Which reagent kit is more stable for extracting low copy BAC plasmids?

The improvement of low copy BAC yield can start from the cultivation conditions. Bacterial strains containing BAC are commonly treated with LB+chloramphenicol, and can be incubated overnight for 12-16 hours. Do not shake until the OD is too high. To increase the yield of low copy plasmids, chloramphenicol can be used to amplify plasmid copy numbers after the bacterial solution reaches a certain density. However, different BAC vectors have different reactions and require small-scale testing first. It is recommended to start with 100 ml of bacterial solution and collect it according to the 'low copy culture volume' of the large extraction kit. Do not use high copy bacterial solution. If one extraction is not enough, two batches can be extracted and eluted together, but the concentration should be measured before merging to avoid introducing impurities.

Operationally, avoiding large plasmid fragmentation is more critical than choosing a kit. Do not vortex, and do not repeatedly pipette with narrow-bore tips. For resuspension, use a cut short wide-bore tip and gently invert. Control lysis time to 3-5 min and do not mix vigorously. After adding the neutralization solution, gently invert and let stand on ice. When passing through the column, use gravity flow as much as possible; do not vacuum or centrifuge at high speed, otherwise fragments above 100 kb are prone to breakage. Elution conditions should also be gentle; prewarming the elution buffer can improve yield, but do not exceed 70 degrees C. After extraction, run pulsed-field electrophoresis or low-voltage agarose to check integrity; ordinary agarose alone may not reveal fragmentation.

BAC low copy of 100 kb or more is not recommended for ordinary small columns, as it has a low yield and is easy to cut. Priority should be given to large plasmid/BAC specific extraction kits, such as Qiagen Large Construct Kit, Lucigen BACMAX, or MN NucleoBond Xtra BAC, which reduce column membrane shear and use gravity drop or wide caliber columns. The yield of low copy BAC from 50-100 ml bacterial solution is usually around 1-5 μ g. It is recommended to conduct preliminary experiments to confirm. Whether the endotoxin removal method is compatible with BAC depends on the column description. Anion exchange type is usually acceptable, but the yield may be lower than ordinary type. It is recommended to extract the untreated endotoxin quality control first, confirm the yield, and then switch to the endotoxin product.

For the endotoxin-free version compatible with BAC, check whether the manual states it is suitable for large plasmids or BAC. Endotoxin removal steps often add salt washing or additional column purification, which generally has little effect on BAC stability, but a higher salt concentration in the buffer may cause large plasmids to bind more tightly to the column, making elution difficult. It is recommended to first extract once with the brand's regular maxiprep and once with the endotoxin-free version, and compare yield and restriction digestion patterns. For restriction digestion identification, use NotI or PI-SceI, etc., to see whether the bands are clear. Before purchasing, ask the distributor for BAC application data. Do not directly make bulk purchases from brands without actual test data; buy a small package for testing first.