FIG. 3.

SLBP1, but not SLBP2, functions in histone pre-mRNA processing in vitro. (A) SLBP1, SLBP2, and human SLBP (hSLBP) were expressed in baculovirus, and 200 ng of protein was analyzed for binding to the stem-loop by mobility shift assay. (B) A 291-nt substrate was synthesized with T7 RNA polymerase from the mouse histone H1t gene. A nuclear extract prepared from mouse myeloma cells was very active in histone pre-mRNA processing (lane 1). The extract was depleted by using an antibody to the C terminus of mouse SLBP (lane 3), or was treated with immunoglobulin G purified from preimmune serum (lane 2). An equal amount (200 ng) of recombinant baculovirus-expressed mouse SLBP (M) (lane 4), frog SLBP1 (X1) (lane 5), or SLBP2 (X2) (lane 6) was added to the depleted extract. The extracts were incubated for 30 min at 27°C, and RNA was prepared and analyzed by electrophoresis on 8% polyacrylamide–7M urea gels.