Purification of plasmid DNA
Plasmid DNA (pBBSK) was purified from E. coli DH1 using a Qiaprep Spin Miniprep kit (Qiagen, Chatsworth, CA). Plasmid DNA was eluted in Tris–EDTA (TE) buffer and dialyzed for 24 h at 4 C against 130 mM NaCl to remove metal ions. UV–vis measurements were acquired from a Shimadzu UV-3101 PC spectrophotometer. Absorbance ratios of A250/A260 6 0.95 and A260/A280 P 1.8 were determined for all DNA used