Ditrect STORM imaging and image reconstruction of the transcription elongation marker P-S2 and nuclear PI(3,4)P2 indirectly immunolabeled with AF647 (red) and AF555 (green) in THZ1 treated cells.

U-2 OS cells were grown in DMEM with 10% FBS at 37°C and 5% CO₂. Cells were plated one day before staining in ~50% confluence on the high-precision 12 mm round coverslips treated with Hellmanex, sonicated, washed, dried and sterilized. Cells were treated for 3h with 1 µM THZ1 (MedChem HY80013) added to the culture media.

U2OS cells were washed twice with PBS (pH 7.4) and fixed for 30 min in 2% PFA in PBS, washed 3-times for 5 min with PBS, then permeabilized in 0.1% Triton X-100 in PBS for 20 min, washed 3-times for 5 min by PBS and blocked in filtered 5% BSA in PBS for 30 min. Cells were incubated for 45 min with rabbit polyclonal IgG anti-RNAPII CTD P-S2 (Abcam ab5095) 3 µg/mL and mouse monoclonal IgG2 anti-PI(3,4)P2 (Z-P034; Echelon Biosci. Inc., USA) 5 µg/mL  in 5% BSA in PBS, washed 3-times for 5 min in PBS and incubated for 30 min with donkey anti-mouse IgG AF555 (Invitrogen A31570) 10 µg/mL and goat anti-rabbit IgG AF647 (Invitrogen A21245) 10 µg/mL diluted in 5% BSA in PBS. Then the cells were washed 3-times for 5 min in PBS, post-fixed for 15 min in 2% PFA in PBS and washed 3-times for 5 min in PBS. All procedures were performed at RT and the cells were stored in PBS in the fridge overnight prior imaging.

Coverslips with cells were mounted in the Chamlide chamber (Live Cell Instrument, Korea) and covered with imaging buffer (PBS pH 7.4, 50 mM MEA). Single-molecule localizations (SMLs) data were acquired by Zeiss Elyra PS.1 equipped with HR Diode 642-150 and HR DPSS 561-200 lasers, Alpha Plan-Apochromat 100x/1.46 oil DIC M27 Elyra objective and Andor EM CCD iXon DU 897 camera and Zeiss ZEN Black 2.1 SP3 software (Zeiss). AF647 and AF555 photo-switching was achieved by HiLo illumination and TIRF HP FOV with 100% power of 642nm or 561nm laser, and the signal was acquired via MBS 642 + EF LP 655 and MBS 561 + EF BP 570-620 / LP 750 filters, respectively. Exposure time was 40 ms and EM gain was 300 for both channels.

SMLs were calculated in 2D by Zeiss ZEN Black 2.1 SP3 software using x,y 2D Gauss fit with point spread function (PSF) half width 177.9 nm, peak mask size 9 pixels and peak intensity to noise 6 and accounted for overlap in 2D with max cluster size 10. SMLs were rendered in ZEN software with 10 nm/px resolution and 1x PSF expansion factor. The data were model-based drift corrected in ZEN. Two channels were aligned using tetraspec beads fiducial markers for affine calibration. Drift-corrected and aligned localization coordinates were exported as text files. Text files were converted into csv files and imported using self-written macro (Hoboth et al., 2021a) into the ImageJ2 (Rueden et al., 2017) plug-in ThunderSTORM, visualized by normalized Gaussian method (Ovesny et al., 2014).