Transcription of single-copy vitellogenin gene of Xenopus involves expression of middle repetitive DNA.

Abstract

The genome of eukaryotes consists of single-copy DNA interspersed with middle repetitive DNA. Both types of DNA are known to be transcribed (reviewed in ref. 1). Several reports have shown that some nuclear RNA is composed of single-copy DNA transcripts covalently linked to sequences transcribed from repetitive DNA2–4, but the significance of these nuclear transcripts is unknown; there is no convincing evidence that these nuclear transcripts containing repetitive DNA sequences are precursors to mRNAs. To elucidate this question we have analysed the structure of the two related vitellogenin genes A1 and A2 of Xenopus laevis5,6. The expression of these genes is initiated in liver cells by oestrogen (reviewed in refs 7–9). Preliminary experiments demonstrated that the flanking region as well as several introns of these genes contain middle repetitive DNA9. We have now identified and mapped six different repetitive DNA sequences within the transcribed introns of the A1 vitellogenin gene. Further characterization of these sequences implies that they are evolutionary unstable. Analysis of nuclear RNA revealed that transcripts of the six sequences are also found in non-vitellogenic liver cells, suggesting that these repetitive DNAs are also present in transcriptional units active in the absence of oestrogen.