Insulators are sequences that uncouple adjacent chromosome domains. Here we have 
shown that Saccharomyces cerevisiae Rap1p and Abf1p proteins are endowed with a 
potent insulating capacity. Insulating domains in Rap1p coincide with previously 
described transcription activation domains, whereas four adjacent subdomains 
spanning the whole of the Abf1p C terminus (440-731) were found to display 
autonomous insulating capacity. That both Rap1p and Abf1p silencing domains 
either contain or largely overlap with an insulating domain suggests that 
insulation conveys some undefined chromosome organization capacity that also 
contributes a function in silencing. Together with Reb1p and Tbf1p, previously 
involved in the activity of Saccharomyces cerevisiae subtelomeric insulators, 
insulating potential emerges as a supplementary common property of General 
Regulatory Factors (GRFs). Thus GRFs, which bind to sites scattered throughout 
the genome within promoters, would not only play a key role in regulating gene 
expression but also partition the genome in functionally independent domains.