Proteins from Drosophila nuclei that bind to regions of alternating C and T residues present in the promoters of the heat shock genes hsp70 and hsp26 and the histone genes his3 and his4 have been purified. These proteins bind to isolated linear DNA, and genomic footprinting analyses indicate that they are bound to DNA in nuclei. In supercoiled plasmids at low pH, some of these DNA sequences adopt triple-helical structures which, if they form in vivo, could significantly affect chromatin structure. The nuclear proteins described here, and not necessarily the deformed conformation of the DNA, may be responsible for maintaining a potentially inducible promoter structure before transcriptional activation.