A point mutation uncouples RNA 3'-end formation and termination during ribosomal gene transcription in Xenopus laevis

Two sites, T2 and T3, in the ribosomal gene spacer of Xenopus laevis both direct RNA 3'-end formation 15 bp upstream of the conserved box sequence GACTTGC. Site T2, which defines the 3' end of the 40S precursor, does not terminate transcription whereas site T3 at the 3' end of the spacer does. Here we show that T2 can be converted into a T3-like site with termination activity by a single point mutation 2 bp downstream of the T2 box. RNA 3'-end formation at T2 is unchanged by this mutation. Conversely, a point mutation 2 bp downstream of the T3 box inhibits termination without affecting 3'-end formation. Our results identify two separable events occurring at the 3' end of the ribosomal genes: (1) RNA 3'-end formation by processing and (2) transcription termination. The two processes are directed by two distinct, but overlapping, signals in the DNA sequence. Site T2 in X. laevis is damaged in the second process by a natural mutation.