Required for light-regulated photosystem II (PSII) biogenesis and grana thylakoids formation by binding to the 5' UTR of PSII subunit mRNAs (e.g. psbJ, psbN and psbA) in a light-dependent manner through a redox-based mechanism, and facilitating the association of HCF173 with target mRNAs, which encodes PSII reaction center proteins (e.g. J, N and D1), thus regulating its expression by modulating ribosome loading. Interacts with HCF173. Predominantly present at thylakoid membranes. Retardation of photoautotrophic growth (PubMed:29891689). Reduced efficiency of photosystem II (PSII) subunit mRNAs (e.g. psbJ, psbN and psbA) ribosome loading and impaired synthesis of PSII reaction center proteins (e.g. J, N and D1) leading to reduced PSII activity and biogenesis, as well as reduced grana thylakoid formation (PubMed:29891689, PubMed:30844105). Reduced production of PSII subunits D1, D2, CP43, CP47, PsbE, PsbF, and PsbO, and, to a lower extent, of PSI subunits PsaA and PsaB (PubMed:29891689, PubMed:30844105). High levels of nonphotochemical quenching (NPQ) (PubMed:29891689). Belongs to the PPR family. P subfamily. Sequencing errors.