Does the Promoter Constitute a Barrier in the Horizontal Transposon Transfer Process? Insight from Bari Transposons

Abstract

The contribution of the transposons' promoter in the horizontal transfer process is quite overlooked in the scientific literature. To shed light on this aspect we have mimicked the horizontal transfer process in laboratory and assayed in a wide range of hosts (fly, human, yeast and bacteria) the promoter activity of the 5' terminal sequences in Bari1 and Bari3, two Drosophila transposons belonging to the Tc1-mariner superfamily. These sequences are able to drive the transcription of a reporter gene even in distantly related organisms at least at the episomal level. By combining bioinformatics and experimental approaches, we define two distinct promoter sequences for each terminal sequence analyzed, which allow transcriptional activity in prokaryotes and eukaryotes, respectively. We propose that the Bari family of transposons, and possibly other members of the Tc1-mariner superfamily, might have evolved "blurry promoters," which have facilitated their diffusion in many living organisms through horizontal transfer.

Keywords: Tc1-mariner, Bari transposons; horizontal transposon transfer; promoter; promoter-luciferase assay.

Fig. 1.

—(A) General structure of the Bari transposons. The sequences tested in the promoter-luciferase assays are showed in the boxes. Position and sequences of the three DRs representing the transposase-binding sites, within the 5′ terminal sequences of both transposons are in red-boldfaced uppercases. (B) Schematic structure of the reporter expression cassettes generated for this study. Arrows indicate the transcription direction.

Fig. 2.

—The Bari promoters in Drosophila cells. The luciferase-promoter assays in S2R+ cells. The promoter activity is lower than the copia promoter (23% Ba1p and 15% Ba3p). Bars represent mean values. Circles represent actual data. **P<0.005.

Fig. 3.

—Promoter activity in human cells. Promoter-luciferase assays in HeLa cels (left, green), Hek293 cells (middle, blue), HepG2 cells (right, red). Bars represent mean values. Circles represent actual data. *P<0.05; **P<0.005; ***P<0.001. Note that the Y axis is in logarithmic scale.

Fig. 4.

—The Bari promoters in yeast. The Ba1p promoter activity is as much as the 5% of the URA3 promoter in Saccharomyces cerevisiae. The activity of Ba3p is not significantly different from the promoter-less construct. Bars represent mean values. Circles represent actual data. **P<0.005. Note that the Y axis is in logarithmic scale.

Fig. 5.

—The activity of the Bari promoters in Escherichia coli can be estimated as the 25% and 20% (respectively for Bari1 and for Bari3) compared with the CAT promoter activity (A). Direct visualization of the bioluminescence from transformed E. coli cultures in a dark room (B) when bacterial lysates are exposed to the luciferase substrate. Bars represent mean values. Circles represent actual data. ***P < 0.001.

Fig. 6.

—Mapping the eukaryotic and prokaryotic promoters in Bari transposons. The two sub-fragments analyzed are shown in (A). (B, C) Promoter activity in S2R+ cells of the two halves of the Ba1p and Ba3p sequences compared with the respective complete sequences. (D, E) Promoter activity in Escherichia coli cells using the two halves of the Ba1p and Ba3p sequences and the respective complete sequences. The relative promoter activity (compared with the respective complete sequence set to 100) is shown on the secondary Y axis.