The structure of cloned histone genes of Xenopus borealis
I have constructed a genomic gene library of EcoRI fragments from Xenopus borealis DNA cloned in lambda-gtWES. The library contains 425,000 phage with a mean insert si ~ of 6.1kb. I have screened this library for histone genes and for a family of repetitive DNA sequences. I have isolated one histone gene clone from the library (Xbhl). It is overabundant in the library, so no other histone gene clones are isolatable. The cloned DNA fragment is 8.55kb long and contains one copy of each histone gene in the order H4-H2A-H2B-HI-H3 H2A is transcribed in the opposite direction to the other four genes. There is a short repetitive element between H2A and H2B present in 5000 copies in the genome, and an inverted pair of larger elements 3' to H2B present in/BOOO copies. I have isolated clones containing homologues to the latter and characterised them and homologous genomic sequences: it is a member of a dispersed, diverse family of elements with some sequence homology to his tome mRNA and present in anurans and urodeles. Xbhl is a minor species in the genome, but is homologous to the major histone gene cluster in the H4-H2A-H2B gene region. Its overall sequence is only 1-2% divergent from three histone gene clones isolated from X.laevis, which shared an ancestor with X.borealis 8 million years ago. The H4, H2A, H1 and H3 genes of Xbhl direct the production of normal proteins when Xbhl is microinjected into oocytes. The H3 gene codes a protein 96% homologous to Calf H3,and contains codons conserved in all H3 genes sequenced. The HI gene codes for a protein identical in 16 out of 21 central amino acids to an HI from X.laevis and homologous to calf and salmon HI proteins. It is possible that the H2B gene is a pseudogene.