Complexes C199 (lanes 3 and 4), G200 (lanes 5 and 6), and Ia (lanes 7 and 8) were radiolabeled at position C199 and treated with rat liver SII (phosphocellulose fraction, 0.5 μ g) and 7 mM MgCl2 for 10 min at 28 °C. Cleavage products were separated from the complexes by centrifugation, butanol-extracted, dried, dissolved in 9 μl of 1 mM MgCl2, and treated with (lanes 4, 6, and 8) or without (lanes 3, 5, and 7) 0.05 unit of calf intestinal alkaline phosphatase (Boehringer Mannheim) for 10 min at 37 °C. Samples were dried and resolved on a 25% polyacrylamide gel. Lanes 1, 2, and M are chemically synthesized, 32P-labeled pApC, pCpG, and pCpGpUpUpUpUpU, respectively.