Fig. 2. Pros26.4 transcription during development. (A) Maternal mRNA is uniformly distributed in a stage-4 embryo (yo, yolk; pc, pole cells). (B) With the onset of neurogenesis, transcripts start to accumulate in presumptive neuronal cells (arrow; stage 11). (C) Nervous-system-specific accumulation of Pros26.4 mRNA is apparent with germ-band retraction (br, brain; vc, ventral chord). (D) At the time of dorsal closure (stage 14), expression is mainly detected in the ventral chord (vc) and the brain (br). (E) A close-up of the developing head of a stage-12 embryo shows a modulated expression in the brain (br). (F) A dorsal view on an extended germ band embryo shows the modulated expression (arrow) in the nervous system anlage (stage 10). (G) A ventral view on a stage-11 embryo shows accumulation of Pros26.4 mRNA in presumptive neuroblasts (arrow). (H) A dorsal view on a stage-14 embryo highlights the strong mRNA expression in the two brain lobes (br) and also enrichment in the posterior spiracles (ps). Embryos are oriented with anterior left and dorsal up unless otherwise noted. Stages are according to Campos-Ortega and Hartenstein (Campos-Ortega and Hartenstein, 1997). (I) Pros26.4 mRNA transcription levels remain high in proliferating neuroblasts of the ventral chord (arrow) and also in the ring gland (rg) of third instar larvae. (J) In the eye disc (ed) expression is mainly observed in cells anterior to the morphogenetic furrow, whereas in the antennal disc (ad) mRNA distribution is more uniform. (K) In the wing disc, expression is observed all over in a modulated pattern; it is weaker in presumptive vein areas (arrow) and the zone of non-proliferation (asterisk). (L) A likewise modulated expression is observed in the leg disc. (M-P) Expression of Pros28.1, which encodes the -subunit of the catalytic 20S core of the proteasome, is very similar when compared with that of Pros26.4.