Ca2+ Promoted the Low Transformation Efficiency of Plasmid DNA Exposed to PAH Contaminants

Fuxing Kang,Qian Wang,Jian Long,Hong Wang,Yanzheng Gao,Andy T. Y. Lau

doi:10.1371/journal.pone.0058238

Abstract

The effects of interactions between genetic materials and polycyclic aromatic hydrocarbons (PAHs) on gene expression in the extracellular environment remain to be elucidated and little information is currently available on the effect of ionic strength on the transformation of plasmid DNA exposed to PAHs. Phenanthrene and pyrene were used as representative PAHs to evaluate the transformation of plasmid DNA after PAH exposure and to determine the role of Ca2+ during the transformation. Plasmid DNA exposed to the test PAHs demonstrated low transformation efficiency. In the absence of PAHs, the transformation efficiency was 4.7 log units; however, the efficiency decreased to 3.72–3.14 log units with phenanthrene/pyrene exposures of 50 µg·L–1. The addition of Ca2+ enhanced the low transformation efficiency of DNA exposed to PAHs. Based on the co-sorption of Ca2+ and phenanthrene/pyrene by DNA, we employed Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and mass spectrometry (MS) to determine the mechanisms involved in PAH-induced DNA transformation. The observed low transformation efficiency of DNA exposed to either phenanthrene or pyrene can be attributed to a broken hydrogen bond in the double helix caused by planar PAHs. Added Ca2+ formed strong electrovalent bonds with “–POO––” groups in the DNA, weakening the interaction between PAHs and DNA based on weak molecular forces. This decreased the damage of PAHs to hydrogen bonds in double-stranded DNA by isolating DNA molecules from PAHs and consequently enhanced the transformation efficiency of DNA exposed to PAH contaminants. The findings provide insight into the effects of anthropogenic trace PAHs on DNA transfer in natural environments.

Highlights

Genetic transformation is a process in which a bacterial recipient takes up exogenous free DNA and incorporates it into its own chromosome by homologous recombination or converts it into an autonomous extrachromosomal replicon [1]
E. coli isolated from the intestine of Japanese patients contained gene segments that originated from the oceanic environment by way of edible seafood [5,6], which indicates that gene transfer between species is ubiquitous in natural environments [7,8,9]
The underlying mechanism of transformation was investigated based on Ca2+-controlled polycyclic aromatic hydrocarbons (PAHs) adsorption and the results of Fourier-transform infrared (FTIR), mass spectrometry (MS), and X-ray photoelectron spectroscopy (XPS)

Summary

Introduction

Genetic transformation is a process in which a bacterial recipient takes up exogenous free DNA and incorporates it into its own chromosome by homologous recombination or converts it into an autonomous extrachromosomal replicon [1]. Upon the death of an organism, intracellular germplasm accompanied by other extracellular materials is released into soil and water environments, transferred to other biological cells, and expressed in the new host. Such horizontal gene transfers (HGTs) among biological species have been widely reported. Up to 10–16% of Escherichia coli DNA was acquired through HGT [3,4]. E. coli isolated from the intestine of Japanese patients contained gene segments that originated from the oceanic environment by way of edible seafood [5,6], which indicates that gene transfer between species is ubiquitous in natural environments [7,8,9]

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Results