Choosing a plasmid sequencing service: why whole plasmid sequencing, and a whole plasmid sequencing service bought as such, has replaced primer walking for most constructs, what long reads settle about repeats and inverted regions, how much material and what purity to send, and what the deliverable has to include

Specifying the work

1. Sequence the whole plasmid, not just the insert. Backbone mutations, lost promoter elements and rearrangements from propagation are real and invisible to a primer walk across the insert. Whole plasmid sequencing catches them for a marginal cost, and it is the reason the method has displaced walking.
2. Use long reads where the construct has repeats. Repeated elements, inverted terminal repeats and long homologous stretches cannot be resolved by short reads alone, and an assembly will quietly collapse them. For viral vector backbones and anything with repeats, long reads are the method rather than a refinement.
3. Send the amount and purity they ask for. Providers state a mass and a purity, and a preparation with residual RNA, protein or salt gives a failed run rather than a poor one. Quantify properly rather than relying on an absorbance reading alone.
4. Send your map, and ask for the comparison. The useful deliverable is an assembly compared against your expected sequence with the differences called, not a raw file. A service returning reads without an annotated comparison has handed you the work rather than done it.
5. Verify before anything expensive depends on it. Sequence the construct before a virus is made, before a stable line is generated and before a study starts. The cost of sequencing is trivial against the cost of discovering a construct was wrong three months in.

What to do with the result

Store the verified sequence with the plasmid record and with the glycerol stock, and treat the verified file as the construct's identity. A map drawn in software and never confirmed is a hypothesis about what is in the tube.

Re-verify after anything that could change the molecule: a new preparation from an old stock, a transfer between laboratories, or a construct that starts behaving differently. Recombination during propagation is real.

Sequencing as part of a build

Where constructs are assembled routinely, sequencing every clone before use is cheaper than the experiments wasted on wrong ones. Build the verification into the protocol rather than treating it as an extra step.

For a synthesis service that delivers a construct, the sequence verification should come with it. Ask what was sequenced and how, because a service verifying only the insert leaves the same gap as a primer walk.

Common questions

Is whole plasmid sequencing worth it over primer walking?

Almost always. It verifies the backbone and catches rearrangements that a walk across the insert cannot see, for a marginal additional cost, which is why it has largely replaced walking.

When do I need long reads?

When the construct contains repeats, inverted repeats or long homologous regions, which includes most viral vector backbones. Short read assemblies collapse repeats silently, which is the failure mode you are trying to avoid.

What should the deliverable include?

An assembled sequence, an annotated comparison against your expected map with every difference called, the coverage, and the raw reads. A file of reads with no comparison is not a verification.

How much plasmid should I send?

What the provider specifies, at their stated purity, quantified by a method that measures the DNA rather than everything that absorbs. Residual RNA is the commonest reason a sample reads as sufficient and then fails.

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