In higher plants, the analysis of ferritin gene expression has been developed into a model to understand the mechanism of Fe-regulated expression (Briat and Lobreaux, 1997; Hell and Stephan, 2003). Phytoferritin genes are regulated mainly at the transcriptional level, in contrast to IRE-based regulation in vertebrates. Two groups of researchers have conducted deletion analyses of phytoferritin gene promoters using transient assay systems, leading to the identification of two types of cis-acting elements that derepress the expression of phytoferritin genes via Fe loading. Wei and Theil (2000) showed that an 86-bp fragment (Fe regulatory element; FRE) controls the Fe-mediated derepression of the soybean ferritin gene. Petit et al. (2001) carried out more precise analyses using maize and Arabidopsis ferritin gene promoters. They determined the 14-bp cis-acting sequence (iron-dependent regulatory sequence; IDRS) by deletion and mutation analysis of the maize ferritin promoter, combined with gel-retardation assays. The IDRS was found to be conserved in the promoter of the Arabidopsis ferritin gene (AtFer1), and the AtFer1 IDRS was shown to be functional both in transient assays with Arabidopsis culture cells and in transgenic Arabidopsis plants. The IDRS and the FRE were not homologous to each other. Nitric oxide was found to be necessary for mediating the transcriptional regulation by the IDRS (Murgia et al., 2002). On the other hand, the tissue-specific or developmental expression of phytoferritin genes was suggested to be regulated by other unknown cis-acting elements (Tarantino et al., 2003).
The finding that the disruption of a bHLH transcriptional regulator (FER) accounts for the Fe inefficiency of the tomato mutant T3238fer (Ling et al., 2002) suggests that the consensus binding site of bHLH transcription factor (E-box; CANNTG) is a candidate for plant cis-acting elements regulating Fe-deficiency-responsive expression. However, to date no experimental analyses have been reported as to whether certain E-box sequences in a given promoter possess a function in Fe-deficiency-responsive expression or binding capacity to the FER protein.
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