Conserved Functional Traits of the Atc Protein System

The Atc protein system (AtcJ, AtcA, AtcB and AtcC) plays a major role in cold adaptation in Shewanella oneidensis. AtcJ, a J-domain protein, interacts with the DnaK chaperone and binds AtcC through a conserved C-terminal PX7W motif in which Trp is crucial. Overproduced AtcB inhibits RNA polymerase, while the AtcJ-AtcC complex recruits DnaK to modulate this inhibition. This study investigates the conservation of these functional traits across divergent bacteria. We characterized several non-canonical Atc systems and demonstrated that the core interaction network remains functionally preserved despite remarkable variability in the AtcJ C-terminal motif, ranging from intact PX7W to degenerate or absent forms, but also in the size of AtcC. Phylogenetic analysis revealed vertical inheritance and co-evolution with host transcriptional machinery. AtcB, AtcC, and AtcJ exhibit strong phylogenetic congruence with the species tree, while AtcA shows greater evolutionary flexibility. The system is enriched in aquatic and psychrophilic lineages but absent in thermophiles. Conditional toxicity assays revealed that AtcB-RNAP interaction depends on specific structural determinants, with DnaK recruitment alleviating toxicity. These findings support a unified molecular mechanism coupling chaperone activity to transcriptional regulation under environmental stress, suggesting that the Atc system represents an ancient and adaptable regulatory module in bacteria.