Fig. 2. CRM1 and YRB2 are required for nuclear export of the small ribosomal subunit. (A) 5' ITS1 accumulates in the nucleoplasm of a crm1 mutant. 5' ITS1 was localized in CRM1 (wild-type, b) and crml(T539C) (e) cells after treatment with leptomycin B for 15 minutes. Chromosomal DNA was labeled with DAPI to identify the location of the nucleus (a,d), and Nomarski optics were used to visualize cell morphology (c,f). Arrows point to DAPI-stained chromatin while arrowheads point to the nucleolus. A magnified view of a cell is shown below the lettered panels. (B) 5' ITS1 accumulates in the nucleoplasm of a yrb2 mutant. 5' ITS1 was localized in YRB2 (wild-type, b,f) and yrb2 (d,h) cells grown at 30°C (b,d) or shifted to the restrictive temperature of 15°C (f,h). Chromosomal DNA was labeled with DAPI to identify the location of the nucleus (a,c,e,g). Arrows point to DAPI-stained chromatin, while arrowheads point to the nucleolus. A magnified view of a cell is shown for e, f, g and h. (C) 20S pre-rRNA processing is delayed in yrb2 cells. YRB2 (lanes 1-6) and yrb2 (lanes 7-12) cells were shifted to 15°C for 24 hours, pulse-labeled for 5 minutes, and chased for 5, 10, 20, 40 or 60 minutes. RNA was extracted and run on a formaldehyde-agarose gel. Sizes of the rRNAs are indicated.

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