C.A. Newton

We have isolated several clones containing Xenopus histone genes from genomic libraries of X. laevis and X. borealis DNA. Each genomic clone has been mapped and the positions of 26 histone genes in seven laevis clones and 5 histone genes in one borealis clone have been determined. In laevis, the histone gene clusters show considerable variation in gene order within a single individual. When the cloned DNAs were microinjected into the nucleus of Xenopus oocytes, expression of cloned genes at the transcriptional and translational level was readily detectable. Previously unknown histone variants were revealed by the microinjection experiments.

We have assessed the response of many histone H3 mRNAs and an H1C mRNA in Xenopus tissue culture cells after treatment with the DNA synthesis inhibitor hydroxyurea. The amount of the histone mRNAs falls rapidly in response to the inhibitor. This response is prevented by cycloheximide. Cloned Xenopus histone genes were transfected into mouse cells and a cell line was obtained in which the Xenopus genes were actively expressed giving rise to mRNA with correct 5'-termini. The Xenopus genes were correctly regulated at the level of mRNA amounts in the mouse cell line. Nuclear microinjection experiments with Xenopus oocytes and S1 nuclease analysis of normal ovary RNA showed that the H1C gene, and probably also two H3 genes, which are replication-dependent in somatic cells are expressed in oocytes and are therefore replication-independent in this cell type. The same promoters are used in both replication-dependent and independent expression.