Background: Translesion synthesis requires the scaffolding function of the Rev1 CTD. Results: We determined the structures of the Rev1 CTD and its complex with Pol and mapped its Rev7-binding surface. Conclusion: Distinct surfaces of the Rev1 CTD separately mediate the assembly of extension and insertion translesion polymerase complexes. Significance: Cancer therapeutics could be developed by inhibiting Rev1 CTD-mediated translesion synthesis.

Phytochromes initiate chloroplast biogenesis by activating genes encoding the photosynthetic apparatus, including photosynthesis-associated plastid-encoded genes ( PhAPG s). PhAPG s are transcribed by a bacterial-type RNA polymerase (PEP), but how phytochromes in the nucleus activate chloroplast gene expression remains enigmatic. We report here a forward genetic screen in Arabidopsis that identified NUCLEAR CONTROL OF PEP ACTIVITY (NCP) as a necessary component of phytochrome signaling for PhAPG activation. NCP is dual-targeted to plastids and the nucleus. While nuclear NCP mediates the degradation of two repressors of chloroplast biogenesis, PIF1 and PIF3, NCP in plastids promotes the assembly of the PEP complex for PhAPG transcription. NCP and its paralog RCB are non-catalytic thioredoxin-like proteins that diverged in seed plants to adopt nonredundant functions in phytochrome signaling. These results support a model in which phytochromes control PhAPG expression through light-dependent double nuclear and plastidial switches that are linked by evolutionarily conserved and dual-localized regulatory proteins.