The genome of higher eukaryotes contains highly repeated DNA sequences including long and short interspersed repeated sequences (LINEs and SINEs, respectively) dispersed in between unique gene sequences throughout the genome (Singer 1982). From the sequence analyses of a number of repetitive sequences, it was suggested that one of the mechanisms involved in the transposition and amplification of repetitive sequences is the dispersal by the integration of reverse transcripts of RNA products into the genomic DNA (Rogers 1985). Several structural features of repetitive sequences suggesting the "retroposition hypothesis" are (i) the presence of the internal split promoter (blocks A and B) for RNA polymerase III in Alu and B2 sequences and the RNA polymerase II promoter in LI, (ii) the presence of transcripts containing these sequences in various tissues, especially in embryonic and germ-line cells (Bachvarova 1988;Skowronski and Singer 1985), (iii) the presence of oligo(dA) stretch at the 3' end, and (iv) the short direct repeats flanking the entire repetitive sequences including oligo(dA) tail, which might have been generated by the target site duplication upon the insertion of repetitive elements.
Recently, we have isolated a genomic clone containing rat histone somatic H2A (sH2A), H2B, and H3 genes in a 9.5-kb insert from a rat genomic HaeIII library established in Charon 4A (Choi and Chae 1993). In this study, we report an approximately 2-kb sequence downstream of sH2A gene containing a B2-L1 composite and histone H3 gene. As shown in Fig. 1, histone H3 gene is located 1514 nucleotides downstream from, and in the same orientation as, sH2A gene. Sequence comparison of rat histone H3 gene with four mouse histone H3 genes revealed the highest homology with the mouse H3.1-221 gene (Taylor et al. 1986). There are 27 base differences within the coding regions of the two H3 genes, and all of them are silent changes. The The nucleotide sequence data reported in this paper have been submitted to GenBank and have been assigned the accession number L19706.