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Appl Environ Microbiol, June 1998, p. 2323-2326, Vol. 64, No. 6
Department of
Biochemistry,1
Department of
Microbiology,4
Department of Soil,
Received 22 December 1997/Accepted 19 March 1998
Pseudomonas sp. strain ADP initiates atrazine
catabolism via three enzymatic steps, encoded by atzA,
-B, and -C, which yield cyanuric acid, a
nitrogen source for many bacteria. In-well lysis, Southern
hybridization, and plasmid transfer studies indicated that the
atzA, -B, and -C genes are
localized on a 96-kb self-transmissible plasmid, pADP-1, in
Pseudomonas sp. strain ADP. High-performance liquid
chromatography analyses showed that cyanuric acid degradation was not
encoded by pADP-1. pADP-1 was transferred to Escherichia coli strains at a frequency of 4.7 × 10 Due to its widespread use over the
last 30 years, for both selective and nonselective weed control
(2, 39), atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-1,3,5-triazine)] and other s-triazine derivatives have been detected in
groundwater and surface water at levels exceeding the Environmental
Protection Agency's maximum contaminant level of 3 ppb
(29).
For the past 3 decades, attempts at isolating bacteria (15,
18) or fungi (28) that mineralize atrazine have been
unsuccessful. In the last several years, however, a number of
laboratories have independently isolated atrazine-degrading bacteria
from sites that were previously exposed to atrazine (1, 4, 5, 10, 31, 32, 35). Our laboratory has studied the genes and enzymes involved in atrazine degradation by Pseudomonas sp. strain
ADP, in which the first three enzymatic steps are now well defined (3, 11, 13, 37) (Fig. 1). The
three genes that encode the enzymes AtzA, -B, and -C have been cloned
and sequenced. The first enzyme, AtzA, catalyzes the hydrolytic
dechlorination of atrazine, yielding hydroxyatrazine (11).
The second enzyme, AtzB, catalyzes hydroxyatrazine deamidation,
yielding N-isopropylammelide (3). The third
enzyme, AtzC (or N-isopropylammelide
isopropylaminohydrolase), transforms N-isopropylammelide to
cyanuric acid and isopropylamine (37). An analogous
catabolic pathway in Klebsiella pneumoniae 99 (25) has been reported to metabolize s-triazine
compounds, but not atrazine. In that strain, the trzC,
-D, and -E genes encode ammelide aminohydrolase,
cyanuric acid aminohydrolase, and biuret aminohydrolase, respectively,
and are located on a 113-kb plasmid (27).
Several studies have conclusively shown that horizontal gene transfer
occurs among microorganisms in natural and laboratory environments
(30). The majority of "natural" conjugal transfer experiments have been carried out in nonsterile soil and involve the
use of highly promiscuous plasmids and introduced organisms (30,
42). In general, these results suggest that plasmid transfer is a
prominent factor in gene flow in natural systems. Previously, horizontal gene transfer has been invoked to explain the appearance of
similar tfd genes in different 2,4-dichlorophenoxyacetic
acid-degrading bacterial strains isolated from different regions
(16, 26). Catabolic plasmids have also been implicated in
the dispersal of genes for 3-chlorobenzoate, chlorocatechol, and
naphthalene biodegradation (17, 22, 34).
Recently, the PCR technique was used to demonstrate the presence of DNA
that is strikingly homologous to the atzABC genes in
atrazine-degrading strains obtained from geographically diverse locations (12). In this study, we report the physical
linkage of the atzA, -B, and -C genes
on a large plasmid, pADP-1, which is self-transmissible to
gram-negative bacteria.
Instability of atrazine degradation phenotype.
Pseudomonas sp. strain ADP has an unstable atrazine-clearing
phenotype during cultivation and propagation on complex laboratory growth media. This phenotypic instability, assayed with the
plate-clearing procedure, was especially conspicuous in cells grown
with NH4Cl in the absence of atrazine as the sole nitrogen
source (9). Upon repeated subculturing, clearing-negative
strains of ADP (Atr Transfer of atrazine degradation ability.
