Regulatory mechanisms of gene expression through the interactions of transcription factors in plants

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Focusing on the transcription factors, HBP-1a and HBP-1b, that bind to the hexamer sequence in the type I element which defines the S phase-specific transcription of a wheat histone H3 gene (TH012), we investigated the structures and functions of the members of the HBP-1 transcription factor family and other transcription factors that interact with these members. Within a research term, we obtained the following results.1. The members of the HBP-1 family differ in their ability of the dimer formation and their affinity for DNA-binding. This finding suggests a possibility that the HBP-1 family members would control the gene expression by changing the partners of the members to from homo- or hetero-dimers through the bZIP domain.2. A DNA-binding protein (OBRF) was identified which specifically interacts with an octamer sequence, another cis-sequence within the type I element, by the DNA mobility shift assay of the nuclear extract from wheat cultured cells. Although OBRF was labile, its DNA-binding activity was the highest at the S phase of cell cycle and parallel with the level of the histone H3 gene expression through the cell cycle.3. A cDNA clone encoding the protein, termed HALF1, was isolated which associates with HBP-1a(17) through the protein-protein interactions by the West-western screening method. Sequence analysis of the cDNA clone revealed that an overall structure of HALF1 resembles that of the HBP-1a subfamily. The DNA-binding experiments showed that HALF1 can not bind to DNA (hexamer sequence) as a homodimer but interact with DNA as a heterodimer containing HBP-1a(17). The results suggest that HALF1 may be involved in the H3 gene regulation through the interaction with the members of the HBP-1a subfamily.