Molecular Mechanism of Regulation of Cell Cycle-Dependent Gene Expression in Higher Plants

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In order to elucidate the molecular mechanisms of S-phase-specific transcription regulation in the division cycle of higher plant cells, my co-workers and I have studied the structures and functions of cis-acting elements and trans-acting factors which are implicated in the regulation of the wheat histone H3 gene transcription. We have obtained the following results in this research.1. As cis-elements for the S-phase specificity of the H3 gene transcription we have identified two motifs (hexamer and octamer sequences) in three cis-elements located within the -185 region of the H3 promoter.2. We have identified the sequence-specific DNA-binding proteins, HBP-1a and HBP-1b, that are interact with the hexamer motif. The CDNA clones encoding each of proteins were isolated and their nucleotide sequences were determined. The amino acid sequences deduced from the nucleotide sequences suggested that the HBP-la and -1b have a bZIP motif at the regions near the amino- and carboxy-termini, respectively. We also have identified a DNA-binding protein, HBP-2, which specifically interacts with another cis-element, nonamer sequence, and this protein was shown to have a Zn-finger motif.3. To study the function of HBP-1a and -1b, we examined the influence of the effector gene consisting of the CaMV 35S promoter and HBP-1a or -1b EDNA on the expression of the reporter gene containing the H3 promoter/GUS coding region in the transient expression system using rice and tabacco cell cultures. We have obtained evidence that the proteins both are regulatory factors involved in the H3 gene transcription.4. We found the hexamer and hexamer-like motifs in the promoter regions of cloned HBP-1a and -1b genes and observed that the motifs can bind HBP-1a and -1b. It is suggested that transcription of these genes may be autoregulated by the factors themselves.