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 h istone gene promoters. Many of the Oct motifs are paired with a distinct mo tif, Hex, TCA or CCAAT-box, constituting the type I element (CCACGTCANCGATC CGCG), type II element (TCACGCGGATC) and type III element (GATCCGCG-N14-ACC AATCA). To clarify the roles of these Oct-containing composite elements (OC Es) in cell cycle-dependent and tissue-specific expression, we performed ga in-of-function experiments with transgenic tobacco cell lines and plants ha rboring a derivative of the 35S core promoter/ beta-glucuronidase fusion ge ne in which three or four copies of an OCE had been placed upstream. Althou gh their activities were slightly different, results showed that each of th e three types of OCEs could confer the ability to direct S phase-specific e xpression on a heterologous promoter. In transgenic plants, the type I and III elements exhibited a similar activity, directing expression in meristem atic tissues, whereas the activity of the type II element appeared to be re stricted to young cotyledons and maturating guard cells. Mutational analyse s demonstrated that the co-operation of Oct with another module (Hex, TCA o r CCAAT-box) was absolutely required for both temporal and spatial regulati on. Thus, OCEs play a pivotal role in regulation of the expression of plant histone genes.