Fig. 2: P1 contributes a modest yet non-negligible fraction of early hb transcription and function.

a Confocal image of a WT Drosophila embryo labeled for P1–5′UTR, P2–3′UTR, CDS of hb mRNA, and DNA at nc13. Scale bar, 50 μm. Inset, magnified view of anterior nuclei. Scale bar, 5 μm. The experiment was repeated twice, independently, with similar results. b Nascent P1–5′UTR, P2–3′UTR, and CDS signals per nucleus (in units of the number of mRNA molecules) were plotted against each other and fitted to a linear function. Data from a single nc12 embryo. c Parameters of the linear fit between P1–5′UTR, P2–3′UTR, and CDS signals during nc11–13. Data are presented as mean ± s.e.m. d Schematic of P1 and P2 transcription. Nascent mRNAs were elongated on the gene with a constant speed and stayed on the transcription site for an extra period before being released. The P1-specific intron was spliced during or after transcription. e, f The contributions of P1 and P2 to the mean (e) and the intrinsic noise (f) of the nascent CDS signal in the position range of 0.2–0.4 EL during nc11–13. g, h Nascent mRNA signal per nucleus as a function of the AP position for Kr (g) and kni (h) in nc14. The profile was binned from the single-nucleus data (bin size: 0.05 EL, step size: 0.025 EL). Shadings indicate s.e.m. Right: the anterior boundary positions of Kr and kni expression domains. Data are presented as mean ± s.e.m. with two-sided t-test (Kr: p = 0.0093; kni: p = 0.013. *p < 0.05; **p < 0.01). c, e, f n = 1, 5, and 4 biologically replicates for nc11–13, respectively (see Methods section and Supplementary Fig. 5a for embryo selection). g, h Kr: n = 4 and 5 biologically independent embryos for WT and ΔP1C, respectively; kni: n = 4 and 3 biologically independent embryos for WT and ΔP1C, respectively. Source data are provided as a Source Data file.