ASSEMBLY, NUCLEAR IMPORT AND FUNCTION OF U7 SNRNPS STUDIED BY MICROINJECTION OF SYNTHETIC U7 RNA INTO XENOPUS OOCYTES

In Xenopus oocytes in vitro transcribed mouse U7 RNA is assembled into small nuclear ribonucleoproteins (snRNPs) that are functional in hist one RNA 3' processing, If the special Sm binding site of U7 (AAUUUGUCU AG, U7 Sm WT) is converted into the canonical Sm sequence derived from the major snRNAs (AAUUUUUGGAG, U7 Sm OPT) the RNA assembles into a pa rticle which accumulates more efficiently in the nucleus, but which is non-functional, U7 RNA with a heavily mutated Sm binding site (AACGCG UCAUG, U7 Sm MUT) is deficient in nuclear accumulation and function, B y UV cross-linking U7 Sm WT RNA can be linked to three proteins, i.e. the common snRNP proteins G and B/B' and an apparently U7-specific pro tein of 40 kDa, As a result of altering the Sm binding site, U7 Sm OPT RNA cannot be cross-linked to the 40 kDa protein and no cross-links a re obtained with U7 Sm MUT RNA, The fact that the Sm site also interac ts with at least one U7-specific protein is so far unique to U7 RNA an d may provide an explanation for the atypical sequence of this site, A ll described RNA-protein interactions, including that with the 40 kDa protein, already occur in the cytoplasm, An additional cytoplasmic pho toadduct obtained with U7 Sm WT and U7 Sm OPT, but not U7 Sm MUT, RNAs is indicative of a protein of 60-80 kDa, The m7G cap structure of U7 Sm WT and U7 Sm OPT RNA becomes hypermethylated, However, the 3mG cap enhances, but is not required for, nuclear accumulation, Finally, U7 S m WT RNA is functional in histone RNA processing even when bearing an ApppG cap.