Interaction of spatially separated protein-DNA complexes for control of gene expression: operator conversions

Two operators, spatially separated from each other and from the promoters, repress the gal operon when bound to Gal repressor. Conversion of either gal operator to a lac operator results in derepression, although both Gal and Lac repressors are present, suggesting that mere occupation of operator sites is not sufficient to cause repression. Conversion of both operators to lac operators restores normal repression in the presence of Lac repressor protein. We propose that normal repression requires interaction between operator-bound like repressor molecules; this generates a DNA loop, which is part of a higher order structure. RNA polymerase and cyclic AMP receptor protein are present in this complex but unable to initiate transcription because of the higher order structure. Such higher order DNA-multiprotein complexes could occur in a variety of genetic regulatory systems that are controlled from distal sites by regulatory proteins.