Human transcription factor USF stimulates transcription through the initiator elements of the HIV-1 and the Ad-ML promoters
Du, H.; Roy, A.L.; Roeder, R.G.
EMBO Journal 12(2): 501-511
1993
ISSN/ISBN: 0261-4189 PMID: 8440240 Document Number: 413797
Earlier in vitro studies identified USF as a cellular factor which activates the adenovirus major late (Ad-ML) promoter by binding to an E-box motif located at position -60 with respect to the cap site. Purified USF contains 44 and 43 kDa polypeptides, and the latter was found (by cDNA cloning) to be a helix-loop-helix protein. In this report, we demonstrate a 25- to 30-fold stimulation of transcription via an upstream binding site by ectopic expresssion of the 43 kDa form of USF (USF-43) in transient transfection assays. More recent data have also revealed alternate interactions of USF-43 at pyrimidine-rich (consensus YYAYTCYY) initiator (Inr) elements present in a variety of core promoters. In agreement with this observation, we show here that USF-43 can recognize the initiator elements of the HIV-1 promoter, as well as those in the Ad-ML promoter, and that ectopic expression of USF-43 can stimulate markedly the corresponding core promoters (TATA and initiator elements) when analyzed in transient co-transfection assays. Mutations in either Inr 1 or Inr 2 reduced the USF-43-dependent transcription activity in vivo. In addition, in vitro transcrption assays showed that mutations in either or both of the Inr 1 and Inr 2 sequences of the HIV-1 and Ad-ML promoters could affect transcription efficiency, but not the position of the transcriptional start site. These results indicate that USF-43 can stimulate transcription through initiator elements in two viral promoters, although the exact mechanism and physiological significance of this effect remain unclear.