Studies on the repetitive sequence transcripts of Xenopus oocytes

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RNA of the mature Xenopus oocyte was hybridized with an isolated repetitive DNA fraction of the Xenopus genome representing about 40% of the DNA and containing a major component with an average sequence-repetition frequency of about 1600. Ribonuelease-resistant hybrids which bind to hydroxyapatite represent the most stable fraction of the repetitive RNA-DNA duplexes. These hybrids were studied with CsCl equilibrium sedimentation, gel electrophoresis, and thermal chromatography from hydroxyapatite. It was concluded that after ribonuclease treatment the hybrids which bind to hydroxyapatite retain relatively long regions of duplex structure (over 10 2 nucleotides) but that, even so, they include significantly more nucleotide mismatch than do non-repetitive DNA-RNA hybrids formed with the same RNA. Hybrids were also detected by Cs 2SO 4 buoyant-density sedimentation in the material failing to bind to hydroxyapatite after ribonuclease treatment. These hybrids apparently retain only short duplex regions. RNA-driven hybridization reactions were carried out to obtain an estimate of the amount of repetitive DNA related in sequence to the RNA stored in the mature oocyte. In this procedure, the hybrids are not exposed to ribonuclease before binding to hydroxyapatite, and hybrids representing about 3.5% of the repetitive DNA were obtained. It was concluded that 2 to 5% of the total oocyte RNA is repetitive sequence transcript and that these transcripts are present in the oocyte at a much higher concentration than non-repetitive sequence transcripts.