Yong Xue

Microbial biodegradation of polyaromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are widespread in various ecosystems and are pollutants of great concern due to their potential toxicity, mutagenicity and carcinogenicity. Because of their hydrophobic nature, most PAHs bind to particulates in soil and sediments, rendering them less available for biological uptake. Microbial degradation represents the major mechanism responsible for the ecological recovery of PAH-contaminated sites. The goal of this review is to provide an outline of the current knowledge of microbial PAH catabolism. In the past decade, the genetic regulation of the pathway involved in naphthalene degradation by different gram-negative and gram-positive bacteria was studied in great detail. Based on both genomic and proteomic data, a deeper understanding of some high-molecular-weight PAH degradation pathways in bacteria was provided. The ability of nonligninolytic and ligninolytic fungi to transform or metabolize PAH pollutants has received considerable attention, and the biochemical principles underlying the degradation of PAHs were examined. In addition, this review summarizes the information known about the biochemical processes that determine the fate of the individual components of PAH mixtures in polluted ecosystems. A deeper understanding of the microorganism-mediated mechanisms of catalysis of PAHs will facilitate the development of new methods to enhance the bioremediation of PAH-contaminated sites.

Genome-wide analysis of the AP2/ERF gene family in Populus trichocarpa.

Populus is a model system for investigating the wood development, crown formation, and disease resistance in perennial plants. AR2/ERF is a large family of transcription factors in plant, encoding transcriptional regulators with a variety of functions involved in the developmental and physiological processes. Here, starting from database of Populus genome, we identified 200 AP2/ERF genes by in silico cloning method using the AP2/ERF conserved domain amino acid sequence of Arabidopsis thaliana as probe. Based on the number of AP2/ERF domains and the function of the genes, those AP2/ERF genes from Populus were classified into four subfamilies named the AP2, DREB, ERF, RAV, and a soloist. Among these genes, the number genes of total AP2/ERF family genes, DREB subfamily, and ERF subfamily from Populus trichocarpa were about 1.4-1.6-fold than those from A. thaliana. The rates were very similar for the putative homologs between Populus and Arabidopsis.

Transgenic Arabidopsis Plants Expressing Tomato Glutathione S-Transferase Showed Enhanced Resistance to Salt and Drought Stress

Although glutathione S-transferases (GST, EC 2.5.1.18) are involved in response to abiotic stress, limited information is available regarding gene function in tomato. In this study, a GST gene from tomato, designated LeGSTU2, was cloned and functionally characterized. Expression profile analysis results showed that it was expressed in roots and flowers, and the transcription was induced by salt, osmotic, and heat stress. The gene was then introduced to Arabidopsis by Agrobacterium tumefaciens-mediated transformation. Transgenic Arabidopsis plants were normal in terms of growth and maturity compared with wild-type plants. Transgenic plants also showed an enhanced resistance to salt and osmotic stress induced by NaCl and mannitol. The increased tolerance of transgenic plants was correlated with the changes in proline, malondialdehyde and antioxidative emzymes activities. Our results indicated that the gene from tomato plays a positive role in improving tolerance to salinity and drought stresses in Arabidopsis.

Transcription factor OsAP21 gene increases salt/drought tolerance in transgenic Arabidopsis thaliana.

Transcription factors play vital roles in stress signal transduction and gene expression modulation during plant growth and development. Sequence analysis showed that OsAP21 contained an AP2/ERF domain of 57 amino acids. By comparison of deduced amino acid sequences of AP2/ERF-related proteins, we deduced that OsAP21 is a transcription factor gene, which belonging to rice AP2/ERF family CBF/DREB subfamily. Further, we report that transgenic Arabidopsisthaliana plants expressing the OsAP21 gene exhibited stronger growth than wild type plants under salt/drought stress. Analysis of RT-PCR for RD29B gene implied that OsAP21 over-expressed plants had a higher expression level of RD29B gene than wild type plants, and drought and salt treatments could enlarge these differences. Collectively, our results indicate that OsAP21 may play an important role in the response of transgenic Arabidopsis plants to salt/drought stresses.

Yeast heat-shock protein gene HSP26 enhances freezing tolerance in Arabidopsis.

In the yeast Saccharomyces cerevisiae, the molecular chaperone Hsp26 is one component of the heat-shock response. Hsp26 has the remarkable ability to directly sense increases in temperature and switch from an inactive state to a chaperone-active state. In this study, we report a functional analysis of Hsp26 in Arabidopsis thaliana and its response to freezing stress. After freezing stress, the HSP26 transgenic plants exhibited stronger growth than the wild-type plants. We found that over-expression of HSP26 in Arabidopsis increased the amounts of free proline and soluble sugars, elevated the expression of stress defense genes, and enhanced Arabidopsis tolerance to freezing stress. Taken together, our results indicate that Hsp26 may play an important role in the response of transgenic Arabidopsis plants to freezing stresses.

Directed evolution of a beta-galactosidase from Pyrococcus woesei resulting in increased thermostable beta-glucuronidase activity

We performed directed evolution on a chemically synthesized 1,533-bp recombinant beta-galactosidase gene from Pyrococcus woesei. More than 200,000 variant colonies in each round of directed evolution were screened using the pYPX251 vector and host strain Rosetta-Blue (DE3). One shifted beta-galactosidase to beta-glucuronidase mutant, named YG6762, was obtained after four rounds of directed evolution and screening. This mutant had eight mutated amino acid residues. T29A, V213I, L217M, N277H, I387V, R491C, and N496D were key mutations for high beta-glucuronidase activity, while E414D was not essential because the mutation did not lead to a change in beta-glucuronidase activity. The amino acid site 277 was the most essential because mutating H back to N resulted in a 50% decrease in beta-glucuronidase activity at 37°C. We also demonstrated that amino acid 277 was the most essential site, as the mutation from N to H resulted in a 1.5-fold increase in beta-glucuronidase activity at 37°C. Although most single amino acid changes lead to less than a 20% increase in beta-glucuronidase activity, the YG6762 variant, which was mutated at all eight amino acid sites, had a beta-glucuronidase activity that was about five and seven times greater than the wild-type enzyme at 37 and 25°C, respectively.

