Fig. 4. (A) Genomic organization of the rols locus. The scheme summarizes the data of our genomic walk, the data from the Drosophila Genome Project and the sequence analysis of rols cDNAs, as well as the determination of the P-element insertion sites. The rols gene extends about 60 kb, as indicated by the scale and is flanked towards the centromer by an open reading frame coding for Semaphorin 5c (CG5661; S, gray box). The exons common to rols7 and rols6 are shown as black boxes, specific exons for rols7 are shown as black and white hatched boxes, while the specific exons for rols6 are shown as white boxes. The transcription initiation site of rols6 is localized within the large second intron of rols7. The translation initiation codons for Rols7 and Rols6 are unique, they are localized in the second exon unique to each message. The P-element in the mutant rols^P1027 is localized 70 bp upstream of the transcription initiation site of rols6, while the P-element P1729 of the rols^P1729 mutant is localized within the common part of the second intron. (B) Stage-specific mRNA levels detected by RT-PCR. Poly (A)⁺ RNA was prepared from embryos collected for the times indicated, and RNA from 0-4 and 4-8 hours was purified twice in order to remove genomic DNA completely. For details of the RT-PCR see Material and Methods. Whereas the ß1-tubulin mRNA (arrow) is present during all stages, rols6 and rols7 are first detected at 4-8 hours, corresponding to embryonic stages 8-11; rols7 reaches maximum levels between 8-12 hours (stages 12-14), while rols6 shows the highest levels at 12-16 hours (stages 15-16), whereas rols7 already declines. During 16 hours until hatching, rols6 is present at relative high levels compared with rols7.