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 mob ilization process. To better understand the regulation of the plant SINE (S hort Interspersed Elements) S1, its promoter sequence was studied using an in vitro pol III transcription system derived from tobacco cells. We show t hat the internal S1 promoter can be functional although upstream external s equences were found to enhance this basal level of transcription. For one p utative 'master' locus (na7), three CAA triplets tin positions -12, -7 and -2) acid two overlapping TATA motifs tin positions -54 to -43) were importa nt 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 in ternal motifs. The CAA triplets only influence transcription in +1 and work in association with the internal motifs. We show that methylation can inhi bit 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 maturatio n of mammalian SINEs.