Tec1 is a transcription factor in the yeast mitogen-activated protein kinase 
(MAPK) pathway that controls invasive growth. Previously we reported that a 
fraction of Tec1 protein is sumoylated on residue lysine 54 in normally growing 
cells. Here we describe regulation and functional consequences of Tec1 
sumoylation. We found that activation of Kss1, the MAPK that directly activates 
Tec1, results in a decrease in Tec1 sumoylation and a concurrent increase of 
Tec1 transcriptional activity. Consistent with a role of sumoylation in 
inhibiting Tec1 activity, specifically increasing sumoylation of Tec1 by fusing 
it to the sumoylating enzyme Ubc9 leads to a dramatic decrease of Tec1 
transcriptional activity. Invasive growth is also compromised in Tec1-Ubc9. In 
contrast, fusing sumoylation-site mutant Tec1, i.e., Tec1(K54R), to Ubc9 did not 
significantly alter transcriptional activation and had a less effect on invasive 
growth. Taken together, these findings provide evidence for regulated 
sumoylation as a mechanism to modulate the activity of Tec1 and validate Ubc9 
fusion-directed sumoylation as a useful approach for studying protein 
sumoylation.