Reiteration Frequency of the Histone Genes in the Genome of the Amphibian, Xenopus laevis

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Abstract
Radioactive RNA was obtained from Xenopus laevis gastrula‐stage embryos which had been labelled by microinjection of [3H]uridine. A 9–12‐S RNA fraction was isolated by phenol extraction of whole embryos followed by sucrose gradient centrifugation of the RNA and polyacrylamide gel electrophoresis. This RNA was identified as histone mRNA on the basis of its chemical and physical properties. On polyacrylamide gels the labelled RNA could be subfractionated into different components with electrophoretic mobilities similar to those of histone mRNAs of the sea urchin. 9 ‐ 12‐S RNA lacked poly(A) sequences. In a protein synthesis system in vitro the RNA fraction directed the synthesis of the histones H4, H2A, H2B, H3 and, possibly, of H 1 protein.The Xenopus histone mRNA was challenged with Xenopus DNA under conditions of vast DNA excess. A cot1/2 of between 110 and 250 mol · s · 1−1 was obtained from the RNA trace curve, indicating that the histone genes were 20–50‐fold reiterated in the haploid genome of the species. This value indicates that although amphibians have several embryological and morphological features in common with sea urchins, the reiteration frequency for Xenopus histone genes is closer to the value of 10–20 established for mammals than the value of 400–1000 determined for sea urchins.