Histone‐Gene Reiteration in the Genome of Mouse

Histone messenger RNA labelled to high specific activity has been isolated and purified from the mouse tissue‐culture cell line L929. It had an electrophoretic mobility equivalent to a mean molecular weight of 1.4 × 10 5 . The synthesis of this RNA was suppressed, but apparently not completely so, by inhibition of cellular DNA synthesis to more than 95% by addition of cytosine arabinoside. The synthesis of other RNA species remained unaffected by this treatment. Poly(A)‐containing mRNA, representing a 20% contaminant of the electrophoretically purified histone mRNA, was removed by poly(U)‐Sepharose chromatography. Histone mRNA thus purified was hybridized to DNA fractions enriched for historic genes of the sea urchin Psammechinus miliaris by actinomycin/CsCI gradient centrifugation. The mRNA was eluted from the hybrids and challenged with mouse DNA in vast DNA excess conditions. A c 0 t 1/2 of 360 mol s1 −1 was obtained from the RNA trace curve, suggesting a 10–20‐fold reiteration of the histone genes in the haploid genome. Thus historic genes in mouse are much less highly reiterated than in sea urchins, but nevertheless are present in considerable excess over the number theoretically necessary for histone synthesis during the S phase of the cell cycle.