Alternative splicing of two leading exons partitions promoter activity between the coding regions of the maize homeobox gene Zmhox1a and Trap (transposon-associated protein)

Elucidation of the exon/intron structure of the maize Zmhox1a homeobox gene revealed two small introns in the homeodomain. Both intron positions are conserved in animal counterparts encoded in the metazoan homeobox gene clusters and thus may indicate a common ancestor. The transcription start of the Zmhox1a gene has been localized far from the protein-coding region. Two distal untranslated leading exons are alternatively spliced to either the Zmhox1a coding exons or an unrelated open reading frame comprising two exons located internally of the large second Zmhox1a intron. Due to significant homology to the C-terminus of the Mutator transposase this alternative gene product was named Trap (transposon-associated protein). Splice site selection may involve two sequence elements conserved at the splice acceptor sites in front of the Zmhox1a and Trap protein-coding regions. The translation of a mRNA species devoid of exon 3 which encodes the Zmhox1a transcription start codon may give rise to an N-terminal deletion polypeptide, Delta Zmhox1a. Ectopic expression experiments in transgenic tobacco indicate a putative function distinct from the full-length Zmhox1a protein.