Fig. 7. In activated primary B cells, OBF-1 protein levels are elevated at the post-transcriptional level. Mice (C57Bl6) were immunized with 150 µg of DNP-KLH adsorbed to aluminium hydroxide, and 10 days later splenic B cells were isolated and centrifuged over Percoll step gradients. High density (HD) and low density (LD) B cells were isolated, and protein and RNA prepared simultaneously. Levels of OBF-1 protein were determined by western blot analysis with an anti-OBF-1 antibody. OBF-1 RNA was measured by northern blotting, also using a probe for β-actin as control. The level of Siah-1 RNA was determined by RNase protection assay, using a probe for the ribosomal protein S16 as control. A control hybridization with yeast RNA is also shown. In the lower part, a Phosphorimager quantification of these data is presented, using the level of S16 RNA for normalization.