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and how different bacteria can contain various combinations of atrazine
degradation genes. The upper pathway genes, atzA, atzB and atzC are located
on different parts of the plasmid backbone, the IncP b plasmid pR751, are not
contiguous, have different mole% G
C content, appear to be constitutively
expressed and are flanked by transposase TnpA-like elements from IS1071, forming
nested catabolic transposon structures. In contrast, the lower degradation
pathway genes, atzDEF, werelocatedinanoperononadifferentportionof
the plasmid under control of a LysR-like regulatory gene, atzR (Garc´a-Gonz´lez
et al. 2005 ). Several lines of evidence indicate that the upper atrazine degradation
pathway genes are acquired by soil bacteria via horizontal gene transfer
and transposition processes, including (1) the genes are surrounded by IS
elements and transposases, (2) different bacterial genera have different sized
plasmids containing the degradation genes, (3) various combinations of atrazine
degradation genes can be found in different micro-organisms, (4) the atrazine
degradation genes can be independently lost following growth without selection
pressure and (5) laboratory and soil microcosm studies indicate that atrazine
degradation genes can be transferred between bacteria (de Souza et al. 1998 ;
Devers et al. 2005 ). Taken together, these observations also suggest that the
atrazine degradation genes in the primal degrader(s) were likely acquired from
different plasmids by independent gene transfer and transposition events due to
selection pressure.
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Gram-positive atrazine degrading bacteria
The presence of atrazine degradation genes on plasmids is not limited to only
gram-negative bacteria, and several gram-positive bacteria capable of degrad-
ing s-triazines have been reported (Giardina et al. 1982 ; Behki et al. 1993 ;
Rousseaux et al. 2001 ; Strong et al. 2002 ; Cai et al. 2003 ; Sajjaphan et al. 2004 ).
As before, insights into the constitution, distribution and assembly of the
atrazine catabolic pathway in gram-positive bacteria came from the genome
sequence analyses of Arthrobacter aurescens strain TC1. This bacterium can utilise
atrazine as a sole source of nitrogen and carbon for growth, excreting cyanuric
acid into the growth medium. Initial studies showed that strain TC1 contained
atzB and atzC, but not atzA, and was metabolically diverse, using 22 s-triazines as
growth substrates, including the herbicides simazine, terbuthylazine, propa-
zine, cyanazine, ametryn, prometryn and terbutryn (Strong et al. 2002 ). We
subsequently showed, by polymerase chain reaction (PCR) analysis, that strain
TC1 initiated atrazine catabolism via the triazine hydrolase, encoded by trzN
(Sajjaphan et al. 2004 ). To date, we have synthesised about 64 different s-triazine
compounds, and these have been shown by functional analyses to be substrates
for the TrzN, AtzB and AtzC. However, based on substrate specificity studies of all
three atrazine degradation genes (de Souza et al. 1996 ; Boundy-Mills et al. 1997 ;
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