Azure A Staining
Azure A is produced when Methylene blue, another Thiazin stain, is oxidised. Methylene blue is less sensitive for staining DNA than its oxidation products, while Azure A demonstrates reduced background gel staining and a reduced required staining time (NCBE, 2003b). The Thiazin family is thought to stain nucleic acids through ionic interactions with the phosphate groups of the sugar-phosphate backbone (NCBE, 2003a); but weak intercalative interactions with DNA have also been described (Paul and Kumar, 2013).
The costs of staining DNA
Sigma-Aldrich sells 10ml of Ethidium bromide (EtBr) solution at a concentration of 10mg/ml for £45/$56 (E1510) (Sigma-Aldrich, 2015). We used a solution of 15ul (10mg/ml) EtBr per 0.5L of TAE running buffer for post-run staining, which works out to ~6666 0.5L stains per bottle of Ethidium. Traditional DNA staining with Ethidium bromide appears to be low-cost when only the expense of buying the stain is considered; with a cost per gel of £0.0067. However the cost to a lab is greater than simply the stock stain; Ethidium stained DNA requires visualisation on a UV-transilluminator, and the model of UV transilluminator available to us comes from a range which begins at £600/$900 (VWR, 2015). UV illumination requires an enclosed space where other people and sensitive items will not be damaged by irradiation. Additionally, imaging an Ethidium stained gel requires specialist camera filters to prevent camera sensor damage by the UV radiation, or the use of an enclosed illumination and photographing apparatus such as the BioRad Gel Doc™ XR system. Disposal of Ethidium bromide stained gels is also costly; EtBr has been found to be a potent mutagen in in vitro testing, thus disposal and handling is treated very seriously. The EtBr disposal policy of the University of Glasgow states that EtBr waste should be disposed of into biohazard marked containers, which must be uplifted by a 3rd party waste disposal service. 10g of Azure A chloride powder costs ~£36/$60. 500ml of 1x stain was reused by Glasgow up to 2x per day, for two weeks, equalling somewhere in the region of 28 gels per 500ml of 1x. 10g of Azure A chloride will produce 25L of 1x stain, or 50 500ml batches. This calculates to an estimated 1,400 gels per 10g of Azure A; or £0.026/$0.042 per gel. At face value this is more expensive per gel than EtBr, but Azure A does not require specialist disposal services, the lightbox which is helpful for visualising bands for gel extraction can vary in price but will not likely reach the price of a UV transilluminator, with a Top of the Range professional white light box for photography purposes costing ~£282/$440 (Calumet: https://www.calphoto.co.uk/product/Calumet-Light-Box-A3-/710-407X) with cheaper alternatives available. Imaging an Azure A gel can be done on any lab or office scanner, if the gel is placed between two sheets of acetate, rather than requiring specialist UV illumination and capture equipment.
Protocol for producing a 2x stock solution of Azure A stainWe maintained a 2x stock solution in line with the recommendation of the NCBE guidance from which we first learned of Azure A staining (NCBE, 2003b). To produce 1L of 2x Azure A (0.08% Azure A/40% Ethanol):
- Dissolve 0.8g Azure A chloride solid in 1L 40% Ethanol
- Store in a glass screw capped bottle out of direct sunlight, or alternatively wrap the bottle in aluminium foil. Azure A will bleach over time in sunlight and lose effectiveness.
- Prior to use, a 1x (0.04% Azure A/20% Ethanol) solution should be prepared by diluting the 2x stock 1:1 with distilled or deionised water to a desired final volume.
- 1x stock should also be stored in a glass screw capped bottle out of direct sunlight.
Protocol for staining with 1x Azure A
In this protocol Azure A is used as a post-electrophoresis gel stain. The effect of putting it in the running buffer is unknown, other than it will definitely stain the gel-kit blue.
- First, transfer the gel to a tray or container.
- Glasgow team used a large clear plastic Tupperware box with a lid. A lidded container prevents accidental overflow of the stain
- Azure A can stain plastics so any tray used for Azure A should not be utilised for other uses unless it has been thoroughly cleaned with 100% Ethanol.
- Pour enough 1x Azure A solution into the container to cover the gel, without overfilling it.
- Shake the container for 10-15 minutes, either by a low setting on a benchtop shaker, or by hand (fun)
- 1x stain was shown to be usable for several weeks before making up a new batch, without any demonstrable contamination issues or reduction in effectiveness.
- After a 10-15min stain, DNA bands of high concentration may already be visible at this stage, but we recommend at least one round of destaining for improved visualisation.
- To destain, cover the gel in the used running buffer from the gel-kit, or distilled/deionised water. Shake for 10-15 mins. The destaining solution should be discarded into a sink drain along with running water.
