  Vital biological processes such as genome repair require fast and efficient
binding of selected proteins to specific target sites on DNA. Here we propose
an active target search mechanism based on "chromophoresis", the dynamics of
DNA-binding proteins up or down gradients in the density of epigenetic marks,
or colours (biochemical tags on the genome). We focus on a set of proteins that
deposit marks from which they are repelled---a case which is only encountered
away from thermodynamic equilibrium. For suitable ranges of kinetic parameter
values, chromophoretic proteins can perform unidirectional motion and are
optimally redistributed along the genome. Importantly, they can also locally
unravel a region of the genome which is collapsed due to self-interactions and
"dive" deep into its core, for a striking enhancement of the efficiency of
target search on such an inaccessible substrate. We discuss the potential
relevance of chromophoresis for the location of DNA lesions.

Subcellular Processes;Soft Condensed Matter;Biological Physics