Bicoid (Bcd) is a Drosophila morphogenetic protein and a transcriptional 
activator. Genetic studies have suggested a role of sumoylation in Bcd function, 
but it is unknown how Bcd activity is affected specifically by its own 
sumoylation status. Here we show that Bcd is sumoylated in Drosophila cells. We 
identify a lysine residue of Bcd as the primary sumoylation site. Using a Bcd 
mutant defective in being sumoylated, we show that sumoylation of Bcd is 
inhibitory to its ability to activate transcription. We provide evidence 
suggesting that the SUMO moiety has an intrinsic inhibitory activity for the 
activator function of Bcd.