Based on the
instability observed with the atrazine degradation phenotype in
Pseudomonas sp. strain ADP, an experiment was designed to
determine if the atrazine degradation genes were located on a plasmid
and were self-transmissible. Mating experiments were done in the
absence of a helper strain with Pseudomonas sp. strain ADP
(Nalr) (Atr+) as the donor and
Escherichia coli AD256 (recA56
srlC300::Tn10; Tetr)
(19) as the recipient. Samples of 2 ml of an overnight
culture of Pseudomonas sp. strain ADP (donor) and of
E. coli AD256 (recipient) (19) were centrifuged
at 10,000 × g for 1 min at 4°C, washed in a solution
containing 0.85% sodium chloride and 0.01% Tween 20, and resuspended
in 0.1 ml of sterile LB broth. Cell suspensions were placed on LB agar
plates and incubated at 30°C overnight. Dilutions of the mating
mixtures were plated on LB agar with atrazine (500 µg/ml) and
tetracycline (15 µg/ml) and incubated at 37°C overnight.
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
The atzABC Genes Encoding Atrazine
Catabolism Are Located on a Self-Transmissible Plasmid
in Pseudomonas sp. Strain ADP
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ABSTRACT
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This suggests a potential molecular mechanism for the dispersion of the
atzABC genes to other soil bacteria.
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FIG. 1.
Pathway for atrazine catabolism to cyanuric acid in
Pseudomonas sp. strain ADP.
) that had spontaneously lost the
ability to degrade atrazine and hydroxyatrazine on Luria-Bertani (LB)
medium (38) containing atrazine or hydroxyatrazine (500 µg/ml) were obtained. These observations suggested that the
Atr
strains were lacking at least the first two enzymes
in the atrazine degradation pathway. PCR analyses done with genomic DNA
from the Atr
strains and the atzA,
-B, and -C primers confirmed that the first three
genes were missing in the Atr
strains. Moreover, Southern
hybridization experiments done with radiolabeled probes specific for
the atzA, -B, and -C genes and total
genomic DNA isolated from wild-type Pseudomonas strain ADP (Atr+) and the Atr
strains confirmed that
Atr
strains did not contain DNA homologous to the
atzA, -B, and -C genes (data not
shown). Phenotypic instability of biodegradation capacity has been
observed in other soil bacteria (33, 40).
2 per
recipient. In addition the plasmid encoding atrazine degradation activity was transferable to several gram-negative soil bacteria (6).
)
and E. coli AD256 were used as negative controls, and these strains failed to yield any protein that reacted with the anti-AtzA antibody.
Plasmid content of bacteria. Previously, using conventional plasmid isolation techniques, we failed to observe a plasmid that contained the atzABC genes in Pseudomonas sp. strain ADP (13). In this study, plasmid profiles were determined on horizontal agarose gels by a modified in-well lysis method (20). Gels were prepared in TBE buffer (89 mM Tris-borate-2 mM EDTA; pH 8.0) with 0.75% (wt/vol) agarose and 1% (wt/vol) SDS. Cells were grown with the appropriate antibiotics as described above. Log-phase cells (400 µl) of Pseudomonas, Sinorhizobium, or the transconjugant E. coli strains were centrifuged, washed with 0.5 M NaCl, resuspended in 50 µl of 20% (wt/vol) sucrose, and added to wells preloaded with modified lysis solution (lysozyme [1.0 mg/ml], RNase [10 µg/ml], and 20% [wt/vol] sucrose in TBE). After 10 min of incubation at room temperature, voltage was applied as follows: 5 V for 30 to 45 min, 14 V for 15 min, 40 V for 60 min, and 80 V for 7 to 8 h. Molecular weights of the Sinorhizobium plasmids (21, 36) were used to estimate the sizes of the plasmids present in the Pseudomonas and the recombinant E. coli strains. The approximate molecular masses in kilobases of the indigenous plasmids in the Sinorhizobium fredii strains were as follows: USDA 191, >455, 347, and 105; USDA 205, >455, 342, 177, 126, and 53; USDA 206, >455, 320, 99, and 85; and USDA 217, >455, 335, and 146 (21, 36).
