Analysis of the SINE S1 Pol III promoter from Brassica;: impact of methylation and influence of external sequences

Transcription is an important control point in the transposable element mobilization process. To better understand the regulation of the plant SINE (Short Interspersed Elements) S1, its promoter sequence was studied using an in vitro pol III transcription system derived from tobacco cells. We show that the internal S1 promoter can be functional although upstream external sequences were found to enhance this basal level of transcription. For one putative 'master' locus (na7), three CAA triplets tin positions -12, -7 and -2) acid two overlapping TATA motifs tin positions -54 to -43) were important to stimulate transcription. For this locus, two transcription initiation regions were characterized, one centered on position +1 (first nucleotide of the Si element) and one centered on position -19 independently of the internal motifs. The CAA triplets only influence transcription in +1 and work in association with the internal motifs. We show that methylation can inhibit transcription at the na7 locus. We also observe that Si RNA is cleaved in a smaller Poly (A) minus product by a process analogous to the maturation of mammalian SINEs.