The organisation and expression of histone genes from Xenopus borealis

We have isolated genomic clones from Xenopus borealis representing 3 different types of histone gene cluster. We show that the major type (H1, H2B, H2A, H4, H3), present at about 60-70 copies per haploid genome (1), is tandemly reiterated with a repeat length of 15 kb. In situ hybridization to mitotic chromosomes shows that the majority of histone genes in Xenopus borealis are at one locus. This locus is on the long arm of one of the small sub-metacentric chromosomes. A minor cluster type with the gene order H1, H3, H4, H2A is present at about 10-15 copies. The genome also contains rare or unique cluster types present at less than 5 copies having other types of organisation. An isolate of this type had the gene order H1, H4, H2B, H2A, H1 (no H3 cloned). Microinjection of all of the clones into Xenopus laevis oocyte nuclei shows that most of the genes present are functional or potentially functional and a number of variant histone proteins have been observed. S1 mapping experiments confirm that the genes of the major cluster are expressed in all tissues and at all developmental stages examined.