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View additional product information for Bac-to-Bac™ Vector Kit - FAQs (10360014)
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庆大霉素浓度可能过高。尝试降低庆大霉素的浓度至5 µg/mL，并在培养基中加入更多菌落。
若出现蓝色菌落，则大肠杆菌含有杆粒和质粒，使细胞能在筛选过程中存活。但是，由于未发生转座，所以LacZ基因未被破坏。靶心菌落表示在菌落生长时发生转座。将从混合克隆白色部分中分离得到的克隆重新划线，应该能够得到发生过转座的菌落。
这是同源性重组较差的典型标志。应检查使用的质粒/线性DNA比例。但是，如果存在一些蓝色空斑，则对那些病毒进行扩增并检测它们的蛋白。根据我们的经验，它们应该是正确的，即使其丰度相对较低。
是的，细胞被单个野生型病毒感染后，会以不同速度生成多角体，直到培养瓶中所有的细胞都被感染。细胞中多角体的形成需要3-4天左右，其大小和数量各不相同，直至达到最大能力并发生细胞破裂，从而将微小的病毒颗粒释放到培养基中。
正常情况下，约5-7天会出现很小的白点，约10天出现1 mm空斑。空斑的大小范围为1 mm至4 mm。
在打算挑选空斑的那一天，制备含Bluo-gal的DMSO溶液，浓度为20 mg/mL。在每个板中加入50 µL，使用玻璃涂布器在无菌条件下涂布。等待30-60分钟，空斑应变为蓝色。
有几种原因可导致培养板变蓝：
•病毒接种量过多。可尝试使用更高的稀释度。
•加入热的融化琼脂糖时，将细胞烫死了。过热的琼脂糖会使细胞裂解，将lacZ释放到琼脂糖中，使其变蓝。应复查铺板温度。如果培养板过湿，蓝色会扩散。
琼脂糖覆层过热。加入琼脂糖覆层后，细胞应仍然保持圆形和健康状态。
是的，这表明空斑上存在吸液问题。琼脂糖覆层“漂浮”，是因为板中的培养基未被全部吸走。将琼脂糖覆盖到细胞上之前，培养板应完全干燥，特别是在将要挑选空斑时。为达到这样的效果，我们通常会将培养板稍微倾斜，用Pasteur移液管沿培养板边缘吸一圈，同时小心不要破坏细胞单层。如果培养板边缘有任何培养基残留（小液滴），则继续吸液。琼脂糖覆层“漂浮”，也可能导致野生型病毒污染。野生型病毒可以迁移到培养板的其他部分，从而污染重组型空斑。野生型病毒比重组型病毒复制得快的多，可迅速超过重组型病毒。
细胞接种过多；我们建议在6孔板的每孔中接种8 x 105个细胞。
通常使用的MOI在5-10之间。如果加入过多病毒，会导致细胞过早死亡，并使蛋白表达水平降低。
感染动力学可能比预期要慢。继续观察培养板，直至感染后第8-9天。如果未出现空斑，则检查下面各项：
•如果细胞不健康，会产生较差的空斑或无空斑产生。理想状态下，细胞应处于对数生长中期，并且存活率大于90%。在感染导致细胞停止生长前，细胞应至少倍增1次。应确保细胞接种量正确，且汇合度约为70%。
•细胞营养不良和健康状态较差，都会抑制病毒复制周期。
•琼脂糖温度也很关键。在琼脂糖覆层后，应将培养板静置1小时，使琼脂糖完全凝固。
•在27°C孵育时，过度冷凝会抑制空斑形成——一旦发生冷凝，就应移除纸巾或打开装有培养板的容器。
•病毒滴度过低：使用较高的病毒滴度。您可能需要再次感染细胞，并收集更高滴度的病毒储液。
如果该低滴度储液为P1或P2储液，则可用于病毒扩增实验方案。如果低滴度储液曾经是高滴度储液，但随着时间或繁殖多次而发生滴度降低，则需要重新制备高滴度储液。如果高滴度储液代数大于P5，则可能有过多的缺陷型干扰颗粒感染细胞，而不能进行适当复制或生成蛋白。如果将已有储液接种后，重新分离得到了新的空斑（DIP不会形成空斑），则该储液可用于制备高滴度储液。
可以，用于制备P2病毒储液的实验方案也可用于制备P3、P4或P5病毒储液。我们不建议制备代数高于P5的储液，因为这会产生更多的缺陷型感染颗粒，并会降低蛋白表达水平。
可以，杆状病毒可以感染哺乳细胞，但是需要非常高的滴度。杆状病毒在肝细胞中的感染能力最强。但是，只有在使用高滴度储液直接感染细胞时，才有交叉感染风险。
