Most bacterial gene families are biased toward specific chromosomal positions

The arrangement of genes along bacterial chromosomes influences their expression through growth rate-dependent gene copy number changes during DNA replication. Although translation- and transcription-related genes often cluster near the origin of replication, the extent of positional biases across gene families remains unclear. We hypothesized that natural selection broadly favors specific chromosomal positions to optimize growth rate-dependent expression. Analyzing 910 bacterial species and proteomics data from Escherichia coli and Bacillus subtilis, we found that about two-thirds of bacterial gene families are positionally biased. Natural selection drives genes mainly toward the origin or terminus of replication, with the strongest selection in fast-growing species. Our findings reveal chromosomal positioning as a fundamental mechanism for coordinating gene expression with growth rate, highlighting evolutionary constraints on bacterial genome architecture.