ELECTROTRANSFORMATION OF STREPTOCOCCUS-PNEUMONIAE - EVIDENCE FOR RESTRICTION OF DNA ON ENTRY

Electrotransformation is a method generally used in biotechnology to i ntroduce recombinant DNA into a wide range of bacteria. However, the m echanism of DNA entry is poorly understood. We report that in Streptoc occus pneumoniae, a naturally transformable species. electrotransforma tion efficiently introduces a plasmid replicon. DNA is strongly restri cted by the restriction-modification systems DpnI and DpnII which degr ade methylated and non-methylated DNA, respectively, at GATC sequences . This suggests that in electrotransformation double-stranded DNA pene trates into these bacteria without a single-stranded DNA step in contr ast to natural transformation. Single-stranded DNA by itself is able t o electrotransform very weakly and linearized double-stranded plasmid DNA yields barely detectable levels of transformants.