Formation of a DNA-protein complex with the SP-1–like element of the mouse CPO promoter. Five-microgram aliquots of nuclear extracts from 745A (lanes 1 to 8) or NIH3T3 (lanes 9 to 16) cells were incubated with the end-labeled oligomers corresponding to the SP-1–like element (-21/-11) (lanes 1 to 4, 9 to 12) or the SP-1 consensus sequence (lanes 5 to 8, 13 to 16). Competition assays were performed with a 250-fold molar excess of unlabeled oligonucleotides containing the SP-1–like element (−21/−11) (lanes 2, 6, 10, 14), SP-1 consensus sequence (lanes 3, 7, 11, 15), and GATA consensus sequence from the human ALAS-E promoter (lanes 4, 8, 12, 16). The arrow and the arrowheads indicate the complexes of the probe and nuclear proteins.