One of the most common forms of functional interaction among transcription factors is the more than additive effect at promoters harboring multiple copies of a response element. The mechanisms that enable or control synergy at such compound response elements are poorly understood. We recently defined a common motif within the negative regulatory regions of multiple factors that operates by regulating their transcriptional synergy. We have identified such a synergy control (SC) motif embedded within the "attenuator domain" of CCAAT/enhancer-binding protein alpha (C/EBPalpha), a key regulator of energy homeostasis and cellular differentiation. A Lys(159) --> Arg substitution within the SC motif does not alter C/EBPalpha activity from a single site but leads to enhanced transactivation from synthetic or natural compound response elements. The sequence of SC motifs overlaps with the recently defined consensus SUMO modification site, and we find that the SC motif is the major site of both SUMO-1 and SUMO-3 modification in C/EBPalpha. Furthermore, the disruption of SC motif function is accompanied by loss of SUMO but not ubiquitin modification. C/EBPalpha interacts directly with the E2 SUMO-conjugating enzyme Ubc9 and can be SUMOylated in vitro using purified recombinant components. Notably, we find that PIASy has E3-like activity and enhances both SUMO-1 and SUMO-3 modification of C/EBPalpha in vivo and in vitro. Our results indicate that SUMO modification of SC motifs provides a means to rapidly control higher order interactions among transcription factors and suggests that SUMOylation may be a general mechanism to limit transcriptional synergy.