A mutant lectin gene is rescued from an insertion element that blocks its expression

The soybean lectin gene Le1 encodes a prevalent seed protein and is highly regulated during the life cycle. The mutant lectin gene allele le1 is not transcribed detectably, contains a 3.5-kb Tgm1 insertion element within its coding region 0.6 kb 3' to the transcription start site, and leads to a lectinless phenotype. To determine whether the Tgm1 element or a secondary mutation was responsible for repressing le1 gene transcription, we eliminated the insertion element by constructing a chimeric lectin gene (le1/Le1) that contained the 5' half of the le1 gene and its promoter region and the 3' half of the wild-type Le1 gene. Transformed tobacco seed containing the le1/Le1 gene produced both lectin mRNA and protein, demonstrating that the mutant lectin gene control region is transcriptionally competent. By contrast, transformed seed containing the le1 gene produced no detectable lectin mRNA. We conclude that the absence of detectable transcription from the le1 gene is due to transcriptional inhibition by the Tgm1 insertion element and that this element acts at a distance to block transcription from an upstream promoter region.