Natural plasmid transformation in Escherichia coli.
Although Escherichia coli does not have a natural transformation process, strains of E. coli can incorporate extracellular plasmids into cytoplasm 'naturally' at low frequencies. A standard method was developed in which stationary phase cells were concentrated, mixed with plasmids, and then plated on agar plates with nutrients which allowed cells to grow. Transformed cells could then be selected by harvesting cells and plating again on selective agar plates. Competence developed in the lag phase, but disappeared during exponential growth. As more plasmids were added to the cell suspension, the number of transformants increased, eventually reaching a plateau. Supercoiled monomeric or linear concatemeric DNA could transform cells, while linear monomeric DNA could not. Plasmid transformation was not related to conjugation and was recA-independent. Most of the E. coli strains surveyed had this process. All tested plasmids, except pACYC184, could transform E. coli. Insertion of a DNA fragment containing the ampicillin resistance gene into pACYC184 made the plasmid transformable. By inserting random 20-base-pair oligonucleotides into pACYC184 and selecting for transformable plasmids, a most frequent sequence was identified. This sequence resembled the bacterial interspersed medium repetitive sequence of E. coli, suggesting the existence of a recognition sequence. We conclude that plasmid natural transformation exists in E. coli.
Tsen
Suh-Der
SD
Department of Microbiology, Graduate Institute of Microbiology and Immunology, National Yang-Ming University, Taipei, Taiwan, ROC. nymutsen@ym.edu.tw
Fang
Suh-Sen
SS
Chen
Mei-Jye
MJ
Chien
Jun-Yi
JY
Lee
Chih-Chun
CC
Tsen
Darwin Han-Lin
DH
D001483:Base Sequence; D002118:Calcium / Q000378:metabolism; D002453:Cell Cycle / Q000502:physiology; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics* / Q000502:physiology; D005838:Genotype; D008274:Magnesium / Q000378:metabolism; D010641:Phenotype; D010957:Plasmids / Q000235:genetics* / Q000378:metabolism; D014169:Transformation, Bacterial
D004269:DNA, Bacterial; D008274:Magnesium; D002118:Calcium