MAPPING OF TRANSCRIPTION INITIATION AND TERMINATION SIGNALS ON XENOPUS-LAEVIS RIBOSOMAL DNA

Cloned derivatives of X. laevis ribosomal genes were injected into X. laevis oocyte nuclei and examined the resulting transcription complexes in the EM. The promoter lies somewhere within a region between -320 nucleotides upstream and +113 nucleotides downstream from the site of transcription initiation. This assignment agrees with inferences based on sequence conservation. The duplicated initiation region sequences located further out in the spacer (Bam islands) may not be required for the normal high densities of RNA polymerase loading seen on ribosomal genes. Concerning termination, the cluster of 4 T that forms part of the HindIII restriction site at the 3'' end of the gene appears to be part of the normal termination signal. Termination still occurs when only 3 T are present, but reduction to 2 T damages termination. Because clusters of 3 T appear at several sites within the gene, sequences adjacent to the T cluster also are probably required for normal termination. A fail-safe termination site is evidently present just upstream from the site of transcription initiation.