TRANSCRIPTIONAL TERMINATION IN THE BALBIANI RING-1 GENE IS CLOSELY COUPLED TO 3'-END FORMATION AND EXCISION OF THE 3'-TERMINAL INTRON

We have analyzed transcription termination, 3'-end formation, and exci sion of the 3'-terminal intron in vivo in the Balbiani ring 1 (BR1) ge ne and its pre-mRNA. We show that full-length RNA transcripts are even ly spaced on the gene from a position 300 bp upstream to a region 500- 700 bp downstream of the polyadenylation sequence. Very few full-lengt h transcripts and no short, cleaved, nascent transcripts could be obse rved downstream of this region. Pre-mRNA with 10-20 adenylate residues accumulates at the active gene and then rapidly leaves from the gene locus. Only polyadenylated pre-mRNAs could be detected in the nucleopl asm. Our results are consistent with the hypothesis that transcription termination occurs in a narrow region for the majority of transcripts , simultaneous with 3'-end formation. Excision of the 3'-terminal intr on occurs before 3'-end formation in about 5% of the nascent transcrip ts. When transcription terminates, 3' cleavage takes place and 10-20 a denylate residues are added, the 3'-terminal intron is excised from ad ditionally about 75% of the pre-mRNA at the gene locus. Our data suppo rt a close temporal and spatial coupling of transcription termination and the cleavage and initial polyadenylation of 3'-end formation. Exci sion of the 3'-terminal intron is highly stimulated as the cleavage/po lyadenylation complex assembles and 3'-end formation is initiated.