| Budget Amount *help |
¥17,000,000 (Direct Cost: ¥17,000,000)
Fiscal Year 2003: ¥7,900,000 (Direct Cost: ¥7,900,000)
Fiscal Year 2002: ¥9,100,000 (Direct Cost: ¥9,100,000)
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| Research Abstract |
Different RNA species are exported from the nucleus by distinct mechanisms. Among the different RNA species, major splicesomal U snRNA precursors and mRNAs share some similarities. They are both transcribed by RNA polymerase II, and both initially acquire m7G-cap structure. In addition, in both cases, there are no conserved internal RNA sequences required for nuclear export. Despite these similarities, export of these two RNAs is mediated by different factors, U snRNAs are exported by cooperation of CBC, PHAX, CRM1 and RanGTP, whereas export of mRNAs does not require these factors but depends upon the REF and TAP proteins. Thus there must be distinguishing features of U snRNAs and mRNAs that are recognised by these cellular factors. In this research project, we perform a systematic search for identity elements used in export of mRNAs. We transplanted the features of mRNAs into U1 snRNA one by one, and looked for elements of mRNA that make the U1 RNA behave like an mRNA in nuclear export process. We found that an mRNA-type introit is one such identity element. We demonstrate that U1 snRNA is exported like an mRNA on insertion of a pre-mRNA intron. mRNA-specific factors are recruited onto the spliced U1 RNA whereas U snRNA-specific factors are not. After export, spliced U1 RNA undergoes cytoplasmic maturation but is not re-imported into the nucleus. These data provide insight into mechanisms for discrimination of different classes of nuclear RNA and demonstrate that two RNAs of identical sequence can have distinct cytoplasmic fates depending their mode of export.
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