Few or No Transformants After Cloning

Symptom

Bacterial transformation yields few or no colonies after restriction digest-based cloning. Vector may not be linearized or insert ends incompatible.

1. 1 Recognition site blocked by Dam, Dcm, or CpG methylation preventing cleavage
2. 2 Incorrect buffer used reducing enzyme activity to <10%
3. 3 DNA contaminants (salts, proteins, polysaccharides) inhibiting enzyme
4. 4 PCR fragment has <6 nucleotides between recognition site and DNA end, preventing cleavage

1. 1 Check enzyme methylation sensitivity; use dam⁻/dcm⁻ strain (NEB #C2925) for methylation-sensitive enzymes
2. 2 Use recommended NEBuffer supplied with restriction enzyme; verify 100% activity
3. 3 Clean up DNA with spin column (NEB #T1030) to remove inhibitors before digestion
4. 4 Design PCR primers with ≥6 nucleotides flanking the recognition site at DNA ends

Related Video (3)

New England Biolabs ★ 88

Cloning With Restriction Enzymes

"Directly covers guidelines for choosing restriction enzymes in cloning, the exact technique where methylation-blocked sites cause failure."

Addgene ★ 82

Restriction Digest Analysis

"Demonstrates performing a restriction digest and analyzing products on a gel, letting researchers verify whether the vector actually linearized."

New England Biolabs ★ 70

How to Perform a Transformation with NEB Competent Cells

"Shows proper transformation with competent cells, the step whose failure (few/no colonies) is the downstream symptom described."