Site Specific Lysine Acetylation of Histones for Nucleosome Reconstitution using Genetic Code Expansion in Escherichia coli

Obtaining modified mononucleosome is crucial for designing epigenetic experiments. Our protocol provides acetylated reconstituted mononucleosomes that can be used in biochemical and binding assays, structure determination, and more. The primary advantage of our technique is its versatility.

This system can be easily modified to incorporate many types of modifications into mononucleosomes. Begin by preparing one liter of autoclaved 2YT media in a culture flask. Inoculate 20 milliliters of the media containing appropriate antibiotics with the co-transformed cell stock that has the tag containing plasmid and pEVOL-AckRS and grow them at 37 degrees Celsius to obtain an OD of 0.6.

Add appropriate antibiotics in one liter of autoclaved 2YT media and inoculate it with 20 milliliters of starter culture. Grow the culture at 37 degrees Celsius for two to three hours to obtain an OD of 0.6, 2.8. Add inducing agents and AcK for a final concentration of one millimolar IPTG, 0.2%arabinose, and five millimolar AcK.

Grow the culture at 37 degrees Celsius for six to eight hours. Then pellet the cells at 2, 700 x g for 15 minutes and store them overnight at minus 80 degrees Celsius. Dissolve the pellet in 50 milliliters of histone lysis buffer per liter of culture and sonicate the cells at 60%amplitude for three minutes with one second on and one second off intervals.

Pellet inclusion bodies at 41, 600 x g for 45 minutes. Discard the supernatant and then resuspend the inclusion bodies in 30 milliliters of histone lysis buffer, then centrifuge again for 30 minutes. Resuspend inclusion bodies again in 30 milliliters of pellet wash buffer and repeat the centrifugation.

Next, dissolve inclusion bodies in 25 milliliters of six molar guanidine hydrochloride buffer. Incubate with agitation at 37 degrees for one hour, then centrifuge for 45 minutes. Incubate the supernatant with one milliliter of nickle-NTA resin equilibrated with six molar guanidinium hydrochloride buffer for two hours and proceed with nickle-NTA purification under denatured conditions.

Wash the column with three column volumes of wash buffer, then elute acetylated histone protein with 10 milliliters of elution buffer. Dialyze the acetylated histone protein against 5%acetic acid buffer to remove salts for three hours at four degrees Celsius, exchanging the buffer a minimum of six times. Aliquot and lipolyze protein and stored indefinitely at minus 80 degrees.

Dissolve aliquots of histone protein pellets H2A, H2B, H3, and H4 so that there's a separate stock of each histone protein in guanidinium hydrochloride buffer for the total volume of 100 microliters. Calculate the concentration of each histone protein by measuring the absorbents at 280 nanometers. If absorbance is greater than one for any protein, dilute with guanidinium hydrochloride buffer to get a more accurate concentration.

Combine H2A with H2B and H3 with H4 proteins at a molar ratio of one-to-one and dilute to a total protein concentration of four micrograms per microliter. Dialyze sequentially at four degrees Celsius against two molar TE buffer overnight, one molar TE buffer for two hours and 0.5 molar TE buffer for five hours. When finished, centrifuge at 16, 800 x g at four degrees Celsius to remove the precipitate.

Calculate the concentration of histone dimers and tetramers by measuring absorbance at 280 nanometers. Mix the dimers and tetramers at a one-to-one molar ratio and adjust the sodium chloride concentration to 2M. Adjust 601 DNA to two molar TE buffer using 100X TE buffer and solid sodium chloride.

Add the solution to the histone octamer at a molar ratio of 0.85, 2.9 to one. Transfer the DNA histone mixture to a dialysis bag and place it at about 200 milliliters of two molar TE buffer with very gentle stirring at four degrees Celsius. Set up peristaltic pump to slowly drip in no-salt TE buffer.

When the volume has roughly doubled, pour out half the volume. Do this at least four times, which can take four to eight hours depending on the starting volume. After the salt concentration is reduced to 150 millimolar, dialyze against a 20 millimolar TE buffer overnight.

Remove precipitates by centrifugation and measure the concentration of the nucleosomes with a plate reader. Add his-TEV protease and incubate overnight at four degrees Celsius to remove all histidine tags. Then remove the histidine tag impurities from the nucleosome solution with nickel-NTA resin pull down.

Incubate at 60 degrees Celsius for one hour to position the nucleosome and homogenize the sample. Store nucleosomes at four degrees Celsius short-term. For long-term storage, dialyze nucleosomes against the storage buffer and store at minus 80 degrees Celsius.

Some of the acetylated tetramers had a lower yield than others as assessed by running a 12%SDS-PAGE gel. The closer to the core region, the lower the observed yield. This was most likely due to the acetylation, interfering with the octamer assembly.

After assembling octamers with the 601 DNA sequence, the nucleosome can be assessed using a 5%1X TBE-native PAGE gel followed by staining with ethidium bromide. Before thermal positioning, the observed nucleosome bands are very broad and may streak. When performing this protocol, keep the proteins on ice at all times.

It is crucial to maintain assembled dimers, tetramers and nucleosome. This procedure has allowed researchers to install the acetyl lysine sites, specifically into mononucleosomes, opening the door to studying post-translational modifications in a chromatin context. It can be easily adapted to install any number of non-canonical amino acids.