- Repeated rounds of destaining will improve visualisation bands of weak concentration, as demonstrated in Figure 3 above.
- Band visualisation is improved by viewing the gel on a white lightbox, either inside a clear plastic staining container, or atop of a sheet of acetate to prevent staining.
Gel extraction using Azure A
Gel extraction after staining with Azure A follows the same protocol as an Ethidium bromide gel extraction, without the need to use a UV transilluminator and associated face protection. As the Glasgow team used a clear plastic Tupperware box to stain our gels, we often proceeded to place them straight on top of our benchtop lightbox after destaining and perform the gel extraction there, without having to manipulate the gel onto another surface.
Figure 4: A typical Azure A gel following extractions with a scalpel Figure 6: The inconsequential colour change observed after addition of Buffer QG from the QIAquick Gel Extraction Kit.
Comparison of Azure A staining to Ethidium Bromide staining
We attempted to demonstrate the DNA staining sensitivity of Azure A and Ethidium bromide through a repeatable, directly comparable experimental protocol. iGEM transformation efficiency control DNA (J04450 (RFP) in pSB1C3) was transformed into chemically competent DH5a cells, and then a resulting transformant colony was inoculated into a 10ml overnight culture. Plasmid DNA was isolated from 4.5ml of an overnight culture following the standard QIAGEN “QIAprep Spin Miniprep Kit” protocol.From the resulting J04450.pSB1C3 miniprep a 1/100 dilution was created and OD260 was measured in a spectrophotometer to determine the DNA concentration. DNA concentration of the miniprep DNA was calculated to be ~285ng/ul from the spectrophotometer reading. Restriction digests were set up with EcoRI-HF enzyme and an “uncut”control Serial dilutions of the cut and uncut DNAwere made in order to get a range of known DNA concentrations to load onto two gels; 1. Stained with Ethidium bromide and imaged in a Bio-rad Gel Doc XR imager, 2. Stained with Azure A (destained repeatedly over 2 hours) and imaged on a standard office scanner between two sheets of acetate. The results (Fig 7 & 8 below) show that Ethidium bromide could resolve bands of DNA as low as 2ng, whereas with Azure A band with ~17ng of DNA was the lowest resolvable on an office scanner after 3 hours of destaining.
Figures 7 & 8: 1% Agarose gels loaded with an identical dilution series of prepared DNA of known concentration; one gel stained with Azure A and one gel stained with Ethidium bromide (For EtBr staining method see: Protocols)
Azure A is an easy to use, visible light DNA stain which does not require expensive equipment or biohazard disposal techniques. For routine DNA manipulation techniques such as gel extractions from high copy number plasmids like pSB1C3, Azure A staining is an effective alternative to Ethidium bromide without the need for UV light visualisation.
Glasgow 2015 iGEM team hopes to promote the use of Azure A in order to extend the reach of iGEM, synthetic biology, and molecular biology in general, further into settings where the need for Ethidium bromide and the associated costs may prevent participation, such as high schools and community labs.
Safety and considerations of Azure A
Azure A Chloride's Material Safety Data Sheet (MSDS) is available from the supplier; it is not deemed to be hazardous. Azure A solution can be disposed of down a conventional sink. Azure A stained Agarose gels can be disposed of in standard laboratory waste. Standard laboratory safety measures for handling a dye; such as gloves, goggles, lab coat, should be followed when using Azure A. Flames should not be used near to preparation of Azure A/Ethanol solution due to flammability. Azure A solutions will stain clothing and surfaces if spilled. Azure A solution can be removed from surfaces (such as a benchtop or staining tray) with 100% Ethanol and paper towel. NCBE. 2003a. National Centre for Biotechnology Education | DNA50 | Staining DNA [Online]. Available NCBE. 2003b. National Centre for Biotechnology Education | DNA50 | Staining DNA Table [Online]. Available PAUL, P. & KUMAR, G. S. 2013. Spectroscopic studies on the binding interaction of phenothiazinium dyes toluidine blue O, azure A and azure B to DNA. Spectrochimica Acta Part a-Molecular and Biomolecular Spectroscopy, 107, 303-310. SIGMA-ALDRICH. 2015. Ethidium bromide solution BioReagent, for molecular biology, 10 mg/mL in H2O | Sigma-Aldrich [Online]. SUMNER, A. T. 1980. DYE BINDING MECHANISMS IN G-BANDING OF CHROMOSOMES. Journal of Microscopy-Oxford, 119, 397-406. VWR. 2015. UV transilluminators, Benchtop series [Online]. Available