Replication coordination marks the domestication of large extrachromosomal replicons in bacteria

Bacterial genomes often include extrachromosomal replicons (ERs), ranging from small plasmids to nearly chromosome-sized elements, that foster genome plasticity and adaptation. Despite their prevalence, the mechanisms underlying ER domestication and their long-term adaptation within bacterial hosts remain largely unexplored. By analyzing over 40,000 complete bacterial genomes, we identified two main ER categories: small ERs with diverse GC content and large ERs (≥10% the size of the main chromosome) that closely match the GC content of the chromosome. Across multiple phyla, marker frequency analyses showed that large ERs maintain a 1:1 copy number with the chromosome. Another key finding of this study is that large ERs terminate replication in synchrony with the chromosome. Hi-C contact maps revealed consistent ori - ori interactions between chromosomes and ERs. In large ERs, inter-replichore and ter - ter interactions, along with the recruitment of key chromosomal segregation motifs, suggest the co-option of chromosome-associated replication and segregation machineries. Together, our findings indicate that as ERs become larger, they become increasingly reliant on chromosome-driven processes for stable inheritance, potentially explaining why they do not exceed the size of the chromosome.