Abundance of Transgene Transcript Variants Associated with Somatically Active Transgenic Helitrons from Multiple T-DNA Integration Sites in Maize

Helitrons, a novel type of mysterious DNA transposons discovered computationally prior to bench work confirmation, are components ubiquitous in most sequenced genomes of various eukaryotes, including plants, animals, and fungi. There is a paucity of empirical evidence to elucidate the mechanism of Helitrons transposition in plants. Here, by constructing several artificial defective Helitron (dHel) reporter systems, we aim to identify the autonomous Helitrons (aHel) in maize genetically and to demonstrate the transposition and repair mechanisms of Helitrons upon the dHel-GFP excision in maize. When crossing with various inbred lines, several transgenic lines produced progeny of segregated, purple-blotched kernels, resulting from a leaky expression of the C1 gene driven by the dHel-interrupted promoter. Transcription analysis indicated that the insertion of different dHels into the C1 promoter or exon would lead to multiple distinct mRNA transcripts corresponding to transgenes in the host genome. Simple excision products and circular intermediates of dHel-GFP transposition have been detected from the leaf tissue of the seedlings in F(1) hybrids of transgenic lines with corresponding c1 tester, although they failed to be detected in all primary transgenic lines. These results revealed the transposition and repair mechanism of Helitrons in maize. It is strongly suggested that this reporter system can detect the genetic activity of autonomic Helitron at the molecular level. Sequence features of dHel itself, together with the flanking regions, impact the excision activity of dHel and the regulation of the dHel on the transcription level of the host gene.