Fig. 1. The localization pattern of 5' ITS1 from the small ribosomal subunit export assay. After the small ribosomal subunit is exported from the nucleus, the 5' ITS1 RNA is cleaved away from the 18S rRNA in the cytoplasm and then degraded by the exonuclease Xrn1. Cells lacking XRN1 accumulate 5' ITS1 in the cytoplasm as long as ribosome assembly and export occurs (a). A xrn1 mutant defective in small ribosomal subunit assembly accumulates 5' ITS1 to the nucleolus, the site of rRNA transcription and processing (c). 5' ITS1 also localizes to the nucleolus of wild-type XRN1⁺ cells (d). A mutant defective in small ribosomal subunit nuclear export accumulates 5' ITS1 throughout the nucleoplasm in both xrn1 (b) and XRN1⁺ strain backgrounds (e). Defects in ribosome assembly cannot be detected in XRN1⁺ strains because 5' ITS1 remains localized to the nucleolus.

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