Properties of a distal regulatory element controlling transcription of the U2 small nuclear RNA

The upstream region of human U2 genes contains a distal transcriptional control element, previously mapped between nucleotide (nt) positions -198 and -258 (Westin et al., 1984b). In the present study we show that it resembles transcriptional enhancers in being active even from a distance of 1.4 kb. However, in contrast to most other enhancers it functions unidirectionally in Xenopus laevis oocytes. The distal control element was further mapped by construction of truncated templates for U2 RNA transcription. The results showed that templates, which extended to either of nt positions -214 and -218, were inactive. Templates comprising sequences to nt positions -225 or -226 displayed an intermediate level of activity whereas templates which extend to nt -258 were fully active. It has previously been shown that the human U2 enhancer contains binding sites for the so-called octamer binding protein and for transcription factor Sp1 [Janson et al., Nucl. Acids Res. 15 (1987) 4997-5016]. The partially active templates included one binding site for the octamer binding protein, whereas the fully active template included, in addition, two Sp1 binding sites, thus indicating that these transcription factors are of importance for U2 RNA transcription. The structure of the enhancer was also probed by inserting a pair of complementary synthetic oligodeoxynucleotides which represented the region between nt positions -235 and -215 into a truncated template which lacked the enhancer. The oligodeoxynucleotide enhanced transcription to approximately 50% of the level obtained with templates extending to position -258.