“Light-derepressible” genes are regulated by metal-protein complexes: A hypothesis

Abstract

The hypothesis is presented that genes whose expression is sensitive to light are regulated by metal-protein complexes. This hypothesis is based on induction of a gene for an Mr 21,000 polypeptide in the prokaryote Arthrobacter sp. by photodynamic conditions, chelating agents, or a decrease in pH. It is proposed that these conditions promote dissociation of a regulatory metal-protein complex by photooxidation of the metal-binding site, lowering the concentration of free metal ion, or protonation of amino acid side-chains in the metal-binding site, respectively. Repression of this gene is restored when cells are returned to the dark, metal ions are added in excess of chelating agents or the pH of the medium is returned to neutral. Restoration of repression is inhibited by chloramphenicol and novobiocin, which suggests that this process requires resynthesis of a regulatory protein and reassociation of the complex with DNA. The hypothesis predicts a number of characteristics of the regulatory system, most of which are found among the effects ofphytochrome. The well-documented effects of transition metal ions on the Pfr form ofphytochrome suggest that changes in gene expression in plants mediated by photoisomerisation of this chromoprotein are initiated by a chelating activity of Pfr-