Transcription termination and processing of transcripts from tRNA-related Xenopus satellite DNA sequences.

Nucleotide sequence homologies have been found between the transcriptional unit of a Xenopus laevis satellite DNA containing 1037-bp monomeric repeats and tRNA-coding genes. Since the tRNA-homologous region forms only part of the transcriptional unit, we have analyzed the size of transcripts being made after injection of cloned DNA into Xenopus oocyte nuclei. Incubation for 6 h mainly results in three RNA species comprising 130, 180 and 196 nucleotides, which originate at the same initiation site but terminate at different T stretches. Longer incubation reveals a complex pattern of smaller RNAs most likely resulting from 3'-end processing of primary transcripts. All these RNAs are related to tRNA, since they contain the tRNA-homologous region which is close to the 5' end of the transcription unit.

Satellite DNA from Xenopus laevis: comparative analysis of 745 and 1037 base pair Hind III tandem repeats.

Highly repetitive Hind III restriction fragments of 0.72-0.76 KBP from total Xenopus laevis genomic DNA are organized in a tandem like arrangement. Cloning of these fragments in pBR 322 with subsequent restriction site mapping and nucleotide sequence analysis of some selected clones showed two different types of sequences. 25-30% of material represent the oocyte specific 5 S DNA repeat units, 70-75% are similar to the recently described repeat elements of satellite 1 DNA. Hybridization of a genomic DNA library to such a 745 BP monomeric repeat unit and investigation of some clones with positive autoradiographic signals revealed structural heterogeneities of repeat elements, in that the 745 BP sequence cross-hybridized with 1037 BP Hind III repeat units. Nucleotide sequence analysis demonstrated that the two types of sequences show a homology of 84.3% and that the 1037 BP sequence additionally contains duplicated elements of the 745 BP sequence as well as apparently unrelated DNA sequences.