Making primers

We often hear that oligos (primers) are a simple product, and they should work every single time. From a producer’s point of view, oligos are arguably the most complicated products among common molecular biology reagents, at least in terms of the number of chemical steps required. DNA synthesis starts from the 3’ end to the 5’ end (opposite to DNA polymerases) on a solid support (e.g. CPG beads). For the addition of each base, the process begins by removing a protection group via “Deblocking”; then activating the last base for coupling with an “Activator”, adding the current nucleotide in the chemical form of a phosphoramidite (4 times in our protocol), blocking un-reacted openings with “Capping A and B solutions” (again 4 times each), forming bonds between bases by oxidization with Iodine, then looping all the way back to the beginning of the cycle. Many of these chemicals are either highly sensitive to moisture or have a short shelf life (can go bad any time).

Coming back from a seminar, sitting at the lab desk, you know you have a new idea and some cloning to do, and of course, it must be done tomorrow. The first thing you do is to send in some oligo sequences online to a local synthesis company late in the afternoon after looking at some maps and sequences. Around noon the next day, the oligos will be delivered in person to your hands. Most times everything will just work out fine as far as experiments involving primers are concerned; others you get stuck here and there along the way of cloning. Chances are you have run into problems with primers not giving PCR signals or clones with mistakes in the primer regions at some points in your research career. Even if this has only happened a few times, the memory, as well as the dissatisfaction and anger, can last for quite some time.

Between ordering and delivering, oligos are made overnight; they are then post-synthesis processed (requiring several hours starting in the early morning), OD’ed, and concentration adjusted. Given that the machine completes the run without any problems (power or computer related), and none of the chemicals run out, the best quick indicator of a good run is a color change from the protection group removed by Deblock at the last base. If there is visible amount of blocking group at the end of the synthesis, as reflected by an orange color from a Trityl group, it is likely that the synthesis was efficient till the end. Unfortunately, the efficiency of adding each base is never 100%–accumulations of missing or, at a lower percentage, mistaken bases will add up, especially in long oligos. Purification will remove some of the oligos with deletions, but not all of the bad oligos. MASS analysis will help determine the approximate percentage of bad oligos, but it will require time and cost not typically chosen by customers. It is our hope that understanding how oligos are made will help with more effective use of oligos when you order oligos, conduct experiments using oligos, or clone with oligos.

New Product of the Week: Phosphate-3′-CPG for oligo synthesis, email oligo@allelebiotech.com for details.

Promotion of the week: Promotion of the week: 10% off on 4-in-1 lentiviral particles for iPSCs generation. Email oligo@allelebiotech.com with promocode: VIRUS, or using online purchase.

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Thursday, April 21st, 2011 oligos and cloning No Comments

3 Ways of Making DNA Libraries through Oligo Synthesis

Pools of DNA molecules of related but non-identical sequences are often used for selecting cDNAs that encode polypeptides with desired functions (such as in antibody screening), or DNA segments as protein binding sites (through SELEX), or DNA molecules that can catalyze reactions (DNA enzymes or deoxyribozymes), etc. The most direct way of creating DNA libraries is to introduce mixed bases during the synthesis of the oligos that will be used in creating the libraries.

1) The most commonly used method of generating degenerate oligos is to use mixed phosphoramidites (aka amidites, the building blocks of oligo synthesis) at desired positions in an oligo, e.g. using “N” to incorporate dA, dC, dG, and dT nucleotides, or “Y” for pyrimidines, “R” for purines. Mixed base oligos from most oligo suppliers are simple to order (and at no extra charge from Allele and a few other sources). During automated chemical synthesis of oligos, the synthesizer consecutively adds dT, dA, dC, or dG in the case of “N” at a pre-set ratio (e.g.25% each). This procedure does not always result in expected usage of each amidite because different amidites have different coupling efficiency, and the order of addition may also bias against amidites that are added later.

2) Using mixed bases like in method 1) leaves little control to achieve ratios of codons for specific amino acids. On the other hand, by using trimer amidites, which can be used for adding 3 nucleotides in each synthesis cycle, one can create oligos encoding selected amino acids at pre-determined percentages. However, this procedure is difficult to perform because trimer amidites are bulky and hard to couple to the elongating oligo; any moisture present during synthesis would have even more severe adverse effects than with regular amidites. Trimer oligo synthesis projects cost several thousand dollars per oligo on materials alone, and the risk is quite high that the oligos would not turn out of desired properties and qualities. For commercial users, this process has another problem—it is patented.

