HIGH-RESOLUTION ELECTRON-MICROSCOPIC STUDIES OF GENETIC-REGULATION

High magnification EM methods were used to study DNA fragments and regulatory proteins binding to DNA fragments containing the .lambda. phage early rightward operon and the lac operon. DNA lengths and repressor or RNA polymerase binding positions could be determined with a precision of .apprx. .+-. 5 base-pairs. DNA and protein were positively stained with uranyl formate. This staining yields reproducible differences in the shapes of bound proteins but shows no reproducible internal structure. RNA polymerase alone was able to bind to the .lambda. promoter pr, but the presence of catabolic gene activator protein was required for detectable RNA polymerase binding to the lac promoter. A < 2% shortening was observed in the measured length of a 203 base-pair lac DNA fragment upon the binding of repressor to operator, and the DNA showed only a slightly increased tendency to bend at the site of repressor or RNA polymerase binding. Similar microscopic studies should be possible for other systems in which proteins bind to DNA.