Organization and bidirectional transcription of H2A, H2B and H4 histone genes in rice embryos.

Our current understanding of the evolution of the histone gene family suffers from a lack of information on plant histone genes. With a view to gathering some much needed information on these genes, we studied a rice genomic clone in pBR322 carrying H2A, H2B and H4 histone genes on a DNA fragment of 6.64 kilobases (kb). A restriction map of the insert was constructed and the organization of the three genes on this insert was determined. H2A and H2B histone genes were located at one end of the insert and H4 gene at the other with a 3.1 kb spacer in between. This cluster of three histone genes was found to be transcribed in a bidirectional fashion with H2A and H2B genes being encoded by one strand and the H4 gene by the other. These results indicate that plant histone gene organization differs from that of the sea urchin, but shows many similarities to the systems in other animals.