Extended Data Fig. 4: TLs manifest co-transcriptional splicing.

Two sequentially positioned introns were labeled with oligoprobes encompassing 1.2–5 kb. The schematics above the panels depict the distribution of oligoprobes (green and red rectangles), labeled introns (green and red lines) and positions of BAC probes used as references (grey lines above genes). After RNA-FISH the intron probes label TLs only partially and sequentially. Since the 5’ and 3’ intron signals do not overlap, the 5’ introns are spliced before the 3’ introns are read. For instance, in Cald1, introns 1 and 3 are separated by intron 2, suggesting that the “green” intron 1 is spliced out before polymerases reach the “red” intron 3, most likely after RNAPII runs over the 3’ splice-site of the first intron. Projections of confocal sections through 2–3.5 µm. Scale bars: 2 µm, in close-ups, 1 µm. Data represent 2 independent experiments.