We have theoretically analyzed the quasibound states in a Mraphene quantum dot （GO, D） with a magnetic flux -φ in the centre. It is shown that the two-fold time reversal degeneracy is broken and the quasibound stat...We have theoretically analyzed the quasibound states in a Mraphene quantum dot （GO, D） with a magnetic flux -φ in the centre. It is shown that the two-fold time reversal degeneracy is broken and the quasibound states of GQD with positive^negative angular momentum shifted upwards/downwards with increasing the magnetic flux. The variation of the quasibound energy depends linearly on the magnetic flux, which is quite different from the parabolic relationship for SchrSdinger electrons. The GQD＇s quasibound states spectrum shows an obvious Aharonov-Bohm （AB） oscillations with the magnetic flux. It is also shown that the quasibound state with energy equal to the barrier height becomes a bound state completely confined in GQD.展开更多
After deregulating the purine and riboflavin synthesis in the Gram-positive bacterium Bacillus subtilis,it is critical to amplify riboflavin operon with appropriate dosage in the host strain for remarkable increase of...After deregulating the purine and riboflavin synthesis in the Gram-positive bacterium Bacillus subtilis,it is critical to amplify riboflavin operon with appropriate dosage in the host strain for remarkable increase of riboflavin production.Bacillus subtilis RH13, a riboflavin-producing strain, was selected as host strain in the construction of engineering strains by protoplast fusion. The integrative plasmid pRB63 and autonomous plasmid pRB49, pRB62 containing riboflavin operon of B.subtilis 24 were constructed and transformed into the host strain respectively. Increasing one operon copy in B.subtilis RH13 results in about 0.4 g/L improvement in riboflavin yield and the appropriate number of operon copies was about 7—8. Amplifying more riboflavin operons is of no use for further improvement of yield of riboflavin. Furthermore, excessive operon dosage results in metabolic unbalance and is fatal to the host cells producing riboflavin.展开更多
OBJECTIVE: To examine the feasibility of linking operons in tandem to enhance expression of heterologous genes in Escherichia coli (E. coli) and clarify the potential control mechanism of the total plasmid DNA amount ...OBJECTIVE: To examine the feasibility of linking operons in tandem to enhance expression of heterologous genes in Escherichia coli (E. coli) and clarify the potential control mechanism of the total plasmid DNA amount in each host cell. METHODS: Two series of expression plasmids, CW11 and CW12, containing 1 to 4 and 1 to 3 heterologous gene operon(s) respectively, were constructed. The molecular size of the CW11 series varied from 5.47 kb to 12.26 kb in 2.25 kb increments. The CW12 series varied from 5.40 kb to 9.72 kb in 2.16 kb increments. The expression level of desired protein was assayed by SDS-PAGE and laser density scanning. Plasmid copy number was determined by incorporation with (3)H-thymidine ((3)H-TdR). RESULTS: No influence of the tandem-joined operons on host growth and plasmid stability was observed. Upon induction, the desired protein accumulations in the CW11 series were 44.9% +/- 3.9%, 51.3% +/- 4.1%, 54.8% +/- 3.3% and 58.2% +/- 3.4% of total cell protein. In the CW12 series, the yields were 32.2% +/- 5.0%, 42.8% +/- 4.1% and 46.9% +/- 4.0% of total cell protein. As size increased, the plasmid copy number decreased, but target gene dosage increased significantly (P 0.05) and restricted to some extent. CONCLUSIONS: Increasing the target gene dosage by tandem linking of operons may enhance the expression level of a desired protein. Although the size (kb) and the copy number of each plasmid are negatively interrelated, for certain plasmids in each series, their total DNA amount per cell seems to be a restricted constant for specific E. coli strains under identical incubation condition.展开更多
We measured in the laboratory compressional wave velocity and electrical resistivity on 434 sediment samples collected from the Yellow Sea to study the joint elastic-electrical properties of marine sediments. Porosity...We measured in the laboratory compressional wave velocity and electrical resistivity on 434 sediment samples collected from the Yellow Sea to study the joint elastic-electrical properties of marine sediments. Porosity was found to reduce both elastic velocity and electrical resistivity of the marine sediments in a non-linear fashion; velocity showed an approximate linear increase with increasing logarithm of resistivity. Various effective medium models either implemented or developed were compared with the new dataset. The model results showed that the combined self-consistent approximation and differential effective medium model using critical porosity of 0.6 and 0.5 for velocity and resistivity respectively gave a reasonable description of the joint elastic-electrical behaviors of the marine sediments. The joint elastic-electrical properties of the marine sediments established would be used to estimate resistivity from measured velocity and vice versa, and could also be suitable for detection of gas hydrate or other suitable targets from joint seismic-resistivity surveys.展开更多
基金Support by National Science Foundation of China under Grant Nos. 60776067 and 10974011
文摘We have theoretically analyzed the quasibound states in a Mraphene quantum dot （GO, D） with a magnetic flux -φ in the centre. It is shown that the two-fold time reversal degeneracy is broken and the quasibound states of GQD with positive^negative angular momentum shifted upwards/downwards with increasing the magnetic flux. The variation of the quasibound energy depends linearly on the magnetic flux, which is quite different from the parabolic relationship for SchrSdinger electrons. The GQD＇s quasibound states spectrum shows an obvious Aharonov-Bohm （AB） oscillations with the magnetic flux. It is also shown that the quasibound state with energy equal to the barrier height becomes a bound state completely confined in GQD.
