Competent cells for protein expression: strain and delivery before the construct

Choosing strain and delivery

1. Separate cloning strains from expression strains. Cloning strains maximise uptake and plasmid stability. Expression strains minimise proteolysis and leaky expression. Their properties are close to opposite and the catalogue names do not always make that obvious.
2. Choose repression for a toxic product. Where the product harms the host, leaky expression before induction selects for cells that have lost the construct. Strains and vectors offering tighter repression are the remedy, not lower induction.
3. Match delivery to the cell, not to the protocol. Chemical transformation suits laboratory strains, electroporation suits difficult ones and high efficiency needs, and suspension mammalian cells need formulations designed for them.
4. Control arcing and cuvette hygiene. Electroporation fails on salt carryover and reused cuvettes. Desalting the DNA and using fresh cuvettes removes most of the failures blamed on the instrument.
5. Use transposon systems where a virus is unnecessary. Transposon based integration gives stable expression without viral containment requirements, which simplifies approvals and handling for many applications.
6. Design the workspace against amplicon carryover. A dedicated setup enclosure with ultraviolet decontamination and a strict direction of travel controls contamination far better than any reagent. Automation helps by removing manual transfers, not by being fast.

The strain is a design decision

Codon usage, protease complement, repression tightness and disulphide forming capacity in the cytoplasm all differ between strains, and each solves a specific expression problem. Choosing by habit means solving none of them.

Screen two or three strains in parallel for a difficult protein. It is a week that regularly replaces a quarter of vector redesign.

Contamination is designed out, not cleaned up

Amplified product is the contaminant, and it travels on gloves, pipettes and air. Separate areas or separate times for reagent preparation, template addition and post amplification work, with dedicated equipment, is the control.

A laboratory that has had an event and cannot say where it came from has a layout problem. Buying new reagents will not fix it.

An electroporation instrument or chemical cells, and what each is for

Chemically prepared cells are convenient, need no equipment and deliver enough transformants for routine subcloning from a ligation. Electroporation reaches efficiencies orders of magnitude higher and is what a library, a large construct or a scarce ligation actually needs.

Buying the highest efficiency stock for routine subcloning wastes it, since a fraction of a vial will do and efficiency falls with every freeze and thaw. Match the stock to the job, and aliquot so that a vial is used once.

Genotype is the specification

The strain's genotype decides whether a plasmid is stable, whether it is methylated, whether repeats survive and whether expression is possible at all. A strain deficient in recombination protects repeats; one deficient in methylation is required where the DNA must be unmethylated; an expression strain carries the polymerase an expression vector needs.

Read the genotype rather than the marketing name and match it to the construct. Most cloning problems blamed on efficiency are strain choices, and they are invisible until a construct rearranges or a digestion fails for no apparent reason.

Cloning strains and expression strains are different animals

A cloning strain is chosen for transformation efficiency and for keeping a plasmid intact: deficient in recombination so repeats survive, and deficient in the nuclease that degrades incoming DNA. It is not built to express anything.

An expression strain carries the polymerase an expression vector's promoter needs and is deficient in proteases that would degrade the product. It transforms less efficiently, which is why constructs are built in one strain and moved to the other.

What the expression genotype implies

A strain carrying a phage polymerase under a controllable promoter gives very high expression on induction, and that strength is also the problem: a toxic or insoluble product accumulates faster than the cell can fold it, giving inclusion bodies.

The usual answers are lowering the temperature after induction, reducing the inducer, and strain variants that suppress expression before induction or supply rare transfer RNAs. Those are all cheaper than moving host, and they are where most optimisation happens.

When to leave bacteria

Yeast folds and secretes proteins that bacteria cannot, performs disulphide formation and glycosylation, and grows cheaply to high density. It is the natural next host when a bacterial expression is insoluble or needs modification, and its glycosylation is not human.

Beyond that the choices are insect and mammalian cells, at higher cost and closer to native modification. Moving host is a bigger step than optimising expression, so it is worth exhausting temperature, inducer and strain variants first.

soc medium, and the hour after a transformation

SOC is a rich medium with magnesium and glucose added, and it exists for one hour in the protocol: the recovery between heat shock or electroporation and plating. The magnesium supports membrane repair, the glucose gives energy without inducing the lac promoter, and the tryptone and yeast extract give the cell what it needs to start expressing the resistance marker before the antibiotic arrives. Skipping the recovery costs transformation efficiency in proportion to how long the marker takes to protect, which is why ampicillin tolerates a short outgrowth and kanamycin does not.

chemically competent e coli, and what the efficiency figure means

Chemically competent cells are prepared with divalent cations and take up DNA after a heat shock, and the efficiency quoted on the tube, colonies per microgram of a supercoiled control plasmid, is a ceiling rather than a prediction. A ligation, a large construct or a library all transform orders of magnitude worse than the control, which is why a library is done by electroporation. What protects the number you have is handling: thaw on ice, add DNA in a small volume of low salt, do not mix by pipetting, and keep the heat shock to its stated time.

Chemically competent cells, and how they are used

Cells made competent with divalent cations take up DNA after a brief heat shock, and they are the routine workhorse for transforming a plasmid or a ligation. Their ceiling is set at preparation: efficiency is quoted against a supercoiled control and a ligation will always do worse. The handling rules are strict because the membrane state is fragile: thaw on ice, add DNA in a few microlitres of low-salt buffer, never pipette to mix, keep the heat shock to its stated time and give the cells their recovery in rich medium before the antibiotic.

e coli competent cells, and choosing the strain

Strain choice is a design decision before it is a purchase. A cloning strain carries mutations that stabilise the plasmid and improve yield, recA and endA deleted, and often a methylation genotype that matters if the DNA will be cut afterwards. An expression strain carries the T7 polymerase and, in its derivatives, features for toxic products, rare codons or disulfide formation. A strain for unstable repeats or for methylated DNA is a separate purchase again. Reading the genotype takes a minute and saves the week a wrong strain costs.

transformation competent cells and the efficiency that matters

transformation competent cells are specified by efficiency in colonies per microgram, and the number is measured with a small supercoiled plasmid, so a large or ligated construct transforms orders of magnitude worse. Chemically competent cells suit routine work; electrocompetent cells are needed for libraries where every molecule counts. Thawing on ice and one use per vial are what preserve either.

Common questions

Why is my protein degraded?

Frequently the strain. Expression strains deficient in major proteases exist for this reason, and switching strain is faster and cheaper than adding more inhibitors to the lysis buffer.

Why do my transformants lose the plasmid?

Leaky expression of a product the host does not tolerate. Tighter repression, a lower copy vector or a lower growth temperature before induction usually recovers it.

When is electroporation worth the equipment?

For difficult strains, library scale transformations where efficiency is the constraint, and primary or suspension cells. For routine cloning into laboratory strains, chemical transformation is simpler.

Do I need high efficiency competent cells?

For a library, a large construct or a scarce ligation, yes. For routine subcloning a standard chemical stock is ample, and a fraction of a vial will do. Aliquot so a vial is used once, because efficiency falls with each freeze and thaw.

Why not clone and express in the same strain?

Because the genotypes pull in opposite directions: a cloning strain protects the plasmid and transforms efficiently, an expression strain carries the polymerase the promoter needs and lacks degrading proteases. Build in one, express in the other.

My protein is in inclusion bodies. What first?

Lower the temperature after induction, reduce the inducer, and try a strain variant that suppresses pre-induction expression. Those are cheaper than changing host, which is the next step if they fail.

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