Final answer:
DNA footprinting is the technique initially used to identify promoter sequences by determining where proteins bind to DNA, protecting it from DNAse treatment.
Step-by-step explanation:
The technique used to initially identify promoter sequences was DNA footprinting. DNA footprinting is a method that identifies the site on a strand of DNA where proteins, such as transcription factors, bind. This is critical in figuring out promoter regions, which are key to starting the process of gene transcription. The process involves selectively treating DNA with DNAse in the presence of the protein of interest. Protected areas, where the protein is bound, can then be analyzed after further processing, helping identify promoter regions.