The ZEM2 family of maize MADS box genes possess features of transposable elements

The expression of certain plant MADS box genes in vegetative tissues suggests that the functions of MADS box genes in plants are not restricted to floral development. As the first example of an alternatively spliced MADS box gene in plants, the maize ZEMa gene was shown to generate certain differentially spliced transcripts accumulating preferentially in leaf and endosperm tissues. The isolation and characterization of MADS box clones from a genomic maize library resulted in the identification of the three genes ZEMa, ZEMb and ZEMc. They constitute a novel subfamily of highly related MADS box genes, Be ZEM2 family. Further members of this subfamily have been detected by DNA blot and genomic PCR analysis. The ZEM2 family genes are characterized by a bipartite structure. While they contain a MADS domain nearly identical to that of AGAMOUS, their C-terminal sequences are unrelated to that of any known plant MADS box gene. Furthermore, the ZEMa gene is located as an insertion in the promoter of a maize polygalacturonase gene. It is flanked by 13 bp perfect terminal inverted repeats (TIR's) of the Spm/En transposon family and also shows the 3 bp target sequence duplication characteristic for insertions of the Spm/En transposon family. Taken together, these data suggest a totally different evolutionary lineage of the ZEM2 family from those already described.