Environmental Engineering Reference
In-Depth Information
has a 39.5 mole% G
C content, well outside the range for total Pseudomonas
and Arthrobacter DNA (Sadowsky et al.
1998
); (2) individual atrazine degradation
genes can be lost from cultures grown without selection pressure (Topp et al.
2000b
; Martinez et al.
2001
; Sajjaphan et al.
2004
); (3) atrazine degradation
genes reside on different size plasmids in different bacteria (Topp et al.
2000b
;
Martinez et al.
2001
; Wackett et al.
2002
); (4) atrazine degradation genes
are flanked by insertion sequence-like elements and transposases (Martinez
et al.
2001
, Devers et al.
2007
); and (5) triazine-degrading bacteria can have
various combinations of atrazine degradation genes, some with complete and
incomplete degradation pathways (Strong et al.
2002
; Devers et al.
2007
;
Shapir et al.
2007
).
In Pseudomonas strain ADP, sequence, Northern hybridization and RT-PCR
analyses indicate that the atzA, atzB and atzC genes are constitutively expressed
(Martinez et al.
2001
; Devers et al.
2004
). However, atrazine degradation in this
bacterium appears to be influenced by inorganic nitrogen sources, such as
nitrate and ammonia, likely under the control of a genome-wide system influ-
encing general nitrogen metabolism in response to decreased nitrogen avail-
ability (Garc´a-Gonz´lez et al.
2003
,
2005
). In Pseudomonas ADP, ammonia
repression of atrazine degradation has been shown to be relieved by growth
of cells in the presence of L-methionine sulfoximine (MSX), an inhibitor of
glutamine synthetase, and in a nitrate assimilation mutant (Garc´a-Gonz´lez
et al.
2003
). This, along with the above observations, is consistent with the
recent acquisition of these atrazine catabolism genes, likely in a time scale of
tens
of years.
In contrast, the atzDEF genes, which are located in an operon-like
structure on pADP-1 (Martinez et al.
2001
) are regulated by a LysR-like regula-
tory element, atzR, located upstream, and divergently transcribed, from atzDEF
(Martinez et al.
2001
; Garc´a-Gonz´lez et al.
2005
). The regulation of cyanuric
acid degradation in Pseudomonas ADP is apparently complex and involves AtzR
activity, the presence of cyanuric acid and Ntr-mediated signal transduction in
response to nitrogen limitation. Nevertheless, the presence of these genes in
an operon suggests that they were acquired/recruited and assembled together
much earlier, in an evolutionary sense, than the other atrazine catabolism
genes. Accordingly, it is not surprising that many soil bacteria have been
shown to have the ability to use cyanuric acid as a growth substrate (Fruchey
et al.
2003
). Recent studies have shown that expression of atzR is induced by
nitrogen limitations, transcription of atzDEF is driven from a sigma70-type
promoter, and atzD expression is induced by cyanuric acid (Garc´a-Gonz´lez
et al.
2005
). Taken together, these results strongly suggest (1) that pADP-1
independently acquired the atzA, atzB, atzC and the atzDEF genes from different
sources, (2) that gene acquisition was likely a recent event, and (3) that the
LysR-controlled atzDEF operon likely evolved much earlier than did the acquisi-
tion and assembly of the upper pathway genes on pADP-1.
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