杆状病毒可以感染果蝇细胞，但不能在果蝇细胞中复制。普通杆状病毒系统中驱动目的基因表达的启动子都是晚期启动子，并且需要来自杆状病毒基因组的早期蛋白。因此，它们在S2细胞中无效，因为无法生成早期蛋白。
通常，每孔加入含0.5 x 106个细胞的培养基2.5–3 mL，是一个良好的开端。第3天开始发生裂解。在第3-7天（90%细胞死亡），可收获和扩增病毒。
我们的研发团队通常会将病毒加到含0.5 x 106细胞/孔的12孔培养板中，每孔总体积为2.5 mL。大约3天后，取出0.75 mL培养液用于制备PCR用DNA，将剩余培养基收集至Eppendorf管中，作为P1病毒储液。此外，也可以挑选一个空斑，保存在Grace’s培养基中。
我们建议在细胞裂解率达到90%时，收获高滴度病毒。大约需要5-7天。如果延长细胞感染时间，则裂解细胞释放的蛋白酶会降解病毒表面蛋白，导致传染性病毒量降低。
这取决于病毒的加入量。如果感染细胞的MOI为5，则细胞通常在24小时被感染，细胞在65小时左右开始裂解。病毒使用量越少，则所需时间越长，反之亦然。
繁殖病毒储液时，为避免缺陷性干扰颗粒（DIP）的影响，应使用低MOI（0.03–0.1）。低MOI可确保每个细胞的病毒颗粒感染量低于1，防止DIP扩增。当细胞存活率为15%时，是合适的病毒收获时间。
注意：DIP是接近正常的病毒衣壳，含缺陷性基因组，无法进行成功的复制。尽管该“颗粒”本身无感染性，但当它与正常病毒颗粒或一些其他类型的DI颗粒共感染时，可以进行复制。
您可使用中性红或MTT对单层细胞进行染色，使空斑更明显。或者，您可延长室温下的空斑形成时间（平均2-5天），增强重组空斑的对比度。但是，中性红空斑染色法不适用于空斑纯化和病毒扩增。
用于表达研究时，我们建议使用滴度为>1 x 108 pfu/mL的病毒储液。
请参见以下公式：
pfu/mL =空斑数量(pfu)/稀释系数x接种体积(mL)
例如，如果病毒稀释度为10-8的孔中含有18个白色空斑，则病毒滴度计算为：
X pfu/mL = 18 pfu/10-8 x 1 mL
X = 1.8 x 109 pfu/mL
以下是空斑实验法的主要步骤：
•在6孔板中接种细胞，并使细胞到达80%融合
•对P1病毒储液（1–10-5）进行连续稀释，并加到细胞中
•在27°C孵育1小时
•在培养基中混合1%融化的琼脂糖
•移除病毒上清液
•在细胞上覆盖含琼脂糖的培养基
•将培养板静置2-3小时，使琼脂糖完全凝固
•培养10-14天
•计算空斑数量
进行该实验时，我们建议：
•细胞应处于非常健康的状态，代数较低（10-20代），处于对数生长期，并且存活率高于95%
•确认病毒储液是无菌的（无污染）
•使用高质量、低熔点的琼脂糖
•琼脂糖培养基的温度非常重要——太热，则细胞会死亡；太冷，则琼脂糖凝固太快
•覆盖琼脂糖培养基后等待2-4小时，使琼脂糖达到100%凝固
•在稀释的板中，空斑数量按下述公式计算：
（1/稀释度）x 空斑数量 = pfu/mL
例如，如果在稀释度为10-6的板中形成50个空斑，则为1/(10-6) x 50 = 5 x 107 pfu/mL
我们建议您采用空斑实验法检测病毒储液的滴度。如果需要，您也可以利用空斑实验纯化单个病毒克隆。
尽管Kozak序列在哺乳细胞翻译起始中的重要性已被证明，但昆虫细胞中是否也严格遵循Kozak规则仍有争议。确定其重要性的唯一方法是，对相同蛋白从不同起始序列的表达进行直接对比。即使这样，一种蛋白实现最佳表达所使用的规则可能并不适合其它蛋白。以下四篇文献证明了Kozak序列对昆虫细胞中的表达效率无任何影响：
•Hills D, Crane-Robinson C (1995) Baculovirus expression of human basic fibroblast growth factor from a synthetic gene: role of the Kozak consensus and comparison with bacterial expression. Biochim Biophys Acta 1260(1):14–20.
