Characterization and expression of a cDNA encoding a seed-specific metallothionein in maize.

Developing embryos of many higher plants undergo a maturation process in which a variety of storage proteins and putative dessication protectants are synthesized and stored. One of the proteins found to accumulate in maturing wheat embryos i s a Zn2+-associated class I1 metallothionein called E, (for early Cys) (Lane et al., 1987). Like many other maturation proteins, the promoter regions of wheat E, genes have been found to contain ABA-response elements, and their expression is ABA inducible (Kawashima et al., 1992). We have characterized a cDNA from maize (Zea mays L.), pMEC (maize EJ, whose sequence, regulation, and expression patterns indicate that it is the equivalent of wheat E, (Table I). Severa1 clones having strong homology with transcripts encoded by the wheat E, genes were isolated from a cDNA library that was differentially screened for messages expressed in maturing wild-type embryos but absent from vpl mutant embryos. These mutant embryos synthesize normal levels of ABA but are not ABA responsive (Neill et al., 1986; Robichaud et al., 1986). The product of the Viviparous-1 locus has been shown to act as a transcriptional activator of promoter targets carrying ABA-response elements (Hattori et al., 1992). We observed complete sequence identity among a11 pMEC isolates and single-band patterns on genomic Southern analysis, indicating that pMEC is probably encoded by a single gene. The approximately full-length clone, pMEC, contains a complete coding region for a polypeptide of 7.8 kD having 77% amino acid identity and 83% similarity with wheat E,. Maize embryos accumulate pMEC mRNA specifically in maturing seeds. On northem blots of RNA isolated from developing maize embryos, pMEC detects an mRNA species migrating at approximately 530 bases. The message leve1 is low in immature embryos and increases to a peak as embryos enter the mid-maturation phase. It remains high throughout later development. We have not assessed the relative contributions of transcription or stability to this persistence. Significant hybridization to pMEC was not found in RNA from seedling shoot, cob, tassle, or adult leaf.