Tissue-dependent enhancement of transgene expression by introns of replacement histone H3 genes of Arabidopsis

Intron-bearing replacement histone H3 genes in Arabidopsis and other plants are highly and constitutively expressed. We demonstrate that the introns l ocated within the 5'-untranslated regions (5'-UTR) of the two Arabidopsis r eplacement H3 genes will abolish the cell cycle dependence of an endogenous histone H4 promoter. We demonstrate that these introns, functionally combi ned with their endogenous promoters, could produce the high and constitutiv e expression of the replacement H3 genes observed in planta. They strongly increase gene expression whatever the promoter, from the strong 35S CaMV pr omoter to complete and resected promoters of cell cycle-dependent and repla cement histone genes. Quantitative analysis of the extent of reporter gene enhancement in different parts of developing transgenic plantlets, ranging from 2-fold to 70-fold, supports the notion that trans-acting factors are r esponsible for this effect. Such factors appear most abundant in roots.