•Ranjan A, Hasnain SE (1995) Influence of codon usage and translational initiation codon context in the AcNPV-based expression system: computer analysis using homologous and heterologous genes. Virus Genes 9(2):149–153.
可以。多种五亚基蛋白，如人类复制因子C，已使用重组杆状病毒进行表达。为实现最佳的多亚基蛋白表达，我们建议生产出每个亚基的、独立的高滴度储液（HTS）。利用这种方法，可通过改变每个亚基HTS的感染复数，控制每个亚基的表达量。请参考以下文献，获取更多信息：
•Chen W and Bhal OP (1991) Recombinant carbohydrate and selnomethionyl variants of human choriogonadotropin. J Biol Chem 266(13):8192–8197.
•Chen WY and Bhal OP (1991) Selenomethionyl analog of recombinant human choriogonadotropin. J Biol Chem 266(15):9355–9358.
•Fabian JR, Kimball SR, Jefferson LS (1998) Reconstitution and purification of eukaryotic initiation factor 2B (eIF2B) expressed in Sf21 insect cells. Protein Expr Purif 13(1):16–22.
MOI或感染复数，是指某个实验中，感染单个细胞的平均病毒颗粒数量。您可使用以下公式计算MOI：
MOI (空斑形成单位(pfu)/细胞) = [滴度(pfu) x 接种物所用病毒储液体积(mL)] / [细胞密度(细胞/mL) x 培养物体积(mL)]
可以，可以进行大规模表达实验。下表描述了不同大规模实验方法、要求、优势和参考文献：
- Stirred bioreactor
- Airlift fermentor
- Insect larvae
如果培养基是无血清的，可加入血清至浓度为10%。血清蛋白可作为蛋白酶的底物，防止病毒外壳蛋白发生降解。将病毒储液置于4°C避光保存。分装储液可保存在–80°C，使用前应检测病毒滴度，因为冻融循环会导致病毒滴度降低10-100倍。
需要。重组DNA存在未切割（occ+）DNA污染，将导致重组病毒随时间而逐渐稀释，因为未切割（野生型，occ+）病毒的感染和复制效率高于重组病毒。同时，使用纯化的、单一病毒群体进行起始表达，将确保得到可重复的结果。
请查看以下关于病毒感染不同阶段的描述：
早期
细胞直径增加——细胞直径可能增加25–50%。
细胞核体积增加——细胞核可能“填满”细胞。
末期
细胞生长停止——与单纯的细胞对照相比，细胞停止生长。
颗粒状外观
病毒出芽迹象——细胞出现泡状外观
病毒包涵体——少量细胞会含有包涵体，表现为昆虫细胞核内出现折射晶体。
脱落——细胞从培养皿或培养瓶上脱落。
晚期的细胞裂解——少量细胞可能会充满包涵体病毒、发生死亡和裂解，留下单层清理迹象。
贴壁Sf9细胞呈聚集状，接触点较小。感染后Sf9细胞聚集悬浮于培养液中，细胞体积变大。
请遵循以下建议：
•细胞应处于非常健康的状态，代数较低（5-15代），处于对数生长期，并且存活率高于95%。
•DNA必须高度纯化，无内毒素
•转染时，不可使用抗生素
•Cellfectin试剂应完全重悬
•设置对照组（培养基对照、DNA对照和转染试剂对照），用于对比和问题排查
昆虫细胞中的蛋白表达高峰，取决于感染复数（MOI）、感染时间和目标蛋白。系统优化包括使用MOI为5-10，表达时间为48-72小时。72小时后表达的蛋白可能是经过异常加工的，因为较高的病毒载量可引起细胞进程故障。
是的，杆状病毒是表达毒性蛋白（即，膜蛋白）的良好候选方案。多角体启动子在感染后18-24小时才能表达至最高水平。多角体启动子在裂解晚期激活。也就是说，其在早期8小时的时候活性极低，所以，如果目的基因毒性很强，可能会造成问题。该问题的解决方法是转换到诱导型表达系统。跨膜蛋白通常难以在任何系统中表达。
ATG通常对于高效的翻译启始是足够的，尽管翻译效率要视目的基因而定。最佳的建议应是保持cDNA中天然起始位点，除非确定这一位点的功能性不理想。如果从表达的角度来考虑，推荐构建并测试两种载体，一个具有天然的起始位点，另一个具有保守的Kozak序列。通常情况下，所有N-端融合型表达载体都已包含了一个RBS或翻译起始位点。
原核生物mRNA含有Shine-Dalgarno序列，也称为核糖体结合位点(RBS)，它是由AUG起始密码子5’端的多嘌呤序列AGGAGG组成。该序列与16S rRNA 3’端的互补，有助于mRNA有效结合到核糖体上。同理，真核生物（特别是哺乳动物）mRNA也含有完成有效翻译所需的重要序列信息。然而，Kozak序列不是真正的核糖体结合位点，而是一种翻译起始增强子。Kozak共有序列是ACCAUGG，其中AUG是起始密码子。-3位的嘌呤(A/G)具有重要作用；若-3位是一个嘧啶(C/T)，翻译过程会对-1、-2和+4位的改变更敏感。当-3位从嘌呤变为嘧啶时，可使表达水平降低多达95%。+4位对表达水平的影响相对较小，可以使表达水平降低约50%。
注：果蝇的最佳Kozak序列稍有不同，酵母完全不遵循这些规则。见下列参考文献：
•Foreign Gene Expression in Yeast: a Review. Yeast, vol. 8, p. 423-488 (1992).
•Caveneer, Nucleic Acids Research, vol. 15, no. 4, p. 1353-1361 (1987).
The concentration of gentamicin might be too high. Try lowering the amount to 5 µg/mL and try adding more of the colony to the culture medium.
