Femtosecond laser–assisted thermal annealing of Ni electrode on SiC substrate

Alloying at the metal–semiconductor interface induced by femtosecond laser irradiation associated with thermal annealing was examined to ascertain whether an ohmic contact was formed on silicon carbide (SiC). In general, the electric field of the femtosecond laser beam destroys the crystal structure, but with lower thermal damage around the irradiated areas. In addition to the laser irradiation, we employed thermal annealing to enhance the diffusion of the metal atoms inside the SiC. After these processes, an ohmic contact was successfully formed on the SiC with thermal annealing at a temperature of 900 °C, which is 100 °C lower than with the conventional thermal annealing method

Specific expression and function of the A-type cytochrome c oxidase under starvation conditions in Pseudomonas aeruginosa.

Pseudomonas aeruginosa has one A-type (caa3) and multiple C-type (cbb3) cytochrome c oxidases as well as two quinol oxidases for aerobic respiration. The caa3 oxidase is highly efficient in creating a proton gradient across the cell membrane, but it is not expressed under normal growth conditions and its physiological role has not been investigated. In the present study, a mutant strain deficient in the coxBA-PA0107-coxC genes encoding caa3 exhibited normal growth under any test conditions, but it had low relative fitness under carbon starvation conditions, indicating that the expression of caa3 is advantageous under starvation conditions. A mutant that lacked four terminal oxidase gene clusters except for the cox genes was unable to grow aerobically because of low expression level of caa3. However, suppressor mutants that grew aerobically using caa3 as the only terminal oxidase emerged after aerobic subculturing. Analyses of the suppressor mutants revealed that a mutation of roxS encoding a sensor kinase of a two-component regulator RoxSR was necessary for the aerobic growth in synthetic medium. Two additional mutations in the 5'-flanking region of coxB were necessary for the aerobic growth in LB medium. Although the expression level of caa3 was higher in the suppressor mutants, their growth rates were lower than when the other terminal oxidases were utilized, suggesting that caa3 was not suited for utilization as the only terminal oxidase. Overexpression of the cox genes also inhibited the aerobic growth of the wild-type strain. These results indicate that caa3 is tightly regulated to be expressed only under starvation conditions at low level and it functions in cooperation with other terminal oxidases to facilitate survival in nutrient starvation conditions

Effect of the overexpression of the <i>cox</i> genes.

<p>Strains transformed with pMMB67EH or pMMB-cox were cultivated aerobically in 4 ml of LB medium <b>(A)</b> and glutamate medium <b>(B)</b> in test tubes shaken at 200 rpm. The media were supplemented with 200 μg/ml carbenicillin and 0.5 mM IPTG. The results shown are representative based on at least three independent experiments. The maximum specific growth rates (μ_max) of WT (pMMB67EH), WT (pMMB-cox), ROX1 (pMMB67EH), and ROX1 (pMMB-cox) were 0.79 ± 0.055, 0.30 ± 0.041, 0.72 ± 0.067, and 0.34 ± 0.103 h⁻¹, respectively, for LB medium and 0.65 ± 0.034, 0.27 ± 0.014, 0.67 ± 0.042, and 0.50 ± 0.013 h⁻¹, respectively, for glutamate medium. The μ_max value was not calculated for the recombinant strains of PTO5, because they did not show apparent growth.</p

Effects of the suppressor mutations on the transcriptional activity of <i>coxB</i>.

<p>Effects of the suppressor mutations on the transcriptional activity of <i>coxB</i>.</p