The organization of the histone genes in the genome of Xenopus laevis.

We have studied the organization of the histone genes in the DNA from several individuals of Xenopus laevis. For that purpose, Southern blots of genomic DNA, that was digested with several restriction enzymes, were hybridized with radioactively labeled DNA fragments from clone X1-hi-1 (14), containing genes for Xenopus histones H2A, H2B, H3 and H4. In the DNA of all animals that were screened we found a major repeating unit of 14 kilobasepairs, which contains genes for histones H2A, H2B, H3 and H4 (H1 not tested) and is represented up to 30 times in the genome. The order of the genes in this major repeating unit is H4 - H3 - H2A - H2B. This order is different from that in the histone DNA of clone X1-hi-1, i.e. H3 - H4 - H2A - H2B. In addition to the genes in the major repeating unit, histone genes are present in unique restriction fragments in numbers that vary from one animal to another. The restriction patterns for the histone genes in these unique fragments were found to be different for all eight Xenopus individuals that were screened. The cloned Xenopus histone gene fragment X1-hi-1 represents such a unique fragment and is not present in the DNA of each single individual. The total number of genes coding for each of the nucleosomal histones is 45-50 per haploid genome.