We have previously shown that the proximal 2-kb sequence of the fatty acid 
synthase (FAS) promoter transfected into rat adipocytes was highly sensitive to 
the cellular context, displaying an overactivity in obese (fa/fa) versus lean 
Zucker rat adipocytes. Using deletional analysis, we show here that FAS promoter 
activity mainly depends on a region from -200 to -126. This sequence exerts a 
strong negative effect on FAS promoter in adipocytes from lean rats but not in 
those from obese rats, resulting in a marked overtranscriptional activity in the 
latter cells. This region, fused to a heterologous promoter, the E1b TATA box, 
induced differential levels of gene reporter activity in lean and obese rat 
adipocytes, indicating it harbors fa-responsive element(s). Whatever the rat 
genotype, adipocyte nuclear proteins were shown to footprint the same protected 
sequence within the fa-responsive region, and supershift analysis demonstrated 
that Sp1 or Sp1-like proteins were bound to this DNA subregion. Compelling 
evidence that the Sp1 binding site contained in this sequence was implicated in 
the differential promoter activity in lean versus obese rats, was provided by 
the observation that mutations at this Sp1 site induced a 2.5-fold increase in 
FAS promoter activity in adipocytes from lean rats, whereas they had no effect 
in adipocytes from obese rats.