IE1 positive feedback sustains expression from the HCMV immediate-early promoter. (A) Map of the MIE region of the recombinant HCMV TB40/E IE1 conditional-mutant dual-reporter virus (IE1-CMDR) showing the MIEP (black) and IE region exons 1–5 (gray), along with the recombinant fusions: Exon 2 is genetically fused at its N terminus to mCherry (mCh, red) and the FKBP degron tag (hashed) that destabilizes the IE1 protein in the absence of the small molecule Shield-1; exon 5 also contains a fusion to EYFP (green) at its C terminus. Translation typically begins at the 5′ end of exon 2 (now FKBP). See also SI Appendix, Fig. S3. (B) Schematic of the putative effects of Shield-1 on IE1-mediated positive feedback. (C) Degradation rate of IE1 in cells infected with IE1-CMDR virus as measured by flow cytometry. Protein synthesis was blocked with cycloheximide at 24 h after infection (time = 0) and the degradation rate was measured ±1 μM Shield-1. Shown is the average of two repeats, and error bars denote SD. Decay rate was calculated by fitting the data to an exponential decay model (solid lines). (D) Representative images from fluorescence time-lapse microscopy of IE-mCherry expression in cells infected with IE1-CMDR virus. Cells were cultured in medium containing 1 μM Shield-1 (Top) or without Shield-1 (Bottom). Cells were cultured in medium with 1 μM Shield-1 (Top, wild-type feedback) or without Shield-1 (Bottom, attenuated feedback) and tracked over time; “bg” represents background autofluorescence image. (E) IE-mCherry expression of cells infected with IE1-CMDR virus. Cells were cultured in medium with 1 μM Shield-1 (Left, wild-type feedback, averaged over 119 cells) or without Shield-1 (Right , attenuated feedback, averaged over 153 cells). Bold line denotes mean (i.e., general trend) of the population with gray shading showing SE. Cell trajectories were digitally synchronized to the first detection of mCherry signal. (Inset) The fraction of cells with sustained (S, STI > 0.5) or transient (T, STI < 0.5) IE-mCherry expression over three biological repeats. (F) HF-ACF of cells infected with IE1-CMDR virus in the presence of 1 μM Shield-1 (dark gray) or without Shield-1 (light gray). Shown is an average over 100 cells each; error bars denote SE. (G and H) IE-mCherry mean expression level and noise (CV) 15 h after infection. Dashed line is the expected value from change in IE1 half-life alone (Δ from τ1/2), whereas the additional difference (Δ) is ascribed to the loss of IE1 positive feedback. (I) Relative IE-mCherry mRNA levels (from RT-qPCR) in cells infected with IE1-CMDR virus ±1 μM Shield-1 (P < 0.01, two-tailed t test). (J) Degradation rate of IE2-EYFP in cells infected with IE1-CMDR virus as measured by flow cytometry. Protein synthesis was blocked with cycloheximide at 24 h after infection (time = 0) and degradation rate was measured ±1 μM Shield-1. Shown is the average of two repeats, and error bars denote SD. Decay rate was calculated by fitting the data to an exponential decay model (solid lines). (K) Flow cytometry for IE2-EYFP levels in cells infected with IE1-CMDR virus. Cells were cultured in medium with 1 μM Shield-1 or without Shield-1. (Inset) Normalized mean fluorescence of IE2-EYFP. (L) Relative IE2-EYFP mRNA levels (from RT-qPCR) in cells infected with IE1-CMDR virus ±1 μM Shield-1 (P = 0.012, two-tailed t test).