Cooperation of two distinct cis-acting elements is necessary for the S phase-specific activation of the wheat histone H3 promoter

We have previously shown that the proximal promoter region (-185 to +57) of the wheat histone H3 gene (TH012) is sufficient for regulating S phase-specific expression of a reporter GUS gene. To define the cis-acting element(s) responsible for S phase-specific expression, GUS fusion genes under the control of wild-type or variously mutated H3 promoters were stably introduced into cultured rice Oc cells and their temporal expression was analyzed during the cell cycle by quantitative S1 analysis. The S phase-specific expression of the full-sized promoter (-1716 to +52) was significantly impaired by short internal deletions disrupting the type I element from -175 to -158 (CCACGTCACCaATCCGCG), composed of the Hex (CCACGTCA) and reverse-oriented Oct (GATCCGCG) motifs. Moreover, the H3 proximal promoters (-184 to +52) harboring base-substitution mutations in either or both of the Hex and Oct motifs could no longer activate gene expression during the S phase. These results indicate that the type I element is the first cis-acting element identified responsible for the S phase-specific expression of plant histone genes. Results also suggested the presence of a redundant cis-acting element(s) responsible for S phase-specific expression in the H3 far-upstream region (-1716 to -185).