FIG. 10.

Model of histone gene expression in the early Drosophila embryo. Maternal histone mRNA is rapidly destroyed at interphase 14. Zygotic histone transcription occurs during S phase of cycle 14 as determined by transcription dots visible with the H3-ds 3′ flanking probe and terminates by the start of G₂. Transcription reinitiates in late G₂ of cycle 14 in each mitotic domain in anticipation of S phase of cycle 15. A low level of histone mRNA can be detected in all cells during G₂ of cycle 14, probably because both the maternal mRNA and mRNA synthesized during S phase 14 are not quantitatively destroyed at the end of S phase 14. dSLBP protein is not provided maternally, and the timing of its accumulation zygotically matches the onset of histone transcription in S phase 14. When zygotic dSLBP is not provided, misprocessed histone messages begin to accumulate immediately in S phase 14 and subsequently in G₂ phase 14.