The role of RNA polymerase initiation and elongation in control of total RNA and histone mRNA synthesis in sea urchin embryos.

The involvement of RNA polymerase initiations in regulating total RNA synthesis and the synthesis of the early histone mRNAs was investigated. Nuclei were isolated from developing sea urchin embryos from 4- to 600-cell stages, and the transcription of already initiated polymerase complexes was studied in a "run-off," or elongation, assay; this assay was optimized by using high levels of ribonucleoside triphosphates. Under these conditions the relative levels of RNA synthesis in isolated nuclei from different stages closely paralleled the known rates of synthesis in vivo. However, if sarkosyl is included in the elongation assay, the nuclei of older stages display greatly stimulated synthesis while early cleavage stage nuclei are not stimulated. Sarkosyl does not reveal any elongated transcripts from the early histone genes in nuclei from later stages of development. This has been interpreted to mean that there are many initiated polymerase II complexes that do not elongate rapidly at later stages, but the early histone genes are inactive at later stages because they do not possess any productively initiated polymerases.