DNA-dependent RNA polymerases from Artemia salina: decreasing polymerase activities and number of polymerase II molecules in developing larvae.

Embryos and larvae of the brine shrimp, Artemia salina, provide a useful biological system for biochemical studies of animal development. Dormant encysted embryos can be cultured readily in the laboratory to provide large quantities of free-swimming nauplius larvae. The rate of synthesis of all classes of RNA in swimming larvae declines markedly between 24 and 72 h after immersion of dormant embryos in sea water. Nuclei were isolated from 24-72 h larvae and RNA polymerase activity was measured under conditions in which the nuclei remained intact. Total RNA polymerase activity of isolated nuclei decreased in parallel with RNA synthesis in vivo. RNA polymerases were solubilized from nuclei and fractionated by chromatography on DEAE-cellulose. The levels of both RNA polymerases I and II also decreased in parallel with RNA synthesis in vivo. The specific activity of highly purified RNA polymerase II was determined by comparison of enzyme activity with the mass of RNA polymerase II subunits displayed on SDS gels. The specific activities of RNA polymerase II preparations from 24 and 72 h larvae were identical. The number of polymerase II molecules was estimated from the mass of the subunits. The number of molecules per nucleus declined from 20,000 at 24 h to 3500 at 72 h.