In the case of a blue colony, the E. coli has the bacmid and the plasmid in it, allowing the cells to survive the selection process. However, because the transposition has not occurred, the LacZ gene is not disrupted. For bulls-eye colonies, this indicates that the transposition took place when the colony was growing. Re-streaking for an isolated clone from the white portion of the mixed colony should yield some colonies where transposition occurred.
This is typically an indication of poor homologous recombination. Check the plasmid/linear DNA ratio you used. If there are some blue plaques, however, expand those viruses and check for their protein. In our experience, they are correct, even if they were in relatively low abundance.
Yes, cells are infected with wild-type virus individually and will develop polyhedra at different rates until all the cells in the flask are infected. The polyhedra in cells will form in approximately 3-4 days, differing in size and number until they reach their maximum capacity and burst the cell, releasing tiny particles of virus into the medium.
Normally, very small white dots show up about 5-7 days and 1 mm plaques show up around day 10. Plaques can vary in size from 1 mm to 4 mm.
On the day you intend to pick plaques, make a solution of Bluo-gal in DMSO at 20 mg/mL. Add 50 µL per plate and spread with a glass spreader under sterile conditions. Wait 30-60 min, and your plaques should turn blue.
There are a few things that can turn plates blue:
- Too much virus when plating. Try a higher dilution.
- Cells are being singed when plated with hot melted agarose. This lyses the cells and releases lacZ into the agarose, turning it blue. Double-check plating temperatures. If plates are too wet, the blue can diffuse.
The agarose overlay was too hot. After addition of the agarose overlay, cells should still be round and healthy.
Yes, this is indicative of an aspirating problem on the plaques. The agarose overlays were “floating” because the medium was not completely aspirated from the plates. The plates need to be completely dry before the agarose is placed over the cells, especially when plaques will be picked. To do this, we typically tip the plate slightly and keep going around the rim of the plate with the Pasteur pipette tip, being careful not to disturb the cell monolayer. If any medium pooling at the rims of the plates (they will be small pools) is seen, continue to aspirate. This “floating” agarose overlay problem may also result in wild-type contamination. The wild-type virus is able to migrate to other portions of the plates and contaminate recombinant plaques. Wild-type virus replicates much faster than recombinant virus, and can quickly overwhelm the recombinant virus.
Too many cells were seeded; we recommend seeding 8 x 10e5 cells per well for a 6-well plate.
An MOI of 5-10 is typically used. If too much virus is added, unfortunately the cells die too soon and the protein expression level goes down.
