PLOS Genetics study shows replication–transcription collisions constrain gene length differently on leading and lagging DNA strands in bacteria

A study published in PLOS Genetics by Anjali Variyar, Samhitha Patil, Jebin Babu, Akanksha Bhat, and T. Sabari Sankar investigates why gene length is constrained differently depending on a gene's orientation relative to the direction of DNA replication in bacteria.

Gene length shows remarkably little variation within domains of life, suggesting conserved underlying constraints. The authors focus on the well-documented observation — in Escherichia coli and Bacillus subtilis — that genes on the lagging strand tend to be shorter than those on the leading strand. The study provides evidence that this asymmetry arises from replication–transcription collisions: when the replication fork and RNA polymerase travel in opposing directions on the lagging strand (head-on collisions), the probability of DNA damage and replication fork stalling increases with gene length. Longer genes therefore face stronger negative selection on the lagging strand.

Leading-strand genes, where replication and transcription proceed in the same direction (co-directional), are subject to less severe collision-associated fitness costs, and can therefore tolerate greater length.

The findings contribute to the mechanistic understanding of how genome architecture is shaped by the interplay between DNA replication and gene expression — a question relevant to understanding mutation bias, genomic organisation, and evolutionary genome biology. The work is applicable to researchers in molecular evolution, microbial genomics, and anyone teaching DNA replication and transcription at undergraduate or postgraduate level.