STRUCTURE AND DIFFERENTIAL EXPRESSION OF 2 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 contra st to ferritin synthesis in animals. Part of the response is mediated through a transduction pathway which involves the plant hormone abscis ic acid. In this work, we report the cloning and sequencing of two mai ze ferritin genes (ZmFer1 and ZmFer2) coding for members of the two fe rritin mRNA subclasses, FM1 and FM2, respectively. Although plant and animal ferritins are closely related proteins, a major difference is o bserved between the organisation of the genes. Both maize ferritin gen es are organised as eight exons and seven introns, the positions of wh ich are identical within the two genes, while animal ferritin genes ar e interrupted by three introns, at positions different from those foun d in maize genes. Sequence divergence between the 3' untranslated regi ons of these genes has allowed the use of specific probes to study the accumulation of FM1 and FM2 transcripts in response to various enviro nmental cues. Such probes have shown that FM1 and FM2 transcripts accu mulate with differential kinetics in response to iron; FM1 mRNA accumu late earlier than FM2 mRNA and only FM2 transcripts accumulate in resp onse to exogenous abscisic acid or water stress. Mapping of the transc riptional initiation region of these two genes defined their 5' upstre am regions and allowed a sequence comparison of their promoters, which appeared highly divergent. This raises the possibility that the diffe rential accumulation of FM1 and FM2 mRNAs in response to iron, abscisi c acid and drought could be due to differential transcription of ZmFer 1 and ZmFer2.