Functional difference between the sites of ribosomal 40S precursor 3' end formation in Xenopus laevis and Xenopus borealis.

In the ribosomal genes of X. laevis, the sequence GACTTGCNC is found about 60bp upstream of the gene promoter (T3) and is necessary and sufficient to cause termination of RNA polymerase I transcription. At the 3' end of the 40S precursor coding region (T2) a sequence differing by one nucleotide, GACTTGCNG, directs RNA 3' end formation but allows polymerase to transcribe on into the intergenic spacer (Labhart and Reeder, 1989, Genes and Dev. 4: 269-276). Sites corresponding to T2 and T3 are also found in a related species, X. borealis. Inspection of the T2 sequence in X. borealis reveals that it contains two copies of the terminator sequence, GACTTGCNC, located 15 and 96 bp downstream of the 3' end of the 40S precursor coding region. Here we present functional tests of those two T2 elements that show that, as predicted from the sequence, they both show termination activity and are functionally indistinguishable from the T3 site in X. laevis. These results suggest that X. laevis T2 is an example of a naturally occurring point mutation, and the inability to terminate transcription at T2 is an exception to the general pattern of ribosomal gene transcription in higher eukaryotes.