Enhancement of bovine growth hormone gene expression by increasing the plasmid copy number

Effect of copy number on the expression of bovine growth hormone gene (bGH) was investigated using the copy number mutants such as pKBJ10, pBJ( tet)10, pUBJ10-1, and pUBJ10 plasmids. The cells harboring plasmids below 84 copies/cell did not produced detectable levels of bGH. When the ColE1 replicon was replaced with the mutated ColE1 replicon originated from pUC19 plasmid, the copy number was increased to about 300 copies/cell and bGH production was enhanced by 11.5% (pUBJ10-1) and 12.3% of total cell protein (pUBJ10). A large amount of mRNA caused by increment of copy number would be needed to overcome some inhibitory threshold and might be an important factor for regulating bGH expression.