Stable transcription activities dependent on an orientation of Tam3 transposon insertions into Antirrhinum and yeast promoters occur only within chromatin.

Transposon insertions occasionally occur in the promoter regions of plant genes, many of which are still capable of being transcribed. However, it remains unclear how transcription of such promoters is able to occur. Insertion of the Tam3 transposon into various genes of Antirrhinum majus can confer leaky phenotypes without its excision. These genes, named Tam3-permissible alleles, often contain Tam3 in their promoter regions. Two alleles at different anthocyanin biosynthesis loci, nivea(recurrensTam3) (niv(rec)) and pallida(recurrensTam3) (pal(rec)), both contain Tam3 at a similar position immediately upstream of the promoter TATA-box; however, these insertions had different phenotypic consequences. Under conditions where the inserted Tam3 is immobilized, the niv(rec) line produces pale red petals, whereas the pal(rec) line produces no pigment. These pigmentation patterns are correlated with the level of transcripts from the niv(rec) or pal(rec) alleles, and these transcriptional activities are independent of DNA methylation in their promoter regions. In niv(rec), Tam3 is inserted in an orientation that results in the 3' end of Tam3 adjacent to the 5' region of the gene coding sequence. In contrast, the pal(rec) allele contains a Tam3 insertion in the opposite orientation. Four of five different nonrelated genes that are also Tam3-permissible alleles and contain Tam3 within the promoter region share the same Tam3 orientation as niv(rec). The different transcriptional activities dependent on Tam3 orientation in the Antirrhinum promoters were consistent with expression of luciferase reporter constructs introduced into yeast chromosomes but not with transient expression of these constructs in Antirrhinum cells. These results suggest that for Tam3 to sustain stable transcriptional activity in various promoters it must be embedded in chromatin.