A novel geminivirus-derived 3' flanking sequence of terminator mediates the gene expression enhancement.

Exploring the new elements to re-design the expression cassette is crucial in synthetic biology. Viruses are one of the most important sources for exploring gene expression elements. In this study, we found that the DNA sequence of the SBG51 deltasatellite from the Sweet potato leaf curl virus (SPLCV) greatly enhanced the gene expression when flanked downstream of the terminator. The SBG51 sequence increased transient GFP gene expression in Nicotiana benthamiana leaves by up to ~6 times and ~10 times compared to the gene expression controlled by the UBQ10 promoter and 35S promoter alone, respectively. The increased GFP gene expression level contributed to the continuous accumulation of GFP protein and GFP fluorescence until 8 days post-inoculation (dpi). The SBG51 sequence also enhanced the gene expression in the transgenic Arabidopsis plants and maintained the spatio-temporal pattern of the FLOWERING LOCUS T (FT) and TOO MANY MOUTHS (TMM) promoters. We identified a 123 bp of AT-rich sequence containing seven "ATAAA" or "TTAAA" elements from the SBG51 DNA, which had the gene expression enhancement effect. Furthermore, the artificial synthetic sequences containing tandem repeated "ATAAA" or "TTAAA" elements were sufficient to increase the gene expression but did not alter the polyadenylation of mRNA, similar to the function of matrix attachment regions (MAR). Additionally, the compact artificial synthetic sequence also had an effect on yeast when the expression cassette was integrated into the genome. We conclude that the geminivirus deltasatellite-derived sequence and the "ATAAA"/"TTAAA" elements are powerful tools for enhancing gene expression.