Novel exon combinations generated by alternative splicing of gene fragments mobilized by a CACTA transposon in Glycine max

Background: The recent discoveries of transposable elements carrying host gene fragments such as the Pack-MULEs (Mutator-like transposable elements) of maize (Zea mays), rice ( Oryza sativa) and Arabidopsis thaliana, the Helitrons of maize and the Tgm-Express of soybeans, revealed a widespread genetic mechanism with the potential to rearrange genomes and create novel chimeric genes affecting genomic and proteomic diversity. Not much is known with regard to the mechanisms of gene fragment capture by those transposon elements or the expression of the captured host gene fragments. There is some evidence that chimeric transcripts can be assembled and exist in EST collections. Results: We report results obtained from analysis of RT-PCR derived cDNAs of the Glycine max mutant flower color gene, wp, that contains a 5.7-kb transposon (Tgm- Express1) in Intron 2 of the flavanone 3-hydroxylase gene (F3H) and is composed of five unrelated host gene fragments. The collection of cDNAs derived from the wp allele represents a multiplicity of processed RNAs varying in length and sequence that includes some identical to the correctly processed mature F3H transcript with three properly spliced exons. Surprisingly, the five gene fragments carried by the Tgm-Express1 were processed through complex alternative splicing as additional exons of the wp transcript. Conclusion: The gene fragments carried by the Tgm inverted repeat ends appear to be retained as functional exons/introns within the element. The spliceosomes then select indiscriminately the canonical intron splice sites from a pre-mRNA to assemble diverse chimeric transcripts from the exons contained in the wp allele. The multiplicity and randomness of these events provide some insights into the origin and mechanism of alternatively spliced genes.