The En/Spm transposable element of Zea mays contains splice sites at the termini generating a novel intron from a dSpm element in the A2 gene.

The A2 locus of Zea mays, identified as one of the genes affecting anthocyanin biosynthesis, was cloned using the transposable elements rcy and dSpm as gene tags. The A2 gene encodes a putative protein of 395 amino acids and is devoid of introns. Two a2-m1 alleles, containing dSpm insertions of different sizes, were characterized. The dSpm element from the original state allele has perfect termini and undergoes frequent transposition. The element from the class II state allele is no longer competent to transpose. It has retained the 13 bp terminal inverted repeat but has lost all subterminal sites at the 5' end, which are recognized by tnpA protein, the most abundant product of the En/Spm transposable element system. The relatively high A2 gene expression of one a2-m1 allele is due to removal of almost all dSpm sequences by splicing. The slightly altered A2 enzyme is still functional as shown by complementation of an a2 mutant with the corresponding cDNA. The 5' and 3' splice sites are constituted by the termini of the dSpm element; it therefore represents a novel intron of the A2 gene.