Ditrect STORM imaging and image reconstruction of the transcription elongation marker P-S2 and nuclear PI(4,5)P2 indirectly immunolabeled with AF647 (red) and AF555 (green) in DRB treated cells.

<p>U-2 OS cells were grown in DMEM with 10% FBS at 37&deg;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 2h with 100 &micro;M DRB (Sigma D1916) added to the cell culture media.</p> <p>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 &micro;g/mL and mouse ascites IgM anti-PI(4,5)P2 2C11 (Z-A045; Echelon Biosci. Inc., USA) 5 &micro;g/mL&nbsp; in 5% BSA in PBS, washed 3-times for 5 min in PBS and incubated for 30 min with goat anti-mouse IgM (&micro;-chain) AF555 (Jackson ImmunoRes. A24126) 10 &micro;g/mL; goat anti-rabbit IgG AF647 (Invitrogen A21245) 10 &micro;g/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.</p> <p>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.</p> <p>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).</p>