We have located chicken histone genes within 14-kilobase pair inserts of two genomal clones, λCH-01 and λCH-02. The genes were mapped with chicken histone cDNA and gene-specific probes, and the identity and polarity of each gene confirmed by DNA sequencing. As previously indicated, histone mRNAs are transcribed from both DNA strands. The gene organization differs in the two clones. Although there is some clustering of genes, no repeating unit is observed. λCH-01 contains all core histone genes, but two copies of H2A occur before the full complement of histone genes is represented. λCH-02 contains no H4 or H2A genes, but notably has two adjacent H3 genes which are divergently transcribed. H2A, H2B and H3 genes all appear more than once in the combined inserts of the two clones, but in no case do they occur in equivalent environments. This indicates a considerable degree of disorder of the his tone genes in the chicken genome.
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