Groucho (Gro) is a Drosophila corepressor required by numerous DNA-binding 
repressors, many of which are distributed in gradients and provide positional 
information during development. Gro contains well-conserved domains at its N- 
and C-termini, and a poorly conserved central region that includes the GP, CcN, 
and SP domains. All lethal point mutations in gro map to the conserved regions, 
leading to speculation that the unconserved central domains are dispensable. 
However, our sequence analysis suggests that the central domains are disordered 
leading us to suspect that the lack of lethal mutations in this region reflects 
a lack of order rather than an absence of essential functions. In support of 
this conclusion, genomic rescue experiments with Gro deletion variants 
demonstrate that the GP and CcN domains are required for viability. 
Misexpression assays using these same deletion variants show that the SP domain 
prevents unrestrained and promiscuous repression by Gro, while the GP and CcN 
domains are indispensable for repression. Deletion of the GP domain leads to 
loss of nuclear import, while deletion of the CcN domain leads to complete loss 
of repression. Changes in Gro activity levels reset the threshold concentrations 
at which graded repressors silence target gene expression. We conclude that 
co-regulators such as Gro are not simply permissive components of the repression 
machinery, but cooperate with graded DNA-binding factors in setting borders of 
gene expression. We suspect that disorder in the Gro central domains may provide 
the flexibility that allows this region to mediate multiple interactions 
required for repression.