Transcriptional Coactivator CBP Facilitates Transcription Initiation and Reinitiation of HTLV-I and Cyclin D2 Promoter

HTLV-I is the etiologic agent for adult T-cell leukemia/lymphoma (ATL) and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Taxi, the major activator of this virus, is a 40- kDa (353 amino acid) phosphoprotein, predominantly localized in the nucleus of the host cell, which functions to trans-activate both viral and cellular promoters. Recently it has been shown that HTLV-I and /or Taxl expressing cells have altered gene expression of some of the cell cycle associated genes. Tax activates HTLV-I as well as number of other cellular promoters through CREB, NF-kB and SRE elements. In this study we analyzed the effect of a general coactivator, namely CBP, which may be involved in activation of many cellular genes. To analyze CBP transcription activation, we have util‌ized an in vitro transcription assay that allows the analysis of transcription initiation and reinitiation in the absence of chromatin effects. In this assay, which utilizes a G-free cassette downstream of the Tax-responsive 21 by repeats, polymerase II molecules responsible for the first round of transcrip‌tion remain at the end of the G-free region, effectively blocking the complete elongation of reinitiated transcripts. Addition of Tax and a 682 amino acid fragment of CBP to the in vitro transcription reac‌tions increased both full-length and shorter transcripts resulting from reinitiation. A CBP deletion mutant lacking the N-terminal activation domain was inactive. Preliminary data is also presented to show that, transactivation of HTLV-I and a cellular promoter, namely cyclin D2, takes place in early GO/G1 before the restriction point, "R", where Rb function has been implicated.