Restriction Enzymes

Restriction enzymes are used to cut plasmids. We have tackled the plasmids in the previous lecture. You can have a full description about the restriction enzymes here.

As a most basic introduction I would say that restriction enzymes are enzymes of the bacteria representing a kind of immune function of the bacteria. They are present in pairs in bacteria: a DNA methylase and a restriction enzyme. They both recognize the same sequence. The bacteria is methylating its own DNA in a sequence specific manner. By this its own DNA is protected against any foreign DNA. Since horizontal gene transfer is quite common in bacteria, the bacterial cell can protect its own genetic material with the help of the restriction enzymes. The foreign DNA entering into the cell will present a different DNA methylation pattern. The unmethylated recognition sites will be cut  by the restriction enzymes and by this destroyed.

Different bacterial species have different restriction enzymes with different recognition sites (certainly each has a DNA methyltransferase, too). The nomenclature of the restriction enzyme reflects their origin. In the most trivial case the name Eco RI enzyme is informing us that it has been isolated from Escherichia coli strain R and it has been the first to have been isolated from this strain.

In the molecular biology lab we use them to cut and manipulate plasmids. They are like scissors that can be directed to specific sites in the plasmid to cleave it. With an appropriate collection of site specific cutting enzymes we can step into the very exciting field of genetic engineering.

Let us have a look to some basic usage of restriction enzymes:



You can check a good introductory video here.

In any case when working with enzymes, please use latex gloves, and keep enzymes on ice!

The unit of a restriction enzyme "U" stands for the amount of enzyme needed to cut 1microgram of plasmid with a single cutting site, in one hour, in ideal environment.

The environment of the reaction is provided by buffers. The enzymes are usually provided in a concentration of 10U/ul (10 units per microliter). The enzymes are supplied in glicerol solution and always stored at -20 C. The buffer may as well come in a 10 fold concentrated solution (10X) and it should also be kept frozen.

A typical restriction enzyme reaction is set up in the following way:

1. Check the map of the plasmid for the distribution of the cutting sites.

2. Measure the concentration of the plasmid solution by spectrophotometer. Your plasmid concentration should be in the range of 1 microgram per microliter.

3. Calculate the volume of the plasmid needed to have the required amount of product at the end. The volume of the reaction should be kept as low as possible, and should not exceed 100 ul/ reaction tube. Use sterile, DNAse free microcentrifuge (so called) "Eppendorf" tubes.

4. Plan the reaction. You should have approx 1 to 10 U of enzyme per microgram of plasmid. In the final volume of the reaction the total volume of the enzyme should be less the 1/10, because higher glicerol concentration might alter the specificity of the reaction. The buffer will be 1/10 of the final volume. Keep the final volume low (less then 100 microliters). If needed, adjust the reaction volume to the planned final volume with nuclease free water. Check the optimal temperature for the reaction. It is usually 37C, but it might differ. Check for possible star activity of the enzyme in its data sheet.

Example:

Mix the following components (ul stands for microliter):


16ul Nuclease Free Water+

1ul Plasmid solution (concentration 1ug/ul)+

2ul 10X Buffer+

1ul Restriction Enzyme (10U/ul)


Total:     20ul


5. Once the reaction is planned, start to do it: bring ice, prepare tubes, melt the buffer in your hands.

6. Pipette the required volumes of water, plasmid and buffer into the tube.

7. Add the enzyme to the tube and mix gently. Do not vortex!

8. Put the reaction into the thermostat set to the required temperature.

9. Put the enzyme and the buffer back to -20C and clean up you bench!

10. After the allocated time has  passed, stop the reaction. We are usually keeping the reaction in the thermostat for 4 hours. You can stop the reaction in several ways: by adding EDTA; by heat inactivating the enzyme at 85C for 10 minutes, or simply by freezing the tube and keeping it frozen until you purify it.

You can have a look on the applications in the video below.

Good luck!