The kinetics of infection may be slower than expected. Observe plates until the 8-9th day after infection. If no plaques appear, investigate the following:
- If the cells are not healthy, then poor-quality or no plaques can result. Ideally, cells should be in mid-log phase and have a viability of greater than 90%. Cells should double at least once before infection stops growth. Ensure that the correct amount of cells was used at ~70% confluency.
- The viral replication cycle can be inhibited due to poor nutritional and physical conditions of the cell.
- The temperature of the agarose is also crucial. After overlaying the agarose, the plates should be left untouched for 1 hour for the agarose to completely solidify.
- Excessive condensation during incubation at 27 degrees C can inhibit plaque formation-remove paper towels or open the container containing plates as soon as condensation appears.
- The viral titer is too low: Use a higher viral titer. You may need to re-infect your cells and collect a higher titer of your viral stock.
If this lower-titer stock is a P1 or P2 stock, a viral amplification protocol can be used. If the low-titer stock was once a high-titer stock, but has dropped titer due to age or the stock was propagated many generations, then it may be necessary to regenerate the high-titer stock. If the high titer stock is >P5, then there may be an excessive amount of defective interfering particles that infect cells but do not properly replicate or produce protein. If the existing stock is plated out and a fresh plaque is re-isolated (DIPs do not form plaques), a new high-titer stock can be established.
Yes, the same protocol used to make your P2 viral stock can be used to make a P3, P4, or P5 viral stock. We don't recommend making the stock higher than P5, as more defective interfering particles will be produced and a decrease in protein expression level will occur.
Yes, baculovirus can infect mammalian cells, although only at very high titers. Baculovirus works best in liver cells. However, there is no danger of cross-contamination unless the cells are directly infected with the high-titer stocks. Bacuolvirus can infect Drosophila cells; however, it will not replicate in these cells. The promoters used to drive expression of your gene in a typical baculovirus system are both late promoters and require earlier proteins from the baculovirus genome. Thus, they will not work in S2 cells since the early proteins are not made.
Typically, 0.5 x 106 cells per well in 2.5-3 mL is a good starting point. Lysis should begin by day 3. Virus may be harvested and amplified between 3 and 7 days (90% cell death).
Our R&D team will typically pick a plug and add it to a 12-well dish with 0.5 x 10e6 cells/well and 2.5 mL total volume per well. After approximately 3 days, remove 0.75 mL to make DNA for PCR and keep the remaining medium in an Eppendorf tube as your P1 viral stock. As an aside, it is okay to pick a plaque and store it in Grace's medium.
We recommend harvesting high-titer virus when there is 90% cell lysis. This takes approximately 5-7 days. If the cells go longer, the proteases released from the lysed cells will start to degrade viral surface proteins and result in less infectious virus.
This is dependent on how much virus is added. If cells are infected at an MOI of 5, usually cells are infected at 24 hours, and cells begin to lyse at around 65 hours. If less virus is used, this takes longer, and more virus takes less time.
When propagating virus stock, use a low MOI (0.03-0.1) in order to avoid effects of defective interfering particles (DIPs). A low MOI, which ensures no more than 1 virion per cell, prevents the amplification of DIPs. A harvest time based on 15% cell viability is appropriate. NOTE: DIPs are nearly normal virus capsids containing genomes that are defective and are unable to undergo successful replication. While this "particle" is not infectious by itself, it can replicate when co-infected with normal virion, or with some other types of DI particles.
You can stain the monolayer with neutral red or MTT to make the plaques more visible. Alternatively, you can allow the plates to develop for a few days longer (2-5 days on average) at room temperature to increase the contrast in recombinant plaques. However, the plaques stained with neutral red cannot be used for plaque purification and viral amplification.
We suggest using a viral stock with a titer of >1 x 10e8 pfu/mL for expression studies.
