Structure and differential expression of two maize ferritin genes in response to iron and abscisic acid

In plants, synthesis of the iron-storage protein ferritin in response to iron is not regulated at the translational level; this is in contrast to ferritin synthesis in animals. Part of the response is mediated through a transduction pathway which involves the plant hormone abscisic acid. In this work, we report the cloning and sequencing of two maize ferritin genes (ZmFer1 and ZmFer2) coding for members of the two ferritin mRNA subclasses, FM1 and FM2, respectively. Although plant and animal ferritins are closely related proteins, a major difference is observed between the organisation of the genes. Both maize ferritin genes are organised as eight exons and seven introns, the positions of which are identical within the two genes, while animal ferritin genes are interrupted by three introns, at positions different from those found in maize genes. Sequence divergence between the 3' untranslated regions of these genes has allowed the use of specific probes to study the accumulation of FM1 and FM2 transcripts in response to various environmental cues. Such probes have shown that FM1 and FM2 transcripts accumulate with differential kinetics in response to iron; FM1 mRNA accumulate earlier than FM2 mRNA and only FM2 transcripts accumulate in response to exogenous abscisic acid or water stress. Mapping of the transcriptional initiation region of these two genes defined their 5' upstream regions and allowed a sequence comparison of their promoters, which appeared highly divergent. This raises the possibility that the differential accumulation of FM1 and FM2 mRNAs in response to iron, abscisic acid and drought could be due to differential transcription of ZmFer1 and ZmFer2.