FIG. 8.

Proteasomal inhibition affects pre-rRNA processing, and impairs the production of mature 28S. (a) ³²P metabolic labeling. Proteasome inhibition has modest effects on the levels of rRNA gene transcription (compare the levels of 47S in control and MG-132-treated cells), leads to reduced formation of 36S/32S pre-rRNA, and impaired production of mature 28S and 18S rRNA. (b) Quantification of the ³²P-labeled 47S and 36/32S pre-rRNAs and ³²P-labeled mature 28S and 18S rRNAs corresponding to the representative experiment shown in panel a. (c) iFRAP experiments on an anti-28S rRNA oligonucleotide microinjected in control and MG-132-treated HeLa cells reveal a slower loss of 28S rRNA fluorescence from the nucleolus after proteasome inhibition. This indicates impaired production and export of mature 28S rRNA. Curves: black, control iFRAP; red, iFRAP after MG-132 treatment (± the SE). Scale bars, 2 μm.