The Spm (En) transposable element controls the excision of a 2-kb DNA insert at the wxm-8 allele of Zea mays

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Abstract
The waxy (Wx) locus of Zea mays was cloned from strains carrying the wild‐type and wxm‐8 mutant alleles. The receptor component of the Suppressor‐Mutator (Spm) controlling element system in the wxm‐8 allele was shown to be a 2 kb long insertion within the transcribed region of the Wx gene. The insertion, termed Spm‐I8, is excised during somatic reversion events induced by the autonomous controlling element Enhancer (En), which is an equivalent to Spm. Integration of Spm‐I8 into the Wx gene generates a 3‐bp target site duplication. Spm‐I8 has a 13 bp long inverted repeat at its termini. The ends of the element can be further folded to build a large double‐stranded structure consisting of five perfectly matching double‐stranded regions of 9–13 bp in length, interrupted by single‐stranded loops. A comparison of the wild‐type and wxm‐8 alleles revealed two additional insertions 6 (insert‐1) and 0.25 (insert‐2) kb in length. No En‐induced excision of insert‐1 and insert‐2 could be detected so far. There is remarkable structure and sequence homology between Spm‐I8 and the transposable elements Tam1 and Tam2 of Antirrhinum majus at their termini, reflecting a possible evolutionary and/or functional relationship between transposons in different plant species.