DNA looping provides for "intersegmental hopping" by proteins: a mechanism for long-range site localization

Studies on how transcription factors and DNA modifying enzymes passively locate specific sites on DNA have yet to be reconciled with a sufficient set of mechanisms that can adequately account for the efficiency and speed of this process. This is especially true when considering that these DNA binding/modifying proteins have diverse levels of both cellular copy numbers and genomic recognition site densities. The monomeric bacterial DNA adenine methyltransferase (Dam) is responsible for the rapid methylation of the entire chromosome (with only ~100 Dam copies per cell) and the regulated methylation of closely spaced sites that controls the expression of virulence genes in several human pathogens. Provocatively, we find that Dam travels between its recognition sites most efficiently when those sites are ~500bp apart. We propose that this is manifested by Dam moving between distal regions on the same DNA molecule, which is mediated by DNA looping, a phenomenon we designate as intersegmental hopping. Importantly, an intermediate found in other systems including two simultaneously bound, looped DNA strands is not involved here. Our results suggest that intersegmental hopping contributes to enzymatic processivity (multiple modifications), which invoke recent reports demonstrating that DNA looping can assist in site finding. Intersegmental hopping is possibly used by other sequence-specific DNA binding proteins, such as transcription factors and regulatory proteins, given certain biological context. While a general form of this mechanism is proposed by many research groups, our consideration of DNA looping in the context of processive catalysis provides new mechanistic insights and distinctions.