3) Another method for making library oligos is the so called “split-and-pool”, which is particularly suitable for having diversified amino acids embedded in otherwise common sequences like the CDRs within antibody variable regions. The latest oligo we made last month was a ~72 nt oligo with 8 locations that have pre-determined composition of amino acids, i.e. 20% Ala, 10% Gly, 12% His, etc. The procedure took us about 8 hours and we estimated the cost to be about $1,000. The subsequent sequencing results confirmed that ~70% of the clones using this oligo have desired degeneracy, compared to a similar oligo made by a bigger oligo company, at only 40%. In addition, we did not see any stop codon interruptions or major abnormalities.

DNA pools can also be generated by error-prone PCR, or more specifically with overlapping PCR using degenerate primers. The bottleneck for a library screening is how to handle big enough a number of colonies to accommodate the population, e.g. 10e10, or at least 10e8 clones are needed for finding high affinity antibodies. The second critical point is to have a robust and consistent selection readout such as fluorescence in cell sorting.

New Product of the Week 090710-091310; loxP-mWasabi reporter T cells, email vivec@allelebiotech.com for details.

Promotion of the Week 090710-091310: 15% off our NEW purified fluorescent proteins (not plasmids); All Expressed from E.coli PROMOCODE: 090910FP

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Thursday, September 9th, 2010 oligos and cloning No Comments

Competition from the Marketplace to the Courtroom

The hottest subject in the biological research equipment field has to be whole genome sequencing; hence it is no surprise that companies execute mergers and acquisitions in order to position themselves to go after their competitors in an attempt to corner this valuable market.

A bit of the background history: Illumina was started a decade ago to build DNA chip arrays by people with experience at Affymetrix, when the latter was the first and absolute leader in the DNA chip field. For years, rather than providing DNA chips, Illumina was known for generating revenue by selling oligonucleotides at 20% of the prevailing market price, essentially starting the low end oligo market. Just three or four years ago, it was a front page promotion on Invitrogen’s website to sell Illumina’s oligos through a production/shipping alliance, a cooperation previously unheard of in our field for such low price, non-commodity products. This move quite probably contributed to the decisions made by the more dedicated oligo company, IDT, to acquire local oligo production houses and move to the West coast (Allele opted out of such an acquisition and later did one of its own by taking over Orbigen and since moved into the viral systems and antibody fields). At that point when whole genome sequencing technologies were becoming mature and marketable, Illumina had performed brilliantly in out competing the previously dominant chip supplier Affymetrix, acquired Solexa, and quickly moved into the whole genome sequencing with Genome Analyzer and Genome Analyzer II, a move Affi’s management probably regretted not making.

In the years roughly around 2005-2007, Applied Biosystems, Inc. (ABI) was developing its own genome analysis equipment, the SOLiD system. It surely had a solid base to build on from its strong leadership in providing sequencer and analyzers for many years. Earlier in the year Invitrogen and ABI merged to form Life Technologies, pitching Invitrogen (now LifeTech) and Illumina in a collision course in battle for dominance in genomic analysis. In September, LifeTech brought suit against Illumina for patent infringement; in October Illumina countered with suits of its own. While the fight in court may be long and only sprinkled with occasional fireworks, the competition in the market could be fierce and should ultimately decide on whose technology is superior and offered at better prices. From the technical presentation made by sales teams to us during on site seminars, Solexa’s science sounded better. I was sitting next to Jay Flatley, CEO of Illumina at a San Diego biotech CEO dinner, and heard him predicting that the technology would advance and in a few years, one could get their own genome sequenced for about a thousand dollars, ~10% of the current cost! That’s simply innovation and competition at work. But watch out, a new wave of sequencing technologies based on single molecule capture might make the Illumina and LifeTech courtroom argument a moot point in the market.

