The Nucleotide Sequence of the Repeating Unit in the Oocyte 5S Ribosomal DNA of Xenopus laevis

To understand a gene from a developmental perspective, it is necessary to identify DNA sequences which influence the quantity and timing of its expression and, eventually, to elucidate the underlying molecular control mechanisms. In tandemly redundant gene clusters, such as the 5S ribosomal RNA genes, the repeating unit of DNA is longer than the transcription unit. The extra sequences, designated “spacers,” probably contain important regulatory signals. The periodicity of the DNA clearly delimits the DNA sequences associated with a single transcription unit, facilitating the task of locating and studying potentially significant extragenic sequences in the immediate vicinity of the gene. Here we summarize the results of nucleotide sequencing studies on the repeating unit of DNA containing the oocyte 5S rRNA genes (5S DNA) in Xenopus laevis. We discuss some evolutionary and functional implications of the sequence organization observed in 5S DNA.