Please see the equation below:
pfu/mL = number of plaques (pfu)/dilution factor x mL of inocula
So, if you have a well with viral dilution of 10-8 containing 18 white plaques, the viral titer is calculated as followed:
X pfu/mL = 18 pfu/10-8 x 1 mL
X = 1.8 x 10e9 pfu/mL
Please see the method below for an outline of the main steps of performing a plaque assay:
- Plate cells at 80% confluency in a 6-well plate
- Make a serial dilution of the P1 viral stock (1-10-5) and add to cells
- Incubate for an hour at 27 degrees C
- Mix 1% melted agarose into the medium
- Remove the viral supernatant
- Overlay the cells with the medium containing agarose
- Leave the plates for 2-3 hours for agar to completely solidify
- Incubate plates for 10-14 days
- Count plaques
When performing this assay, we suggest:
- Use cells that are in excellent health, of low passage (10-20) in log-phase growth, and high viability (>95%)
- Check viral stock for sterility (free of contamination)
- Use high-quality, low melting point agarose
- The temperature of the medium with agarose is crucial-too hot, cells will die; but if too cold, it will solidify too quickly
- Wait 2-4 hours before removing the plate after overlay so that the agarose can 100% solidify
- Count plaques on a dilution plate where (1/dilution) x # of plaques = pfu/mL
e.g., if you have 50 plaques on the 10-6 plate, then you have 1(10-6) x 50 = 5 x 10e7 pfu/mL
We recommend you perform a plaque assay to determine the titer of your viral stock. You may also perform a plaque assay to purify a single viral clone, if desired.
While the importance of a Kozak consensus sequence in translation initiation has been demonstrated in mammalian cells, there seems to be some debate as to whether the Kozak rules are as stringent in insect cells. The only way to determine its importance would be a direct comparison of expression of the same protein from different initiation sequences. Even then, the rules for optimal expression of one protein may not hold for another. Here are two references which indicate that a Kozak consensus sequence does not have any effect on efficiency of expression in insect cells:
- Hills D, Crane-Robinson C (1995) Baculovirus expression of human basic fibroblast growth factor from a synthetic gene: role of the Kozak consensus and comparison with bacterial expression.
- Biochim Biophys Acta 1260(1):14-20.
- Ranjan A, Hasnain SE (1995) Influence of codon usage and translational initiation codon context in the AcNPV-based expression system: computer analysis using homologous and heterologous genes. Virus Genes 9(2):149-153.
Yes, it is possible. Several five-subunit proteins, such as human replication factor C, have been expressed using recombinant baculovirus. We recommend that a separate high-titer stock (HTS) of each subunit be produced to optimally express the multi-subunit protein. This way, the amount of each subunit expressed can be controlled by varying the multiplicity of infection (MOI) of each subunit's HTS. Please refer to the following articles for more information:
- Chen W and Bhal OP (1991) Recombinant carbohydrate and selnomethionyl variants of human choriogonadotropin. J Biol Chem 266(13):8192-8197.
- Chen WY and Bhal OP (1991) Selenomethionyl analog of recombinant human choriogonadotropin. J Biol Chem 266(15):9355-9358.
- Fabian JR, Kimball SR, Jefferson LS (1998) Reconstitution and purification of eukaryotic initiation factor 2B (eIF2B) expressed in Sf21 insect cells. Protein Expr Purif 13(1):16-22.
The MOI, or multiplicity of infection, is the average number of viral particles that infect a single cell in a specific experiment. You can calculate the MOI with the following equation:
MOI (pfu/cell) = [titer (pfu) x viral stock volume (mL) used in inocula] / [cell density (cells/mL) x culture volume (mL)]
Yes, large-scale expression experiments can be performed. Please see below for different large-scale methods, requirements, added benefits, and references:
- Stirred bioreactor
- Airlift fermentor
- Insect larvae
If the medium is serum-free, add serum to 10%. Serum proteins act as substrates for proteases and therefore prevent degradation of viral coat proteins. Store viral stocks at 4 degrees C, and protect from light. Aliquots can be stored at -80 degrees C, but viral titer should be checked before use, as freeze/thaw cycles of the virus can result in a 10- to 100-fold decrease in viral titer.
Yes. Contamination of your recombinant DNA with uncut occlusion body positive (occ+) DNA will lead to dilution of your recombinant virus over time because, in general, uncut (wild-type, occ+) virus infects and replicates at higher efficiency than recombinant virus. Also, initiating expression studies with a pure, single virus population will ensure reproducible results.
Please see the description below of the different stages of viral infection:
Early
- Increased cell diameter-a 25-50% increase in the diameter of the cells may be observed.
- Increased size of cell nuclei-the nuclei may appear to "fill" the cells.
Late
- Cessation of cell growth-cells appear to stop growing when compared to a cell-only control.
- Granular appearance
- Signs of viral budding-vesicular appearance of cells.
