RT Journal Article SR Electronic T1 Chloroplast-localized translation for protein targeting in Chlamydomonas reinhardtii JF bioRxiv FD Cold Spring Harbor Laboratory SP 2021.12.27.474283 DO 10.1101/2021.12.27.474283 A1 Yi Sun A1 Shiva Bakhtiari A1 Melissa Valente-Paterno A1 Yanxia Wu A1 Christopher Law A1 Daniel Dai A1 James Dhaliwal A1 Khanh Huy Bui A1 William Zerges YR 2021 UL http://biorxiv.org/content/early/2021/12/28/2021.12.27.474283.abstract AB Translation is localized within cells to target proteins to their proper locations. We asked whether translation occurs on the chloroplast surface in Chlamydomonas and, if so, whether it is involved in co-translational protein targeting, aligned spatially with localized translation by the bacterial-type ribosomes within this organelle, or both. Our results reveal a domain of the chloroplast envelope which is bound by translating ribosomes. Purified chloroplasts retained ribosomes and mRNAs encoding two chloroplast proteins specifically on this “translation domain”, but not a mRNA encoding a cytoplasmic protein. Ribosomes clusters were seen on this domain by electron tomography. Activity of the chloroplast-bound ribosomes is supported by results of the ribopuromycylation and puromycin-release assays. Co-translational chloroplast protein import is supported by nascent polypeptide dependency of the ribosome-chloroplast associations. This cytoplasmic translation domain aligns localized translation by organellar bacterial-type ribosomes in the chloroplast. This juxtaposition the dual translation systems facilitates the targeting and assembly of the polypeptide products.One-Sentence Summary Translation is localized to a domain of the chloroplast envelope for co-translational protein targeting in Chlamydomonas.Competing Interest StatementThe authors have declared no competing interest.