Evolution of Bacterial Genome under Changing Mutational Pressure - Computer Simulation Studies

The main force shaping the structure of bacterial chromosomes is the replication-associated mutational pressure which is characterized by distinct nucleotide substitution patterns acting on differently replicated DNA strands (leading and lagging). Therefore, the composition of DNA strands is asymmetric and it is important at which strand a gene is located and into which strand it could be translocated. Thus, the mutational pressure restricts also intragenomic translocations. To analyze this effect, we have elaborated a simulation model of bacterial genome evolution assuming translocation of protein coding genes and different types of selection acting on their sequences. The ’negative’ selection eliminated individuals if the coding signal of any gene in its genome dropped below the acceptable range, whereas the ’stabilizing’ selection did not allow for the decrease in the coding signal of any gene below its original value. Under the ’negative’ selection more genes stayed or were translocated to the lagging strand, whereas under the ’stabilizing’ selection more genes preferred the leading strand. The ’stabilizing’ selection eliminated more individuals because of the coding signal loss and slightly fewer because of the stop codon generation. The ’stabilizing’ selection allowed also for much less gene translocations between strands than the ’negative’ selection.