Results of the in-well lysis studies indicated that Pseudomonas sp. strain ADP (Atr+) contained at least two plasmids, pADP-1 and pADP-2, of approximately 96 and 53 kb, respectively (Fig. 2). The pADP-1 plasmid was missing in the Atr
Pseudomonas sp. ADP
strains. The E. coli transconjugants (Tetr) that
acquired atrazine degradation ability gained a plasmid of approximately
the same size (Fig. 2). Plasmid pADP-2
did not hybridize with any of the atzA, -B, and
-C gene probes.
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Presence of atrazine- and cyanuric acid-metabolizing enzymes.
To determine if pADP-1 contained a gene that encoded cyanuric acid
degradation, resting cells and cell extracts from
Pseudomonas sp. ADP strains (Atr+ and
Atr
), E. coli 3A (Atr+), and
E. coli AD256 (Atr
) were tested for their
abilities to catabolize atrazine and cyanuric acid. High-performance
liquid chromatography analysis was done on supernatants from whole
resting cells and cell extracts incubated with atrazine or cyanuric
acid (200 µg/ml) with a Hewlett-Packard HP 1090 liquid chromatograph
system as described previously (11). Atrazine and its
metabolites were resolved with a Nova-Pak analytical C18
reverse-phase high-performance liquid chromatography column (4-µm
spherical packing, 150 by 3.9 mm; Waters Corp.) and an acetonitrile gradient in water at a flow rate of 1.0 ml/min as described previously (11). Cyanuric acid was resolved with an analytical
normal-phase column (Lichrosorb RP-18 column, 5-µm spherical packing,
250 by 4.6 mm; Alltech, Deerfield, Ill.) as described previously
(37). Authentic atrazine and cyanuric acid were analyzed
simultaneously. Cells were incubated with 200 µg of atrazine per ml
and 200 µg of cyanuric acid per ml at 30°C for 12 h.
)
and E. coli AD256 cells, 80 and 100% of the atrazine,
respectively, were recovered. Moreover, while 88 to 98% of the
cyanuric acid was recovered after 12 h in cultures of E. coli 3A or E. coli AD256, with Atr+ and
Atr
Pseudomonas sp. ADP strains, only 30 and
49%, respectively, of the cyanuric acid were detected at the end of
the experiment. Similar results were obtained with the cell extracts.
These results indicated that the transconjugant E. coli
strain 3A (Atr+) acquired only the atzABC genes
and not genes encoding enzymes involved in the degradation of cyanuric
acid. Moreover, Pseudomonas sp. strain ADP
(Atr
) that lacked atzA, -B, and
-C retained the ability to degrade cyanuric acid. These
results suggest that the gene(s) encoding the degradation of cyanuric
acid is not located on pADP-1.
In summary, bacterial growth on cyanuric acid is thought to be a
relatively common phenotype in soil (7, 8, 14, 24, 25, 43).
However, fewer bacteria are thought to catabolize atrazine and these
have only been identified recently (1, 4, 5, 10, 31, 32,
35). The atzABC genes confer on a host bacterium the
ability to metabolize atrazine to cyanuric acid. The identification of
the self-transmissible plasmid pADP-1 containing the atzABC
genes demonstrates a mechanism for conferring atrazine-mineralizing ability on bacteria capable of metabolizing cyanuric acid. In this
context, it is important to further delineate the structure and
evolution of pADP-1 and related plasmids. Such studies are in progress.
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ACKNOWLEDGMENTS |
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This work was supported in part by a grant from Novartis Crop Protection (formerly Ciba-Geigy Corporation) and by grant 94-34339-1122 from the U.S. Department of Agriculture-BARD program.
We thank Janis McFarland and Steven Dumford of Novartis Crop Protection for providing s-triazine compounds; William Koskinen, David Gartner, and Mark Sanders for experimental assistance; and Olga Selifonova and Jennifer Seffernick for helpful discussions.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Soil, Water, and Climate, Biological Process Technology Institute and Center for Biodegradation Research and Informatics, University of Minnesota, 1991 Upper Buford Circle, Borlaug Hall, St. Paul, MN 55108. Phone: (612) 624-2706. Fax: (612) 625-2208. E-mail: sadowsky{at}soils.umn.edu.
Article 981250043 in the University of Minnesota Agricultural
Experiment Station series.
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