The androgen receptor (AR) can be small ubiquitin-like modifier (SUMO)-ylated in its amino-terminal domain at lysines 385 and 511. This SUMO-ylation is responsive to several agonists, but is not induced by the pure antagonist hydroxyflutamide. We show that the main site of interaction of Ubc9, the SUMO-1 conjugating enzyme, resides in transcription activation unit 5. Overexpression of SUMO-1 represses the AR-mediated transcription, and this effect is abolished after mutating both SUMO-1 acceptor sites. On the other hand, the mutation of lysine 385 clearly affects the cooperativity of the receptor on multiple hormone response elements. Lysine 511 is not implicated in this function. Surprisingly, these effects on cooperativity clearly depend on the nature of the response elements. When selective androgen response elements, which are organized as direct repeats of 5'-TGTTCT-3'-like sequences, were tested, the lysine 385 mutation did not increase the androgen response. Point mutations changing the direct-repeat elements into inverted-repeat elements restored the effects of the lysine 385 mutation on cooperativity. In conclusion, SUMO-ylation of the AR might have a differential function in the control of cooperativity, depending on the conformation of the AR dimer bound to DNA.