Concentrations were determined spectrophotometrically at 260 nm. Plasmid separation was done by agarose gel electrophoresis (SCIE-PLAS, UK), using Agarose Multi Purpose (MP) gels from Roche Applied Science (Germany)
and 50 TAE buffer (0.04 mol/l Tris-acetate, 0.002 mol/l EDTA, pH 8.0).