A family of RRM-1 RNA binding proteins enables cold adaptation and environmental resilience in Bacteroides

Bacteria use post-transcriptional regulatory mechanisms to rapidly adjust gene expression during environmental change. In the gut-associated genus Bacteroides, these mechanisms remain poorly defined as these organisms lack canonical RNA chaperones like Hfq and CsrA that coordinate post-transcriptional stress responses in many well-studied model bacteria. Most Bacteroides possess conserved RNA recognition motif-1 (RRM-1) domain-containing RNA-binding proteins (more common in eukaryotes than bacteria) that have been proposed to act as global RNA chaperones. Here, we show that these RNA binding proteins (RBPs) are central to cold stress adaptation. Simultaneous deletion of all rbp genes produces a cold-sensitive growth defect across multiple Bacteroides species, while single deletions do not, revealing conserved functional redundancy. RBP transcripts and proteins accumulate rapidly after temperature downshift, and loss of RBPs extensively reprograms the transcriptome. Cold sensitivity of Bacteroides rbp mutants is not caused by defects in ribosome assembly or rRNA maturation. Instead, we find that in Bacteroides thetaiotaomicron, RBPs act together with BT1884, the sole canonical cold shock protein possessed by this organism. The combined loss of RBPs and BT1884 produces a synthetic severe cold sensitivity phenotype, defining two functionally redundant cold stress systems belonging to unrelated protein families. Strains lacking RBPs show reduced survival under simultaneous cold and oxygen stress, the conditions Bacteroides cells are expected to encounter during host-to-host transmission. Together, these findings establish RRM-1 RBPs as non-canonical cold shock proteins that enable cold adaptation and environmental survival in Bacteroides and suggest how these organisms withstand the stresses of transmission between hosts. IMPORTANCEBacteroides species are among the most abundant and stable members of the human gut microbiome, and they are also among the most readily transmitted between people. Reaching a new host requires surviving conditions outside the gut, including cold and oxygen exposure, yet how these bacteria withstand such stress is not well understood. Most bacteria manage stress using a well-defined set of RNA-binding proteins, but Bacteroides lack these canonical factors. We show that Bacteroides instead rely on a different family of RNA-binding proteins, more typical of eukaryotes than bacteria, to survive cold stress, and that these proteins promote survival under the conditions encountered during transmission. This work identifies a molecular system that allows an abundant and ecologically successful gut bacterium to endure the environmental challenges of moving between hosts.