Replication orientation affects the rate and direction of bacterial gene evolution

In many bacterial genomes, the leading and lagging strands have different s kews in base composition; for example, an excess of guanosine compared to c ytosine on the leading strand. We find that Chlamydia genes that have switc hed their orientation relative to the direction of replication, for example by inversion, acquire the skew of their new "host" strand. In contrast to most evolutionary processes, which have unpredictable effects on the sequen ce of a gene, replication-related skews reflect a directional evolutionary force that causes predictable changes in the base composition of switched g enes, resulting in increased DNA and amino acid sequence divergence.