The specific 3′ termini of a number of eukaryotic mRNAs have been shown to be generated by the post-transcriptional processing of primary transcripts or pre-mRNAs1-4. The sequence AAUAAA, present in the 3′ region of nearly all eukaryotic mRNAs, appears to be involved in the cleavage and subsequent polyadenylation of the primary transcript5. An exception to this general rule is the case of the histone mRNAs, which lack the AAUAAA sequence and are not normally polyadenylated. Histone mRNAs do, however, contain a highly conserved 23 base pair sequence at their 3′ termini6, which is required for correct 3′ end formation 7. The similarity between this conserved sequence, which can be drawn as a hairpin loop, and bacterial transcription terminators8 has led several investigators to suggest that the specific 3′ end of histone mRNA is formed by termination of transcription9,10. So far, however, experimental results have not been presented which make it possible to distinguish between a post-transcriptional processing or a transcription termination mechanism for the formation of histone mRNA 3′ termini. We have investigated this issue by synthesizing in vitro unprocessed histone pre-mRNAs that extend past the normal 3′ terminus. These in vitro synthesized pre-mRNAS were injected into frog oocyte nuclei to study their fate. The results demonstrate that correct 3′ ends of chicken histone H2B mRNAs can be formed by RNA processing of longer synthetic pre-mRNAs.
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