Effect of DNA methylation on protein-DNA interactions upstream of the gamma-glutamyl transpeptidase gene.

We have used exonuclease III and DNase I protection assays to study proteins which bind to potentially regulatory elements located in the 5'-flanking region of the gamma-glutamyl transpeptidase gene. With both liver and kidney nuclear extracts, exonuclease III barriers were located at -566 in the coding strand and at -585 in the noncoding strand, and footprints were found from -595 to -566 and from -600 to -562, respectively. When the DNA was methylated in the CG dinucleotides, the exonuclease III barriers disappeared and the footprints were greatly reduced. The transcription factor Sp1 bound to this DNA region but did not seem to be involved in the binding activity. Since the gamma-glutamyl transpeptidase presents different levels of methylation in liver and kidney, associated with different levels of expression, these results suggest that the binding activity could play a role in the control of the expression of the gamma-glutamyl transpeptidase gene in liver and kidney.