One Shot™ TOP10/P3 Chemically Competent E. coli

E. coli harboring the plasmid P3 enable the selection and maintenance of plasmids that encode the tRNA suppressor F gene (supF). P3, a low-copy 60 kb episomal plasmid, encodes the kanamycin resistance gene as well as amber mutants of the tetracyline and ampicillin resistance genes. The amber mutations are point mutations in the resistance markers which inactivate the expression of resistance in the strain unless the tRNA supressor F (supF) is present. Therefore, strains that harbor P3 alone are resistant to kanamycin, but sensitive to both tetracycline and ampicillin. But when the E. coli carrying the P3 plasmid are transformed with supF-containing plasmids (e.g. pcDNA1 and pCDM8), they are rendered resistant to both tetracycline and ampicillin (as well as kanamycin) by suppression of the amber mutations.

The rate of spontaneous reversion of the amber point mutations on the P3 episome is fairly high, so it is important to select supF clones with both tetracycline and ampicillin resistance to reduce background growth. To avoid enriching for revertants, it is recommended that relatively low concentrations of tetracycline (7.5 to 10 ug/ml) and ampicillin (25 to 40 ug/ml) be used.

nupG is a mutation for the transport of nucleosides. The nupG site is next to endA on the chromosome, and when endA was mutated by transposon insertion, the nupG site was unintentionally mutated as well. There are no apparent effects of this mutation on cell function or growth.

References: 1) Nghiem, Y. et al. PNAS 85: 2709-2713. 2) Westh Hansen, S.V. et al. Eur. J. Biochem. 168: 385-391.

The P3 plasmid is approximately 60 kb and usually is present at only 1 or 2 copies per cell. It is similar in properties to the F' episome. It can be recovered along with your vector of interest in a plasmid prep and might be detectable on a gel. The very large plasmid would be seen high on the gel near the wells.

MC1061/P3 is the strain that was historically used for most supF selection applications. It is referenced by Brian Seed in his library construction paper.

For library transformations, we do still recommend MC1061/P3 because of its slightly higher transformation efficiency when transforming pcDNA1.1. However, for propagating plasmid for transfection, we strongly recommend using the TOP10/P3 instead, which is endA- and yields much cleaner plasmid DNA in purification. DNA prepared from MC1061 cells is typically not suitable for transfection.