E. coli One Shot™ TOP10 Electrocomp™

The only difference between TOP10 and TOP10F' cells is that the latter contain the F' episome that carries the tetracycline resistance gene and allows isolation of single-stranded DNA from vectors that have an f1 origin of replication. The F' episome also carries the lacIq repressor for inducible expression from trc, tac, and lac promoters using IPTG. TOP10F' cells require IPTG induction for blue/white screening.

If the insert is potentially toxic to the host cells, here are some suggestions that you can try:

- After transforming TOP10 or DH5? cells, incubate at 25-30°C instead of 37°C. This will slow down the growth and will increase the chances of cloning a potentially toxic insert.
- Try using TOP10F' cells for the transformation, but do not add IPTG to the plates. These cells carry the lacIq repressor that represses expression from the lac promoter and so allows cloning of toxic genes. Keep in mind that in the absence of IPTG, blue-white screening cannot be performed.
- Try using Stbl2 cells for the transformation.

It is imperative that a cloning strain such as TOP10 be used for characterization of the plasmid, propagation, and maintenance. BL21 cells are wild-type for endA and recA, which could result in poor miniprep quality and a greater chance of plasmid rearrangements due to recombination. In addition, BL21 cells contain the T7 RNA polymerase gene which is expressed at low levels even in the absence of inducer. If the gene is toxic to E. coli, plasmid instability and/or cell death can result.

Yes, TOP10 has the hsdRMS mutation, so this strain can be used to clone DNA from PCR reactions and other non-methylated sources. hsdRMS is a mutation in the system that E. coli uses to recognize foreign DNA. There are two parts to this system, methylation and restriction. E. coli methylate DNA at certain sequences, and if the DNA is not methylated at these sequences it will be recognized as foreign and restricted. Thus, if unmethylated DNA is transformed into E.coli that does not carry the hsdRMS genotype, it is recognized as foreign and enzymatically degraded.

TOP10 cells are lacIq- (minus). They do not have the lacIq gene and therefore do not produce the lacIq repressor protein. lacIq is most commonly found on an F' episome, and therefore is present in TOP10F', JM101, JM109, and NM522 strains.