Tax protein of HTLV-1 inhibits CBP/p300-mediated transcription by interfering with recruitment of CBP/p300 onto DNA element of E-box or p53 binding site.

Tax protein of human T-cell leukemia virus type 1 (HTLV-1) is a potent transcriptional regulator which can activate or repress specific cellular genes and has been proposed to contribute to leukemogenic processes in adult T-cell leukemia. The molecular mechanism of Tax-mediated trans-activation has been well investigated. However, trans-repression by Tax remains to be studied in detail, although it is known to require a specific DNA element such as E-box or p53 binding site. Examining possible mechanisms of trans-repression, we found that co-expression of E47 and p300 activated E-box dependent transcription and this activation was efficiently repressed by Tax. In this system, Tax bound to p300 and decreased the level of p300 complexed on the E-box element. Similarly, Tax inhibited transcription directed by p53 and CBP, reducing the level of CBP on the p53 binding site. These results indicate that Tax interferes with recruitment of CBP/p300 into protein complexes on E-box and p53 binding site through its binding to CBP/p300. In contrast to these findings, we observed that Tax increased the level of CBP on the viral 21-bp enhancer which is trans-activated by Tax. From these observations, we propose a universal mechanism for Tax-mediated trans-repression and trans-activation of transcription in which Tax binds to CBP/p300 and determines the accessibility of CBP/p300 to protein complexes on specific DNA element.