Synthetic photocontrolled proteins could be powerful tools for probing cellular chemistry. Several previous attempts to produce such systems by incorporating photoisomerizable chromophores into biomolecules have led to photocontrol but with incomplete reversibility, where the chromophore becomes trapped in one photoisomeric state. We report here the design of a modified GCN4-bZIP DNA-binding protein with an azobenzene chromophore introduced between Cys residues at positions 262 and 269 (S262C, N269C) within the zipper domain. As predicted, the trans form of the chromophore destabilizes the helical structure of the coiled-coil region of GCN4-bZIP, leading to diminished DNA binding relative to wild type. Trans-to-cis photoisomerization of the chromophore increases helical content and substantially enhances DNA binding. The system is observed to be readily reversible; thermal relaxation of the chromophore to the trans state and concomitant dissociation of the protein-DNA complex occurs with tau(1/2) approximately 10 min at 37 degrees C. It appears that conformational dynamics in the zipper domain make the transition state for isomerization readily available so that retention of reversible switching is observed.

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(1) The electrophoretic band patterns (sodium dodecyl sulfate (SDS) – polyacrylamide) for either isotopically labeled or dye-stained proteins in a wash fraction obtained by rinsing germinated wheat embryos with any of a variety of buffers are different from those observed for proteins in either the soluble or pellet fractions of embryo homogenates. Interestingly, the wash fraction is enriched with respect to germin, a marker protein for the onset of growth in germinating wheat embryos.(2) There is selective accumulation of a novel form of germin in the wash fraction. This novel form migrates just ahead of the more usual form of germin during electrophoresis in SDS–polyacrylamide gel, being about 5% smaller, assuming a linear relation between gel mobility and log (relative mass), but it yields the same methionine-labeled peptides when denatured and digested by Staphylococcus aureus protease.(3) Quasi-quantitative analyses of dye-stained proteins suggest that, in the case of embyros cultured in 5% sucrose, which show greatest growth, 50% or more of the total germin is present, in the novel form, in the wash fraction, which contains less than 5% of the total proteins. By way of contrast, germin is barely detected in the wash fraction of embryos excised from germinated grains, unless the excised embryos are incubated in water or 5% sucrose for an additional 24 h.(4) Allied studies of (possibly epiphytic) bacteria, found in wash fractions prepared following germination of preisolated embryos or grains, are described.(5) The significance of the present findings for our continuing efforts to define a phyiological role for germin are discussed at some length.