Fig. 1. Rbf1 activity is controlled by phosphorylation in the early embryo. (A-G) In situ hybridization of stage 10 embryos with an RnrS probe. (A) Sibling control embryo from a collection expressing UAS Rbf1 with prd-Gal4. (B) UAS Rbf1/prd-Gal4. Arrow marks paired-expressing segment. (C) UAS Rbf-280/prd-Gal4. Arrow denotes the precocious termination of RnrS expression in a paired-expressing segment. (D) UAS Rbf-280/prd-Gal4 embryo pulse labeled for 15 minutes with BrdU (green). RnrS expression was detected by FISH (red). Arrow and arrowhead indicate cells in cycle 15 and 16, respectively. (E) Sibling embryo from a collection expressing UAS Rbf-280 with arm-Gal4 VP16. (F) UAS Rbf1/arm-Gal4 VP16. (G) UAS Rbf-280/arm-Gal4-VP16. (H) Rbf1 was immunoprecipitated from 0- to 4-hour-old and 5- to 8-hour-old w¹¹¹⁸ embryo extracts, and the IPs were probed for the presence of E2f1 and Dp by western blotting. (I) Schematics of the embryonic cell cycle program and the regulation of E2f1 activity. Scale bar: 200 µm.