Several mechanisms that increase the rate of mutagenesis across the entire genome have been identified; however, how the rate of evolution might be promoted in individual genes is unclear. Most genes in bacteria are encoded on the leading strand of replication. This presumably avoids the potentially detrimental head-on collisions that occur between the replication and transcription machineries when genes are encoded on the lagging strand.
A new study now describes the ubiquitous (core) genes in Bacillus subtilis and determine that 17% of them are on the lagging strand. The scientists find a higher rate of point mutations in the core genes on the lagging strand compared with those on the leading strand, with this difference being primarily in the amino-acid-changing (nonsynonymous) mutations. They determine that, overall, the genes under strong negative selection against amino-acid-changing mutations tend to be on the leading strand, co-oriented with replication. In contrast, on the basis of the rate of convergent mutations, genes under positive selection for amino-acid-changing mutations are more commonly found on the lagging strand, indicating faster adaptive evolution in many genes in the head-on orientation. Increased gene length and gene expression amounts are positively correlated with the rate of accumulation of nonsynonymous mutations in the head-on genes, suggesting that the conflict between replication and transcription could be a driving force behind these mutations. Indeed, using reversion assays, the scientists show that the difference in the rate of mutagenesis of genes in the two orientations is transcription dependent. Altogether, their findings indicate that head-on replication–transcription conflicts are more mutagenic than co-directional conflicts and that these encounters can significantly increase adaptive structural variation in the coded proteins. The researchers propose that bacteria, and potentially other organisms, promote faster evolution of specific genes through orientation-dependent encounters between DNA replication and transcription.