文摘After deregulating the purine and riboflavin synthesis in the Gram-positive bacterium Bacillus subtilis,it is critical to amplify riboflavin operon with appropriate dosage in the host strain for remarkable increase of riboflavin production.Bacillus subtilis RH13, a riboflavin-producing strain, was selected as host strain in the construction of engineering strains by protoplast fusion. The integrative plasmid pRB63 and autonomous plasmid pRB49, pRB62 containing riboflavin operon of B.subtilis 24 were constructed and transformed into the host strain respectively. Increasing one operon copy in B.subtilis RH13 results in about 0.4 g/L improvement in riboflavin yield and the appropriate number of operon copies was about 7—8. Amplifying more riboflavin operons is of no use for further improvement of yield of riboflavin. Furthermore, excessive operon dosage results in metabolic unbalance and is fatal to the host cells producing riboflavin.
基金ThisstudywassupportedbyagrantfromtheNationalHighTechnologyResearchandDevelopmentProgram (No .10 2 0 8 0 20 2)
文摘OBJECTIVE: To examine the feasibility of linking operons in tandem to enhance expression of heterologous genes in Escherichia coli (E. coli) and clarify the potential control mechanism of the total plasmid DNA amount in each host cell. METHODS: Two series of expression plasmids, CW11 and CW12, containing 1 to 4 and 1 to 3 heterologous gene operon(s) respectively, were constructed. The molecular size of the CW11 series varied from 5.47 kb to 12.26 kb in 2.25 kb increments. The CW12 series varied from 5.40 kb to 9.72 kb in 2.16 kb increments. The expression level of desired protein was assayed by SDS-PAGE and laser density scanning. Plasmid copy number was determined by incorporation with (3)H-thymidine ((3)H-TdR). RESULTS: No influence of the tandem-joined operons on host growth and plasmid stability was observed. Upon induction, the desired protein accumulations in the CW11 series were 44.9% +/- 3.9%, 51.3% +/- 4.1%, 54.8% +/- 3.3% and 58.2% +/- 3.4% of total cell protein. In the CW12 series, the yields were 32.2% +/- 5.0%, 42.8% +/- 4.1% and 46.9% +/- 4.0% of total cell protein. As size increased, the plasmid copy number decreased, but target gene dosage increased significantly (P 0.05) and restricted to some extent. CONCLUSIONS: Increasing the target gene dosage by tandem linking of operons may enhance the expression level of a desired protein. Although the size (kb) and the copy number of each plasmid are negatively interrelated, for certain plasmids in each series, their total DNA amount per cell seems to be a restricted constant for specific E. coli strains under identical incubation condition.
基金supported by the Oceanic Special Public Sector Research Project (Grant No. 200805008)
文摘We measured in the laboratory compressional wave velocity and electrical resistivity on 434 sediment samples collected from the Yellow Sea to study the joint elastic-electrical properties of marine sediments. Porosity was found to reduce both elastic velocity and electrical resistivity of the marine sediments in a non-linear fashion; velocity showed an approximate linear increase with increasing logarithm of resistivity. Various effective medium models either implemented or developed were compared with the new dataset. The model results showed that the combined self-consistent approximation and differential effective medium model using critical porosity of 0.6 and 0.5 for velocity and resistivity respectively gave a reasonable description of the joint elastic-electrical behaviors of the marine sediments. The joint elastic-electrical properties of the marine sediments established would be used to estimate resistivity from measured velocity and vice versa, and could also be suitable for detection of gas hydrate or other suitable targets from joint seismic-resistivity surveys.
