Identification of cis-elements regulating the expression of an Arabidopsis histone H4 gene.

Protein-DNA interactions in the proximal region of an Arabidopsis H4 histone gene promoter were analyzed by DMS in vivo footprinting combined with LMPCR amplification. Interactions were identified over six particular sequence motifs, five of which were previously shown to bind proteins in maize histone H3 and H4 promoters and are commonly found in the corresponding regions of other plant histone gene promoters. These motifs are located within a 126 bp fragment which was previously shown to confer preferential expression in meristems of transgenic plants. The contribution of each cis-element to the overall expression level and specificity was investigated by testing individual or combined mutations in transgenic Arabidopsis plants. All five motifs behaved as positive cis-elements of unequal strength. The GCCAAT-like sequence GCCACT behaved as a strong positive cis-element but had no influence on the specificity. In contrast, the nonamer AGATCGACG and to a lesser extent the closely linked hexamer CCGTCG proved to be essential for meristem-specific expression. Involvement of the highly conserved histone-specific octamer CGCGGATC in specific expression was revealed at some stages of meristem development. Importance of these three cis-elements, nonamer, hexamer, and octamer, was further confirmed by the fact that combining mutations of two of them either abolished the promoter activity or completely modified the promoter specificity. Mutation of the fifth cis-element, a degenerate copy of the octamer, little perturbed the promoter function. However disruption of both octamers had a dramatic negative effect, thus suggesting that the two copies cooperate to achieve maximal function in the wild-type promoter, possibly by mobilizing the proliferation-specific factors binding to the nonamer and CCGTCG cis-elements.