Electrotransformation of Streptococcus pneumoniae: evidence for restriction of DNA on entry

Electrotransformation is a method generally used in biotechnology to introduce recombinant DNA into a wide range of bacteria. However, the mechanism of DNA entry is poorly understood. We report that in Streptococcus pneumoniae, a naturally transformable species, electrotransformation efficiently introduces a plasmid replicon. DNA is strongly restricted by the restriction-modification systems DpnI and DpnII which degrade methylated and non-methylated DNA, respectively, at GATC sequences. This suggests that in electrotransformation double-stranded DNA penetrates into these bacteria without a single-stranded DNA step in contrast to natural transformation. Single-stranded DNA by itself is able to electrotransform very weakly and linearized double-stranded plasmid DNA yields barely detectable levels of transformants.