Figure 3.

CBC contributes to B complex stability. A ³²P-labeled m⁷GpppG-capped pre-mRNA containing the HIV-1 poly(A) site was incubated with either CBC-depleted (lanes 1–5) or mock-depleted (lanes 6–10) nuclear extract. 3′ processing complexes were allowed to form for 10 min at 30°C. An aliquot was removed, denoted time 0, and treated with heparin at 0°C. A 100-fold molar excess of unlabeled m⁷GpppG-capped pre-mRNA was added to the remaining reaction and aliquots were removed after an additional incubation for 5, 10, 20, and 40 min. The RNA/protein complexes were resolved on a nondenaturing 3% polyacrylamide gel. The B complex contains both CPSF and CstF (9).