Gene duplication and transfer events in plant mitochondria genome.

Gene or genome duplication events increase the amount of genetic material available to increase the genomic, and thereby phenotypic, complexity of organisms during evolution. Gene duplication and transfer events have been important to molecular evolution in all three domains of life, and may be the first step in the emergence of new gene functions. Gene transfer events have been proposed as another accelerator of evolution. The duplicated gene or genome, mainly nuclear, has been the subject of several recent reviews. In addition to the nuclear genome, organisms have organelle genomes, including mitochondrial genome. In this review, we briefly summarize gene duplication and transfer events in the plant mitochondrial genome.

A Profile of Ring-hydroxylating Oxygenases that Degrade Aromatic Pollutants

Numerous aromatic compounds are pollutants to which exposure exists or is possible, and are of concern because they are mutagenic, carcinogenic, or display other toxic characteristics. Depending on the types of dioxygenation reactions of which microorganisms are capable, they utilize ring-hydroxylating oxygenases (RHOs) to initiate the degradation and detoxification of such aromatic compound pollutants. Gene families encoding for RHOs appear to be most common in bacteria. Oxygenases are important in degrading both natural and synthetic aromatic compounds and are particularly important for their role in degrading toxic pollutants; for this reason, it is useful for environmental scientists and others to understand more of their characteristics and capabilities. It is the purpose of this review to address RHOs and to describe much of their known character, starting with a review as to how RHOs are classified. A comprehensive phylogenetic analysis has revealed that all RHOs are, in some measure, related, presumably by divergent evolution from a common ancestor, and this is reflected in how they are classified. After we describe RHO classification schemes, we address the relationship between RHO structure and function. Structural differences affect substrate specificity and product formation. In the alpha subunit of the known terminal oxygenase of RHOs, there is a catalytic domain with a mononuclear iron center that serves as a substrate-binding site and a Rieske domain that retains a [2Fe-2S] cluster that acts as an entity of electron transfer for the mononuclear iron center. Oxygen activation and substrate dihydroxylation occurring at the catalytic domain are dependent on the binding of substrate at the active site and the redox state of the Rieske center. The electron transfer from NADH to the catalytic pocket of RHO and catalyzing mechanism of RHOs is depicted in our review and is based on the results of recent studies. Electron transfer involving the RHO system typically involves four steps: NADH-ferredoxin reductase receives two electrons from NADH; ferredoxin binds with NADH-ferredoxin reductase and accepts electron from it; the reduced ferredoxin dissociates from NADH-ferredoxin reductase and shuttles the electron to the Rieske domain of the terminal oxygenase; the Rieske cluster donates electrons to O2 through the mononuclear iron. On the basis of crystal structure studies, it has been proposed that the broad specificity of the RHOs results from the large size and specific topology of its hydrophobic substrate-binding pocket. Several amino acids that determine the substrate specificity and enantioselectivity of RHOs have been identified through sequence comparison and site-directed mutagenesis at the active site. Exploiting the crystal structure data and the available active site information, engineered RHO enzymes have been and can be designed to improve their capacity to degrade environmental pollutants. Such attempts to enhance degradation capabilities of RHOs have been made. Dioxygenases have been modified to improve the degradation capacities toward PCBs, PAHs, dioxins, and some other aromatic hydrocarbons. We hope that the results of this review and future research on enhancing RHOs will promote their expanded usage and effectiveness for successfully degrading environmental aromatic pollutants.

Analysis of gene expression profile of Arabidopsis genes under trichloroethylene stresses with the use of a full-length cDNA microarray

Trichloroethylene (TCE) is a widespread and persistent environmental contaminant. Plants are able to take up a range of harmful organic compounds, including some of the most abundant environmental pollutants like TCE. In this study, complementary DNA microarrays were constructed to have a better view of transcript expression in Arabidopsis thaliana during TCE-induced stress. The microarray analysis demonstrated the complexity of gene expression patterns resulting from TCE. A total of 1,020 transcripts were differentially up-regulated by TCE. Those genes might specifically contribute to the TCE transformation, conjugation, and compartmentation in plant. This study provides informative preliminary data for more in-depth analyses of TCE tolerance in Arabidopsis thaliana.

Gene expression profile of Arabidopsis under sodium bisulfite treatment by oligo-microarray analysis

Low concentrations of sodium bisulfite (NaHSO3) can enhance some plant photosynthetic efficiency. To investigate the growth process of Arabidopsis seedlings in response to NaHSO3 treatment and to obtain a better understanding the mechanism of NaHSO3 treatment toward Arabidopsis, microarray experiments on Arabidopsis thaliana were performed in the present study. Seedlings growth and physiological responses of Arabidopsis to NaHSO3 were investigated. The results indicated that the NaHSO3 response was related to photosynthesis pathways and reactive oxygen species (ROS) accumulation. Our findings could provide valuable gene resources and theoretical information for understanding the physiological responses and dissecting the NaHSO3 response pathways in higher plants.