Essential large transcripts of the maize Spm transposable element are generated by alternative splicing.

We used in vitro mutagenesis and cDNA cloning to identify new Suppressor-mutator (Spm) transposable element genes. Frameshift mutations in the ORFs of the tnpA gene's first intron markedly reduce Spm activity in transgenic tobacco, indicating that intron sequences encode essential gene products. Evidence is given that Spm encodes large alternatively spliced transcripts, designated tnpB (4.9 kb), tnpC (5.7 kb), and tnpD (5.8 kb), comprising all of the tnpA exons, most of the tnpA intron 1 ORF1 sequence, and either none, part, or all of the intron 1 ORF2 sequence. Two alternative splice donor sites were identified at the end of exon 1, and the structure of the different exon 1 sequences suggests that Spm employs a novel mode of translational regulation.