- Viral occlusions-few cells will contain occlusion bodies, which appear as refractive crystals in the nucleus of the insect cell.
- Detachment-cells release from the dish or flask.
Very late
- Cell lysis-a few cells may fill with occluded virus, die, and burst, leaving signs of clearing in the monolayer.
Adherent Sf9 cells round up and show a smaller contact point. Infected Sf9 cells in suspension culture round up and look larger when infected.
Please follow the recommendations below:
- Cells should be in excellent health, of their low passages (5-15), in log-phase growth, with viability >95%
- DNA must be of high purity, free of endotoxin
- No antibiotics should be used during transfection
- Cellfectin reagent has to be completely resuspended
- Include controls (media control, DNA control, and transfection reagent control) for comparison and troubleshooting
Peak expression of protein in insect cells is dependent on the multiplicity of infection (MOI), expression time, and the protein being expressed. Guidelines to optimize your system include using an MOI of 5-10 and an expression time of 48-72 hours. Protein expressed at times later than 72 hours may be processed aberrantly, because the large virus load can cause a breakdown of cellular processes.
Yes, baculovirus is a good candidate for the problem of expressing toxic proteins (i.e., membrane proteins). The polyhedron promoter does not express at maximal levels until 18-24 hr after infection. The polyhedron promoter is active late in the lytic cycle. That being said, it is minimally active as early as 8 hours, so if the gene is very toxic, there may be a problem. The solution in that case would be to switch to an inducible expression system. Transmembrane proteins can often be difficult to express in any system.
ATG is often sufficient for efficient translation initiation although it depends upon the gene of interest. The best advice is to keep the native start site found in the cDNA unless one knows that it is not functionally ideal. If concerned about expression, it is advisable to test two constructs, one with the native start site and the other with a Shine Dalgarno sequence/RBS or consensus Kozak sequence (ACCAUGG), as the case may be. In general, all expression vectors that have an N-terminal fusion will already have a RBS or initiation site for translation.
Find additional tips, troubleshooting help, and resources within our Protein Expression Support Center.
Prokaryotic mRNAs contain a Shine-Dalgarno sequence, also known as a ribosome binding site (RBS), which is composed of the polypurine sequence AGGAGG located just 5’ of the AUG initiation codon. This sequence allows the message to bind efficiently to the ribosome due to its complementarity with the 3’-end of the 16S rRNA. Similarly, eukaryotic (and specifically mammalian) mRNA also contains sequence information important for efficient translation. However, this sequence, termed a Kozak sequence, is not a true ribosome binding site, but rather a translation initiation enhancer. The Kozak consensus sequence is ACCAUGG, where AUG is the initiation codon. A purine (A/G) in position -3 has a dominant effect; with a pyrimidine (C/T) in position -3, translation becomes more sensitive to changes in positions -1, -2, and +4. Expression levels can be reduced up to 95% when the -3 position is changed from a purine to pyrimidine. The +4 position has less influence on expression levels where approximately 50% reduction is seen. See the following references:
- Kozak, M. (1986) Point mutations define a sequence flanking the AUG initiator codon that modulates translation by eukaryotic ribosomes. Cell 44, 283-292.
- Kozak, M. (1987) At least six nucleotides preceding the AUG initiator codon enhance translation in mammalian cells. J. Mol. Biol. 196, 947-950.
- Kozak, M. (1987) An analysis of 5´-noncoding sequences from 699 vertebrate messenger RNAs. Nucleic Acids Res. 15, 8125-8148.
- Kozak, M. (1989) The scanning model for translation: An update. J. Cell Biol. 108, 229-241.
- Kozak, M. (1990) Evaluation of the fidelity of initiation of translation in reticulocyte lysates from commercial sources. Nucleic Acids Res. 18, 2828.
Note: The optimal Kozak sequence for Drosophila differs slightly, and yeast do not follow this rule at all. See the following references:
- Romanos, M.A., Scorer, C.A., Clare, J.J. (1992) Foreign gene expression in yeast: a review. Yeast 8, 423-488.
- Cavaneer, D.R. (1987) Comparison of the consensus sequence flanking translational start sites in Drosophila and vertebrates. Nucleic Acids Res. 15, 1353-1361.
Find additional tips, troubleshooting help, and resources within our Protein Expression Support Center.
You will get the antisense strand with pProEXHT.
You will get the + or sense strand with pFastBac I of pFastBac HT.