A 11 ecosystems and human societies depend on a healthy and tlroductive natural environment that contains diverse plant and animal species. The earth's biota is comtlosed of an estimated 1 0 million species of plants, animals, and microbes (Pimm et al. 1995). In the United States, there are an estimated 750.000 stlecies. of which small organisms, such as arthropods and microbes, make up 95%.' Although approximately 60% of the world's food supply comes from rice, wheat, and corn (Wilson 1988), as many as 20,000 other plant species have been used by humans as food. Some vlants and akimals provide human; with essential medicines and other diverse, useful ~r o d u c t s. For instance. some plants i n d microbes help to d&rade chemical pollutants and organic wastes and recycle nutrients throughout the ecosystem. The rapidiy growing world population and increased human activity threaten many of these species. The current extinction rate of species ranges from approximately 1000 to 10,000 times higher than natural extinction rates (Kellert and Wilson

The C hnRNP proteins bind to nascent premRNA and are thought to participate in an early step of the pre-mRNA spling pathway. We report here that C hnRNP proWeins are phosphorylated by a casein kinase U activity in a HeLa cell nudear extra and that dephosphorylation of C hnRNP proteins Is Inhibited by the specific protein-serine/threoniphosphatase 1/2A inhibitor okadaic acid. We further find that dephosphorylation of C hnRNP proteins Is required for their binding to adenovirus and human 3-globin pre-mRNAs. These results indicate that the participation of C hnRNP proteins in pre-spliceosome assembly is coupled to a dynmk cycle of their phosphorylation and dephosphorylation.Pre-mRNA splicing takes place in complex ribonucleoprotein particles termed spliceosomes (1-5). The first described spliceosome proteins were called hnRNP proteins, based on their association with large heterogeneous nuclear RNA transcripts now known to include unspliced pre-mRNAs (6-12). The C-group hnRNP proteins are abundant nuclear proteins of Mr 42,000-44,000 that have been implicated in splicing (13,14) and are known to be phosphorylated in vivo (8,(15)(16)(17) by a casein kinase II-type activity (18,19 ATP, and 20 mM creatine phosphate. Streptavidin-bound pre-mRNA was then added to the nuclear extract and incubation was continued for an additional 30 min (30°C). The streptavidin-bound RNA and associated proteins were collected by centrifugation and the supernatant unbound fraction was also saved.Selection of C hnRNP Proteins from Streptavidin-Bound RNA and Associated Proteins. The assembled RNA-protein complexes on streptavidin beads were washed extensively with binding buffer, and the RNA was released by nuclease digestion [micrococcal nuclease (400 units/ml) and RNase A (200 pg/ml) in the presence of 3 mM CaCl2 for 45 mi at 30°C]. The unbound fraction was treated similarly. Heparin (2 mg/ml) was then added to both the unbound and bound fractions and they were incubated for 10 min at room temperature before immunoselection with 4F4. The selected

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