The ETS domain transcription factor Elk-1 is a direct target of the MAP kinase pathways. Phosphorylation of the Elk-1 transcriptional activation domain by MAP kinases triggers its activation. However, Elk-1 also contains two domains with repressive activities. One of these, the R motif, appears to function by suppressing the activity of the activation domain. Here, we demonstrate that SUMO modification of the R motif is required for this repressive activity. A dynamic interplay exists between the activating ERK MAP kinase pathway and the repressive SUMO pathway. ERK pathway activation leads to both phosphorylation of Elk-1 and loss of SUMO conjugation and, hence, to the loss of the repressive activity of the R motif. Thus, the reciprocal regulation of the activation and repressive activities are coupled by MAP kinase modification of Elk-1.