Opposing action of photosystem II assembly factors RBD1 and HCF136 underlies light-regulated <i>psbA</i> translation in plant chloroplasts

The D1 subunit of photosystem II (PSII) is subject to light-induced damage. In plants, D1 photodamage activates translation of chloroplast <i>psbA</i> mRNA encoding D1, providing D1 for PSII repair. Three D1 assembly factors have been implicated in the regulatory mechanism: HCF244 and RBD1 activate <i>psbA</i> translation, whereas HCF136 represses <i>psbA</i> translation in the dark. To clarify the regulatory circuit, we analyzed <i>psbA</i> ribosome occupancy in dark-adapted and illuminated <i>rbd1</i> and <i>rbd1;hcf136</i> double mutants in Arabidopsis and in Zm-<i>hcf244</i> and Zm-<i>hcf244;</i>Zm-<i>hcf136</i> double mutants in maize. The results show that RBD1 is required for light-induced <i>psbA</i> translation but has only a small effect on <i>psbA</i> ribosome occupancy in the dark. RBD1 is not required for <i>psbA</i> translation when HCF136 is absent, indicating that RBD1 activates <i>psbA</i> translation in the light by inhibiting HCF136's repressive effect. By contrast, HCF244 is required to recruit ribosomes to <i>psbA</i> mRNA in light, dark, and in the absence of HCF136. We demonstrate further that HCF244 is not required for the translational activator HCF173 to bind the <i>psbA</i> 5'UTR. These results show that RBD1 is central to the perception of the D1 photodamage that triggers D1 synthesis and that it activates <i>psbA</i> translation by relieving repression by an HCF136-dependent assembly intermediate. HCF244 activates downstream of those events without impacting HCF173's binding to <i>psbA</i> mRNA. The results implicate a feature of nascent D1 that is affected by both HCF136 and RBD1 as the signal that reports D1 photodamage to regulate <i>psbA</i> translation rate as needed for PSII repair.