Environmental Engineering Reference
In-Depth Information
Sadowsky et al.
1998
;Seffernicket al.
2002
;Shapiret al.
2002
;Frucheyet al.
2003
), A. aurescens TC1 can theoretically metabolize 560 s-triazine compounds
(Shapir et al.
2007
).
Sequence analysis of a 160-kb genomic DNA region of strain TC1 cloned in
bacterial artificial chromosomes (BACs), and PCR and hybridisation studies,
established that this bacterium contained the trzN, atzB and atzC genes, but
not the lower pathway, localised on a
380-kb plasmid, pTC1 (Sajjaphan et al.
2004
). Like the arrangement of atrazine catabolizing genes on pADP-1, those
on pTC1 are not contiguous and not organised in an operon-like structure.
To get a better understanding of the genomic context of the atrazine catabolism
genes relative to the rest of the genome, the complete genome of A. aurescens TC
1 was sequenced (Mongodin et al.
2006
). The genome of this bacterium consists
of a 4.59- Mbp circular chromosome, and two circular plasmids, pTC1 and
pTC2, which are 408 kb and 300 kb, respectively. As predicted by hybridisation
studies, sequence analysis confirmed that the trzN, atzB and atzC genes are
localised on pTC1. Genomic analyses indicated that the combination of
three, plasmid-encoded, atrazine degradation genes, along with a variety of
chromosome-borne amine-catabolizing enzymes, and plasmid localised Ipu
pathway gene clusters, make this bacterium very metabolically diverse. The
later pathway is likely involved in the catabolism of ethyl- and propyl-amines
liberated from s-triazine compounds by the action of AtzB and AtzC, resulting
in TC1's ability to grow on the large range of s-triazines as C and N sources
(Shapir et al.
2007
). Interestingly, genome sequence analysis indicated that A.
aurescens strain TC1 contains six copies of trzN located within six identical direct
tandem repeats of about 16 kb (Mongodin et al.
2006
). This increase in the copy
number of trzN may enhance catabolism, via gene dosage effects, or provide a
competitive advantage to strain TC1 versus other atrazine-degrading micro-
organisms, such as Pseudomonas ADP, which contains a single triazine hydrolase
gene. This may be reflected in the growth rate differential between these two
organisms, strain TC1 grows much faster on atrazine than Pseudomonas ADP.
Moreover, multiple copies of the catabolism initiating enzyme may protect the
bacterium from complete loss of atrazine degradation potential in growth
conditions lacking adequate selection pressure.
Regulation of atrazine catabolism in Pseudomonas ADP reflects
the recent evolution and assembly of the degradation pathway
Several lines of evidence support the contention that the atrazine degradation
pathway in Pseudomonas ADP, and other gram-negative and -positive bacteria,
was recently assembled. This include reports that (1) the mole% G
þ
C data
showing that while atzA, atzB and trzN have mole% G
C contents of 58.3, 64.1
and 63.1%, respectively, within the range of mole% G
þ
C contents found in
total Pseudomonas sp. DNA (58 to 70%) and Arthrobacter (59 to 66%), the atzC gene
þ
