Modification of Escherichia coli RNA polymerase by diethylpyrocarbonate. II. Binding and unwinding of double-stranded DNA

E. coli DNA dependent RNA polymerase was modified by diethylpyrocarbonate. Binding to a double-stranded DNA and unwinding of the DNA at the enzyme binding site by the modified enzyme were examined. It was found that RNA polymerase reversibly lost the ability to unwind DNA helix as well as the RNA synthetic activity when 9 to 11 histidyl residues of the enzyme were modified. In addition ot modification of the most reactive sulfhydryl or amino groups of the enzyme accompanying histidyl residues modification results in irreversible decrease of the salt concentration which is necessary to remove the enzyme from DNA cellulose column. Further modification of the less reactive sulfhydryl or amino groups leads to irreversible loss of the DNA binding ability and to the enzyme structure alteration.