In-vivo degradation pathway of an excised intervening sequence

A study of the distribution in heterogeneous nuclear RNP (hnRNP) of the different forms of the first intervening sequence (IVSI) from the E3 transcription unit of adenovirus serotype 2 suggests a three-step pathway for IVS excision and degradation. First, the lariat IVS is formed in a large RNP of the size of premessenger RNP where it is resistant to nuclease attack. Second, the lariat IVS is released from this large structure as a small RNP where it is very susceptible to enzymic degradation. Third, the lariat IVS is linearized after the release of the small RNP. Our studies suggest that only the intact lariat and linear IVS are involved in the excision-degradation pathway in vivo, whereas the lariat nicked in its circular part and the IVS trimmed at its 3' end are not pre-existing but generated during isolation of hnRNA or hnRNP.