Whether NF-κB promoter transactivation by the human T-cell leukemia virus type 1 
(HTLV-1) Tax protein requires Tax SUMOylation is still a matter of debate. In 
this study, we revisited the role of Tax SUMOylation using a strategy based on 
the targeting of Ubc9, the unique E2 SUMO-conjugating enzyme. We show that 
either a catalytically inactive form of Ubc9 (Ubc9-C93S) or Ubc9 small 
interfering RNA (siRNA) dramatically reduces Tax conjugation to endogenous 
SUMO-1 or SUMO-2/3, demonstrating that as expected, Tax SUMOylation is under the 
control of the catalytic activity of Ubc9. We further report that a 
non-SUMOylated Tax protein produced in 293T cells is still able to activate 
either a transfected or an integrated NF-κB reporter promoter and to induce 
expression of an NF-κB-regulated endogenous gene. Importantly, blocking Ubc9 
activity in T cells also results in the production of a non-SUMOylated Tax that 
is still fully functional for the activation of a NF-κB promoter. These results 
provide the definitive evidence that Tax SUMOylation is not required for 
NF-κB-driven gene induction.
IMPORTANCE: Human T-cell leukemia virus type 1 is able to transform CD4(+) T 
lymphocytes. The viral oncoprotein Tax plays a key role in this process by 
promoting cell proliferation and survival, mainly through permanent activation 
of the NF-κB pathway. Elucidating the molecular mechanisms involved in NF-κB 
pathway activation by Tax is therefore a key issue to understand HTLV-1-mediated 
transformation. Tax SUMOylation was initially proposed to be critical for 
Tax-induced NF-κB promoter activation, which was challenged by our later 
observation that a low-level-SUMOylated Tax mutant was still functional for 
activation of NF-κB promoters. To clarify the role of Tax SUMOylation, we set up 
a new approach based on the inhibition of the SUMOylation machinery in 
Tax-expressing cells. We show that blocking the SUMO-conjugating enzyme Ubc9 
abolishes Tax SUMOylation and that a non-SUMOylated Tax still activates NF-κB 
promoters in either adherent cells or T cells.