BglII: what to check before ordering, and how restriction enzyme quality is actually specified on a supplier's data sheet

Reading a restriction enzyme data sheet

1. The unit definition is supplier specific. A unit is the amount that digests a stated quantity of a stated substrate in a stated time under stated conditions, and suppliers vary all four. Comparing price per unit between two catalogues without comparing the definitions underneath is how buyers convince themselves one enzyme is cheaper.
2. Buffer and the double digest. Every enzyme has an optimum buffer, and a double digest needs conditions where both work acceptably. Suppliers publish compatibility tables, and where no shared condition exists the answer is sequential digestion with a clean-up between rather than a compromise buffer that halves both activities.
3. Star activity and how to avoid it. Relaxed specificity under the wrong conditions produces cuts at similar but incorrect sites, which is the usual cause of an unexplained extra band. Excess enzyme, extended incubation, high glycerol from a large enzyme volume and the wrong ionic strength are the common triggers, and all four are avoidable.
4. Methylation sensitivity. Site methylation can block cutting entirely, and whether it does depends on the enzyme and the source of the DNA. Check the supplier's methylation table against your plasmid's host strain before concluding that a digest failed for another reason.
5. Quality specifications that matter. Look for stated freedom from contaminating nuclease and from residual endonuclease, and for a ligation and recutting assay result. Those tests are what tell you an enzyme will work in a cloning workflow rather than merely cut on a gel.

High fidelity and time saver formats

Several suppliers offer engineered variants with reduced star activity and formats that digest in a very short incubation. They cost more per unit and are worth it for difficult substrates, for long digests and where an extra band would be expensive to chase.

For routine diagnostic digests on plasmid DNA the standard enzyme is normally sufficient. Buying the premium format for every reaction is a common way to inflate a consumables budget without improving any result.

Storage, handling and shelf life

Restriction enzymes live in glycerol at low temperature and lose activity with repeated warming. Keep them in a cooling block on the bench rather than on ice in a rack where they drift, and return them promptly.

A failing digest that used to work is more often a tired enzyme aliquot than a changed protocol. Keep a fresh reference aliquot and test against it before redesigning anything.

Common questions

What does BglII cut?

It recognises a six-base palindromic site and leaves a four-base overhang compatible with several other enzymes, which is why it appears frequently in cloning strategies. Check the supplier's data sheet for the exact site and compatible overhangs.

Why is my digest incomplete?

Common causes are insufficient units for the DNA quantity, the wrong buffer for a double digest, site methylation in the host strain, or a degraded enzyme aliquot. Check the methylation table and run a fresh aliquot before anything else.

What is star activity?

Cutting at sites similar to but not identical with the recognition sequence, caused by excess enzyme, long incubation, high glycerol or the wrong ionic strength. It presents as unexpected extra bands.

How do I compare enzyme prices between suppliers?

Read each supplier's unit definition first, because the substrate, quantity, time and conditions differ. Price per unit is not comparable until the definitions are.

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