A 168 bp derivative of Suppressor-mutator/Enhancer is responsible for the maize o2-23 mutation

From a directed transposon tagging of the maize Opaque-2 gene (O2), we have isolated a stable mutant o2 allele, o2-23. Cloning and molecular analysis of the allele revealed a 168 nucleotide insertion in the third exon of o2. The sequence of this small insertion indicated identity with the 5' and 3' ends of the 8.3 kb Suppressor-mutator/Enhancer (Spm/En) transposable element. This represents the smallest deletion derivative of Spm (dSpm) thus far characterized in maize. Genetic crosses of plants homozygous for o2-23 with plants homozygous for both an o2 null allele (o2-R) and an autonomous Spm produce stable opaque seed having no apparent sectors of vitreous endosperm. DNA fragments of the size expected if the dSpm were to excise were not detectable by Southern analysis, suggesting that this element is unable to transpose. Northern analysis detected an o2-23 mRNA that was much more abundant in o2-23 seeds lacking Spm than in o2-23 seeds containing Spm, consistent with the idea that Spm transacting functions can suppress the accumulation of the o2-23 transcript.