Characterization of histone genes isolated from Xenopus laevis and Xenopus tropicalis genomic libraries

Using a cDNA clone for the histone H3 we have isolated, from two genomic libraries of Xenopus laevis and Xenopus tropicalis, clones containing four different histone gene clusters. The structural organization of X. laevis histone genes has been determined by restriction mapping, Southern blot hybridization and translation of the mRNAs which hybridize to the various restriction fragments. The arrangement of the histone genes in X. tropicalis has been determined by Southern analysis using X. laevis genomic fragments, containing individual genes, as probes. Histone genes are clustered in the genome of X. laevis and X. tropicalis and, compared to invertebrates, show a higher organization heterogeneity as demonstrated by structural analysis of the four genomic clones. In fact, the order of the genes within individual clusters is not conserved.