtzC (N-isopropylammelide isopropylamino hydrolase), resulting in
the formation of cyanuric acid (Martinez et al. 2001 ; Shapir et al. 2007 ). Cyanuric
acid is now regarded as a central metabolic intermediate in s-triazine meta-
bolism (Cook et al. 1985 ; Fruchey et al. 2003 ), and the enzymes catabolising
atrazine to cyanuric acid (AtzA, AtzB and AtzC) constitute the upper degrad-
ation pathway. All three enzymes have been purified to homogeneity (de Souza
et al. 1996 ; Boundy-Mills et al. 1997 ; Shapir et al. 2002 ; Fruchey et al. 2003 ), and
sequence analyses revealed that all three enzymes belong to the amidohy-
drolase superfamily, whose members are defined by having an ( ab )8 barrel
structure, and conserved reaction mechanism and substrate binding ligands
(Seffernick et al. 2002 ). More surprisingly, initial sequence analysis studies
indicated that the atzABC genes were highly conserved, at nearly 99 % amino

acid identity, in diverse bacterial genera isolated from across the United States
and the world (de Souza et al. 1998 ; Topp et al. 2000a , b ; Devers et al. 2004 , 2007 ;
Shapir et al. 2007 ). This suggests that the atrazine metabolism was rapidly
spreading from bacterium to bacterium.
Not all bacteria initiate atrazine catabolism via AtzA. Results from studies
done by Topp and colleagues indicate that some bacteria initiate atrazine

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