Molecular Mechnisms of Meristematic Tissue-Specific Expression of Plnat Histone Genes Introduced by a Ti-Plasmid Vector, Especially concerning Cis-Acting Control Elements for the Initiation and Termination of

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During the last two years, in order to study the molecular mechanisms of the transcriptional regulation of the genes which are specifically experssed at the S phase during cell cycle, we have examined the cis- and trans-acting Control elements of the histone genes using the cloned wheat histone H3 and H4 genes. To examine the transcriptional activity of the histone gene, the Ti-plasmid mediated gene transfer system was used; the wheat histone genes were introduced into sunflower hypocotyl cells via Ti-plasmid and the transcripts of the wheat genes in transformed cells were measured by the S1 assay.The analysis of the transcriptional activities of the 5' deletion mutants of the H3 and H4 genes revealed that some cis-elements promoting a high efficiency of the transcription are located between the positions -668 and 191 and that the hexameric sequence just upstream from the octameric sequence which exists in all plant histone genes examined so far seemd to have some connection with the transcriptional regulation. On the other hand, the octameric sequence was not shown to be a motif to regulate the transcription of the histone genes.Several experiments using the gel retardation assay, methylation interference assay and DNase I footprint method have demonstrated that there exist nuclear protein(s) specifically binding to the hexameric sequence in wheat cells. The nuclear protein Which was tentatively termed HBP-1 are more abundantly contained in germinating than dormant germs. Furthermore, in wheat seedling in which the cell cycle was partially synchronized by aphidicoline treatment, HBP-1 was persent in the highest concentration in the nuclear extract from an S phase-abundant cell population. These data have suggested that the hexameric sequence and HBP-1 are cis- and trans-acting control elements, respectively, involved in cell cycle-dependent transcription of the wheat histone genes.