Calcium-mediated DNA adsorption to yeast cells and kinetics of cell transformation by electroporation

Abstract

Detailed kinetic data suggest that the direct transfer of plasmid DNA (YEp 351, 5.6 kbp, supercoiled, Mr approximately 3.5 x 10(6)) by membrane electroporation of yeast cells (Saccharomyces cerevisiae, strain AH 215) is mainly due to electrodiffusive processes. The rate-limiting step for the cell transformation, however, is a bimolecular DNA-binding interaction in the cell interior. Both the adsorption of DNA, directly measured with [32P]dCTP DNA, and the number of transformants are collinearly enhanced with increasing total concentrations [Dt] and [Cat] of DNA and of calcium, respectively. At [Cat] = 1 mM, the half-saturation or equilibrium constant is KD = 15 +/- 1 nM at 293 K (20 degrees C). The optimal transformation frequency is TFopt = 4.1 +/- 0.4 X 10(-5) if a single exponential pulse of initial field strength E0 = 4 kV cm-1 and decay time constant tauE = 45 ms is applied at [Dt] = 2.7 nM and 10(8) cells in 0.1 ml. The dependence of TF on [Cat] yields the equilibrium constants KCazero = 1.8 +/- 0.2 mM (in the absence of DNA) and K'Ca (at 2.7 nM DNA), comparable with and derived from electrophoresis data. In yeast cells, too, the appearance of a DNA molecule in its whole length in the cell interior is clearly an after-field event. At Eo = 4.0 kV cm-1 and T = 293 K, the flow coefficient of DNA through the porous membrane patches is Kto = 7.0 +/- 0.7 x 10(3)S-1 and the electrodiffusion of DNA is approximately 10 times more effective than simple diffusion: D/D0 approximately 10.3. The mean radius of these pores is rp = 0.39 +/- 0.05 nm, and the mean number of pores per cell (of size ø approximately 5.5 microns) is Np = 2.2 +/- 0.2 x 10(4). The maximal membrane area that is involved in the electrodiffusive penetration of adsorbed DNA into the outer surface of the electroporated cell membrane patches is only 0.023% of the total cell surface. The surface penetration is followed either by additional electrodiffusive or by passive (after-field) diffusive translocation of the inserted DNA into the cell interior. For practical purposes of optimal transformation efficiency, 1 mM calcium is necessary for sufficient DNA binding and the relatively long pulse duration of 20-40 ms is required to achieve efficient electrodiffusive transport across the cell wall and into the outer surface of electroporated cell membrane patches.