Soil Biology & Biochemistry 39 (2007) 1007–1013
Soil colloids-bound plasmid DNA: Effect on transformation of E. coli
and resistance to DNase I degradation
, Q. Huang
, W. Chen
, D. Zhang
, K. Wang
, D. Jiang
, W. Liang
State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, China
Key Laboratory of Subtropical Agricultural Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China
Received 4 September 2006; received in revised form 12 November 2006; accepted 17 November 2006
Available online 18 December 2006
The adsorption and binding of plasmid p34S DNA on four different colloidal fractions from a Brown soil and clay minerals in the
presence of various Ca
concentrations, the ability of bound DNA to transform competent cells of CaCl
-treated Escherichia coli, and
the resistance of bound DNA to degradation by DNase I were studied. DNA adsorption on soil colloids and clay minerals was promoted
in the presence of Ca
. Kaolinite exhibited the highest adsorption afﬁnity for DNA among the examined soil colloids and clay minerals.
In comparison with organo-mineral complexes (organic clays) and ﬁne clays (o0.2 mm), DNA was tightly adsorbed by H
clays (inorganic clays) and coarse clays (0.2–2 mm). The transformation efﬁciency of bound DNA increased with increasing
concentrations of Ca
at which soil colloid or clay mineral-DNA complexes were formed. DNA bound by kaolinite showed the lowest
transformation efﬁciency, and especially no transformants were observed with kaolinite-DNA complex prepared at 5–100 mM Ca
Compared to organic clays and ﬁne clays, DNA bound on inorganic clays and coarse clays showed a lower capacity to transform E. coli
at different Ca
concentrations. The presence of soil colloids and minerals provided protection to DNA against degradation by DNase
I. Montmorillonite, organic clays and ﬁne clays showed stronger protective effects for DNA than inorganic clays and coarse clays. The
protection mechanisms as well as the differences in transforming efﬁciency of plasmid DNA molecules bound on various soil colloidal
particles are discussed. The information obtained in this study is of fundamental signiﬁcance for the understanding of the horizontal
dissemination of recombinant DNA and the fate of extracellular DNA in soil environments.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Plasmid DNA; Soil colloid; Escherichia coli; Transformation
Transformation is a mechanism of gene transfer in soil,
sediment and other environments. In a transformation,
DNA may be either integrated into genome or stay
extrachromosomal elements in the form of plasmids, or
phages (Stewart, 1989; Lorenz and Wackernagel, 1994).
The application of genetically engineered microorganisms
(GEMs) carrying speciﬁc genetic modiﬁcations on plasmid
DNA in the environment has raised concerns about
transformation as a possible method for the horizontal
dissemination of recombinant DNA molecules (Paget and
Simonet, 1994; Cai et al., 2005).
In soil environments, extracellular DNA molecules such
as chromosomal and plasmid DNA were excreted actively
by living cells or released from lysed cells (Reanney et al.,
1983). These released DNA molecules, physically or
chemically adsorbed by clay minerals, sand, humic acids,
montmorillonite-humic acids-aluminum or iron hydroxy-
polymers and soils are partially protected against degrada-
tion by the nuclease and retain the capacity to transform
competent bacterial cells (Romanowski et al., 1991;
Khanna and Stotzky, 1992; Crecchio and Stotzky, 1998;
Stotzky, 2000; Crecchio et al., 2005; Cai et al., 2006a). For
example, Romanowski et al. (1992) monitored plasmid
pUC8-ISP DNA for up to 60 days after introduction into
loamy sand soil, clay soil and silty clay soil with four
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Corresponding author. State Key Laboratory of Agricultural
Microbiology, Huazhong Agricultural University, Wuhan 430070, China.
Tel.: +86 27 87671033; fax: +86 27 87280670.
E-mail address: email@example.com (Q. Huang).