The RUS1 (ROOT UVB SENSITIVE 1) Protein Is Required for Cold Resistance in Chlamydomonas reinhardtii

Low temperature critically influences cellular metabolism by impairing processes such as membrane fluidity, enzyme activity, and protein folding. However, the comprehensive genetic landscape and regulatory mechanisms governing cold acclimation remain poorly understood. Here, we performed high-throughput, pooled genetic screening in the model alga Chlamydomonas reinhardtii (C. reinhardtii) to identify genes essential for cold acclimation. Our screening revealed numerous candidate genes implicated not only in early cold response pathways but also in core cellular processes, including DNA dynamics, protein homeostasis, metabolic regulation, and substrate transport. Notably, we identified a member of the RUS (ROOT UVB SENSITIVE) family, encoding a conserved DUF647 domain protein, designated CrRUS1. CRISPR-generated rus1 mutant alleles in C. reinhardtii display a phenotype consistent with our screening: the mutants did not exhibit any visible growth defects, but show severe growth defects at low temperature. Interestingly, the cold-induced phenotypic changes in rus1 can be reversed by dark conditions, suggesting that CrRUS1 likely promotes cold acclimation in C. reinhardtii through a light-dependent pathway. Our work provides novel genetic resources and mechanistic insights into cold acclimation in C. reinhardtii, with potential translational relevance for enhancing cold tolerance in crop species.