EFFECTS OF REPETITIVE AND NONREPETITIVE RAT RDNA ENHANCER ELEMENTS ONIN-VIVO TRANSCRIPTION BY RNA-POLYMERASE-I AND RNA-POLYMERASE-II

Previous study has demonstrated that a far upstream 174-bp spacer sequ ence of the rat rRNA-encoding (rDNA) gene can function as an enhancer in vitro in an orientation- and distance-independent manner [Dixit et al., J. Biol. Chem. 262 (1987) 11616-11622]. To demonstrate that this element can also function in vivo, two rat rDNA-cat plasmids, one with the 174-bp element and the other without this sequence, were construc ted and transfected into CHO cells. Primer extension analysis of the t ranscripts produced after transfection showed that transcription initi ation occurred at the + 1 site of the rDNA. The 174-bp sequence stimul ated the rat poll promoter activity in cis 4-5-fold over the control ( with the promoter alone). This RNA polymerase (polI) enhancer also sti mulated the mouse metallothionein-I (MT-I) and SV40 promoter activitie s in vivo, irrespective of its distance and orientation. Further disse ction of the 174-bp element revealed that the stimulatory activity on the RNA polymerase II (polII) promoter resides within the 37-bp and 43 -bp domains at the 3' end of the 174-bp element. Unlike this spacer en hancer, the 130-bp repeat element (RE) proximal to the rat promoter [G hosh et al., Gene 125 (1993) 217-222] was unable to modulate the polII promoter activity in vivo. These data show that while the non-repetit ive enhancer sequence of rat rDNA is interchangeable for the poll and polII promoters, the RE is polI-specific.