Low light-regulated intramolecular disulfide fine-tunes Arabidopsis PTOX role.

Disulfide-based regulation links the activity of numerous chloroplast proteins with photosynthesis-derived redox signals. The Plastid Terminal Oxidase (PTOX) is a thylakoid-bound plastoquinol oxidase that has been implicated in multiple roles in the light and in the dark, which could require different levels of PTOX activity. Here we show that Arabidopsis PTOX contains a conserved C-terminus domain (CTD) with cysteines that evolved progressively following plants colonization of the land. Furthermore, the CTD contains a regulatory disulfide that is in the oxidized state in the dark and is rapidly reduced, within 5 min, in low, 1-5 µE*m-2 *s-1 , light intensity. The reduced PTOX form in the light was reoxidized within 15 min after the transition to the dark. Mutation of the cysteines in the CTD prevented the formation of the oxidized form. This resulted in higher levels of reduced plastoquinone when measured at transition to the onset of low light. This is consistent with the reduced state of PTOX exhibiting diminished PTOX oxidase activity under conditions of limiting PQH2 substrate. Our findings suggest that AtPTOX-CTD evolved to add light-dependent regulation of PTOX activity for the adaptation of plants to terrestrial conditions.