title: High-level expression of foreign genes via multiple joined operons and a new concept regarding the restricted constant of total amount of plasmid DNA per Escherichia coli cell.
abstract: 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 < 0.01). Further calculation showed that the total amount of plasmid DNA per cell was not significantly different in each series (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.
authors.affiliation: National Laboratory of Medical Molecular Biology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing 100005, China.
authors.forename: Weijing
authors.initials: W
authors.lastname: Chen
authors.forename: Mei
authors.initials: M
authors.lastname: Hong
authors.forename: Dan
authors.initials: D
authors.lastname: Li
authors.forename: Shengdong
authors.initials: S
authors.lastname: Lu
mesh_terms: D004247:DNA / Q000032:analysis; D004926:Escherichia coli / Q000235:genetics*; D018628:Gene Dosage; D009876:Operon; D010957:Plasmids
chemical_list: D004247:DNA