Conservation of the Restricted Expression of Brassicaceae B(sister)-Like Genes in Seeds Requires a Transposable Element in Arabidopsis thaliana

Changes in transcription factor binding sites (TFBSs) can alter the spatiotemporal expression pattern and transcript abundance of genes. Loss and gain of TFBSs were shown to cause shifts in expression patterns in numerous cases. However, we know little about the evolution of extended regulatory sequences incorporating many TFBSs. We compare, across the crucifers (Brassicaceae, cabbage family), the sequences between the translated regions of Arabidopsis B(sister) (ABS)-like MADS-box genes (including paralogous GOA-like genes) and the next gene upstream, as an example of family-wide evolution of putative upstream regulatory regions (PURRs). ABS-like genes are essential for integument development of ovules and endothelium formation in seeds of Arabidopsis thaliana. A combination of motif-based gene ontology enrichment and reporter gene analysis using A. thaliana as common trans-regulatory environment allows analysis of selected Brassicaceae B(sister) gene PURRs. Comparison of TFBS of transcriptionally active ABS-like genes with those of transcriptionally largely inactive GOA-like genes shows that the number of in silico predicted TFBS) is similar between paralogs, emphasizing the importance of experimental verification for in silico characterization of TFBS activity and analysis of their evolution. Further, our data show highly conserved expression of Brassicaceae ABS-like genes almost exclusively in the chalazal region of ovules. The Arabidopsis-specific insertion of a transposable element (TE) into the ABS PURRs is required for stabilizing this spatially restricted expression, while other Brassicaceae achieve chalaza-specific expression without TE insertion. We hypothesize that the chalaza-specific expression of ABS is regulated by cis-regulatory elements provided by the TE.