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Friday, October 16th, 2009 State of Research 6 Comments

HPLC Purified siRNA with Known RNAi Effects at $149/12.5nmol

RNA oligo is significantly more difficult to synthesize than DNA oligos, mainly because the efficiency of coupling each new ribonucleotide during RNA synthesis is a few fold lower than deoxyribonucleotide during DNA synthesis. Typically, there is an ~10% chance a DNA oligo of 21 bases will have a mutation (most frequently a deletion mutation); for an RNA oligo of 21 bases, as in an siRNA pair, such chance is much higher. Furthermore, after combining the sense and antisense siRNA strands, some RNA molecules will remain as single-stranded thereby not fitting for the RNAi apparatus.

RNA interference is a dose-sensitive process — specificity of gene silencing is meaningful only relative to the active concentration of siRNA used. When the concentration is too low, even the most effective siRNAs would fail to cause gene expression knockdown; when too high, non-specific effects will be duly observed. Therefore, it is essential that the concentrations of siRNAs are measured correctly. When doing so, one must consider not only what the apparent concentrations are by OD260 reading, but also whether the RNA strands are of full-length and whether only dsRNA molecules are counted. This issue might not affect data interpretation if appropriate controls are included in one set of RNAi experiments, but it could have significant influence on conclusions if data from different experiment sets or labs are compared or combined.

HPLC purification currently provides the best means to remove RNA molecules with deletions or remain single-stranded, however, the price tag added by most reagent providers for such treatment has been prohibiting because manufacturers either need to start synthesis at a much bigger scale to obtain promised amount, or they do not promise the delivery quantity at all. The phosphoramidites (oligo building blocks) for RNA synthesis can be 10 times or more expensive than for DNA. Some companies offer alternative purification methods such as a cartridge type device, but they can only remove salt and small impurities, not RNA oligos of shorter lengths accumulated at each cycle of amide coupling. The AllHPLC siRNAs within Allele’s RNAi product line, pre-validated or custom made, are uniformly HPLC purified with 5 OD or 12.5 nmol of double-stranded, annealed siRNA delivered. Allele passes to customers the cost savings from manufacturing our own RNA amidites and other reagents for oligo synthesis. The pre-validated HPLC purified double-stranded siRNA is offered today at $149/12.5 nmol.

Before purchasing siRNAs, even at a low cost of $29 per pair of HPLC purified control siRNA from Allele, researchers still need to consider how well their cells can be transfected. For hard-to-transfect cells, lentiviral vectors carrying a shRNA expressing cassette is often a better choice. To establish stable cell lines, plasmid vectors should be considered. For low cost target screening, the PCR format linear siRNA expression cassettes have advantages.

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Thursday, September 17th, 2009 oligos and cloning, Uncategorized No Comments

Economy and Your Research: Carpets and Oligos

Do you believe in six-degrees of separation? If you really don’t care how close you are related to Roger Tsien or Bill Gates or the dean of your graduate school, maybe you are still curious about how the economy downturn, oil production, and floor carpet production got to do with you–not just in the sense how the job market is shaping up, but also how your lab research budget and how your DNA oligos are served.

To illustrate how events far and away can influence your daily activities, just use oligos as an example. Starting in 2008 when the oil price was still near its peak (remember paying $4+/gallon?), it became too expensive for carpet producers to continue using petroleum for manufacturing carpets. They switched to some other source or halted business altogether. Side effect was production and supply of Acetonitrile (ACN) dried up. Yes, one of the most commonly used organic solvent is a by-product from making carpets. That, combined with facility shutdowns in Northern China in preparation of the summer Olympics (for clean air) and in Florida by a major hurricane, the price of 4 liters of Acetonitrile changed from ~$40 to about $400 plus lots of begging. This event alone pushed individual customer based (as compared to large scale or prefabricated) oligo businesses like Allele’s to be at a loss.

Eventually the situation changed, price went back to about $90/4L, but not before a long period when Acetonitrile was completely unavailable and alternative solvent had to be used. Long story short, that was some storm to whether! If you didn’t feel it in price or service from Allele Oligo, good, that means we did a fair job shielding the wind and shouldering the pressure from the collapsing roof.

Everything really is connected, sometimes by a few degrees less than you would imagine.

Allele’s mottos: care about the environment, help everybody whenever we can, do the right thing even when nobody is looking, have fun, and contribute to the good of mankind through science and innovation.

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Friday, August 14th, 2009 oligos and cloning, State of Research No Comments