Specific binding of the λ phage repressor to λ DNA.

Ptashne was the first to demonstrate specific binding between protein and DNA.  Abstract for the paper: "Genetic experiments show that a group of genes may be switched off by the product of a regulator gene, called a repressor. An isolated repressor is shown here to bind specifically and with high affinity to DNA, strongly suggesting that, in vivo, repressors block the transcription from DNA to RNA by binding directly to the DNA."