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Using transcriptome and proteome techniques and association analysis, we identified several key genes involved in the formation of Ca 2+ -mediated E. coli DH5α competent cells. We used Red homologous recombination technology to construct three single-gene deletion strains and found that the transformation efficiencies of yiaW , ygiZ , and osmB deletion strains for different-size plasmids were significantly increased.

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The preparation of Escherichia coli competent cells by calcium chloride is a common method in molecular biology, but the mechanism is poorly understood. In a previous study, using transcriptomics and proteomics approaches, we found that the expression pattern of the gene loiP was upregulated by CaCl2. In order to investigate the function of the loiP gene in Ca2+ mediated formation of competent cells of E. coli DH5α, the loiP gene deletion strains were constructed by the lambda-derived Red homologous recombination technique. Then the cell morphology, transformation efficiency and cell membrane changes of the competent cells of the mutant strain were further explored. Compared with the wild-type E. coli DH5α, the mutant strains have no significant differences in the morphology, growth characteristics and the permeability of the intracellular membrane. However, the transformation efficiencies of the mutant strains to plasmids of different sizes significantly reduced, and the permeability of the outer membrane decreased by 2.94 times. These results indicate that the deletion of gene loiP may directly affect the transformation efficiency and outer membrane permeability of E. coli competent cells.

In this study, the Cu2+ (120 mg/L) and Cr6+(80 mg/L) removal rate of S. rhizophila JC1 reached at 79.9% and 89.3%, respectively. Scanning electron microscopy showed that Pb2+ and Zn2+ had no obvious effect on cell structure, but the cells became smaller and brighter under Cu2+ stress, and many nanoparticles formed on the cell surface under Cr6+ stress. The physiological response analyses demonstrated that moderate change of membrane permeability was necessary for adsorption. FI-IR and EDS analyses showed that exopolysaccharides (EPS) and the replacement of basic elements (ie., C, O) might be the main means of heavy metals adsorption by strain. In addition, 323 transport proteins were predicted in the genome of S. rhizophila JC1. Among them, two, six and five proteins of the cation diffusion facilitator, resistance-nodulation-division efflux and P-type ATPase families were respectively predicted. The expression of genes showed that the synergistic action of transport proteins played an important role in the process of adsorption. The comparative genomics analysis revealed that S. rhizophila JC1 has long-distance evolutionary relationships with other strains, but the efflux system of S. rhizophila JC1 contained the same types of metal transport proteins as other metal-resistant bacteria.