The primary function of a redundant Sp1 binding site in the mouse aprt gene promoter is to block epigenetic gene inactivation

The promoter region of the mouse adenine phosphoribosyltransferase (aprt) gene contains one nonconsensus Spl binding site at its 5' end followed by three consensus Spl binding sites. The two 3'-most binding sites are sufficient for maximal expression of aprt, suggesting that the non-consensus and consensus binding sites at the 5' end are redundant, However, the two 3' sites are not sufficient to block epigenetic inactivation, which led to the hypothesis that the redundant consensus and/or non-consensus 5' Spl binding sites are required to block inactivation events, To test this hypothesis, promoter region constructs were made in which the two 5' Spl binding sites were mutated alone or in tandem, and then each construct was tested for its ability to withstand epigenetic inactivation, A cis-acting methylation center that is normally located 1.2 kb upstream of the promoter was used to induce inactivation, The results demonstrate that the presence of the redundant consensus Spl binding site is required to block methylation-associated gene inactivation, Therefore, the Spl binding sites comprising the mouse aprt promoter have evolved two distinct functions, one to promote transcription and the other to block epigenetic inactivation.