Unusual Sp1-GC box interaction in a parvovirus promoter.

The P4 promoter of the parvovirus minute virus of mice contains a single degenerate GC box sequence which binds the transcription factor Sp1 with high affinity. The two promoters of murine Sp1 were affinity purified, and their interactions with the P4 promoter were examined. Several unusual features were observed. Methylation interference experiments demonstrated that Sp1 makes contacts with both DNA strands, including the central guanine as well as an adenine residue on the cytidine-rich strand of the GC box. UV photocross-linking revealed that the 95- and the 105-kDa promoters cross-link exclusively to opposite strands of the GC box. These results suggest that the phosphorylation of the 95-kDa Sp1 promoter results in a change in the way Sp1 is positioned on the P4 GC box and identifies a high-affinity GC box motif.