Distal effects on the in vivo repression of the deo operon are thought to be mediated by the deoR repressor with DNA loop formation. Such loops are easily observed by electron microscopy when the oligomeric deoR repressor is added to a DNA fragment carrying the three genetically defined operators at their chromosomal distances. Upon binding of deoR to any two operators, single loops are formed, 280, 600, and 880 bp in size. With the deo operon, double loops are also formed, which are the combination of the 280 bp and 600 bp loops and the result of simultaneous binding of the protein to its three sites. The formation of both single and double loops is consistent with the long-range effects observed in vivo and with the cooperative involvement of all three operator sites in the repression.