This post was contributed by guest blogger, Scott Messenger, Technical Support Scientist 2 at Promega Corporation.
It’s always an exciting time in the lab when you find a new assay to answer an important research question. Once you get your hands on the assay, it is always good to confirm it will work for your experimental setup. Repeating the control experiment shown in the technical manual is a great way to test the assay in your hands.
After running that first experiment of your assay, it looks pretty good. The trends of control and treatment are consistent. Time to get on with the experiments…but wait—the RLUs (Relative Light Units) are two orders of magnitude lower than the example data! I can’t show this data to my colleagues; it doesn’t match. What did I do wrong?
This is a concern that we in Technical Services hear frequently. The concern is real, and I had this same thought when doing some of my first experiments using luminescence. When a question like this comes in, a Technical Service Scientist will make sure the experiment was performed as we described, and in most cases it is. We then start talking about RLUs (Relative Light Units).
Bioluminescent reporter assays are an excellent choice for analyzing gene regulation because they provide higher sensitivity, wider dynamic range and better signal-to-background ratios compared to colorimetric or fluorescent assays. In a typical genetic reporter assay, cells are transfected with a vector that contains the sequence of interest cloned upstream of a reporter gene, and the reporter activity is used to determine how the target sequence influences gene expression under experimental conditions. A second control reporter encoded on the same or a different plasmid is an essential internal control. The secondary reporter is used to normalize the data and compensate for variability caused by differences in cell number, lysis efficiency, cell viability, transfection efficiency, temperature, and measurement time.
For genetic reporter assays, using a secondary control vector with a weak promoter like PGK or TK to ensures that the control does not interfere with activation of your primary reporter vector. Transfection of high amounts of the control plasmid or putting the control reporter under control of a strong promoter like CMV or SV40 often leads to transcriptional squelching or other interference with the experimental promoter (i.e., trans effects). Reporter assays can also be used to quantitatively evaluate microRNA activity by inserting miRNA target sites downstream or 3´ of the reporter gene. For example, the pmirGLO Dual-Luciferase miRNA Target Expression Vector is based on dual-luciferase technology, with firefly luciferase as the primary reporter to monitor mRNA regulation and Renilla luciferase as a control reporter for normalization.
Here in Technical Services we often talk with researchers who are just starting their project and looking for advice on designing their genetic reporter vector. They have questions like:
How much of the upstream promoter region should be included in the vector?
How many copies of a response element will be needed to provide a good response?
Does the location of the element or surrounding sequence alter gene regulation?
Each luminescent assay plate represents precious time, effort and resources. Did you know that there are three things about your detection instrument that can impact how much useful information you get from each plate? Instruments with poor sensitivity may cause you to miss low-level samples that could be the “hit” you are looking for. Instruments with a narrow detection range limit the accuracy or reproducibility you needed to repeat your work. Finally, instruments that let the signal from bright wells spill into adjacent wells allow crosstalk to occur and skew experimental results, costing you time and leading to failed or repeated experiments.
One of the nice things about working in a biotechnology company is the opportunity to learn about new products as they are developed and to get exposed to scientific disciplines outside of my original area of expertise. Since I came to Promega I have had the opportunity to learn about a large number of products for widely differing research applications including cell biology, forensics, nucleic acid purification, and drug screening.
Over the years I have seen quite a few products that made me think “I wish I had that back when I was in the lab”. Even although there are a lot of “sexier” products around, the number 1 item that would have made my life in the lab better is still the 10-minute plasmid prep kit. In the labs I worked in we tried to save money and do our own minipreps, and it was tedious indeed. It turns out I am not alone in this sentiment, home-made plasmid minipreps came in at #9 on this list of “techniques we are most glad we don’t have to do any more” on BiteSize bio.
Home made minipreps. I may not have been the greenest-fingered scientist that ever lived but the failure rate of my home-made minipreps was pretty big, especially when I did many of them at once. Thank goodness for miniprep kits.
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