Identification of three kinds of mutually related composite elements conferring S phase-specific transcriptional activation

Conservation of the Oct motif (CGCGGATC) is a remarkable feature of plant histone gene promoters. Many of the Oct motifs are paired with a distinct motif, Hex, TCA or CCAAT-box, constituting the type I element (CCACGTCANCGATCCGCG), type II element (TCACGCGGATC) and type III element (GATCCGCG-N14-ACCAATCA). To clarify the roles of these Oct-containing composite elements (OCEs) in cell cycle-dependent and tissue-specific expression, we performed gain-of-function experiments with transgenic tobacco cell lines and plants harboring a derivative of the 35S core promoter/beta-glucuronidase fusion gene in which three or four copies of an OCE had been placed upstream. Although their activities were slightly different, results showed that each of the three types of OCEs could confer the ability to direct S phase-specific expression on a heterologous promoter. In transgenic plants, the type I and III elements exhibited a similar activity, directing expression in meristematic tissues, whereas the activity of the type II element appeared to be restricted to young cotyledons and maturating guard cells. Mutational analyses demonstrated that the co-operation of Oct with another module (Hex, TCA or CCAAT-box) was absolutely required for both temporal and spatial regulation. Thus, OCEs play a pivotal role in regulation of the expression of plant histone genes.     

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 (CCACG-TCA) 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).     

Individual Xenopus histone genes are replication-independent in oocytes and replication-dependent in Xenopus or mouse somatic cells.

We have assessed the response of many histone H3 mRNAs and an H1C mRNA in Xenopus tissue culture cells after treatment with the DNA synthesis inhibitor hydroxyurea. The amount of the histone mRNAs falls rapidly in response to the inhibitor. This response is prevented by cycloheximide. Cloned Xenopus histone genes were transfected into mouse cells and a cell line was obtained in which the Xenopus genes were actively expressed giving rise to mRNA with correct 5'-termini. The Xenopus genes were correctly regulated at the level of mRNA amounts in the mouse cell line. Nuclear microinjection experiments with Xenopus oocytes and S1 nuclease analysis of normal ovary RNA showed that the H1C gene, and probably also two H3 genes, which are replication-dependent in somatic cells are expressed in oocytes and are therefore replication-independent in this cell type. The same promoters are used in both replication-dependent and independent expression.