Fitness Effects of Replichore Imbalance in Salmonella enterica

Abstract

A fitness cost due to imbalanced replichores has been proposed to provoke chromosome rearrangements in Salmonella enterica serovars. To determine the impact of replichore imbalance on fitness, the relative fitness of isogenic Salmonella strains containing transposon-held duplications of various sizes and at various chromosomal locations was determined. Although duplication of certain genes influenced fitness, a replichore imbalance of up to 16°did not affect fitness.The bacterial chromosome is a dynamic molecule that can undergo various types of rearrangements, including inversions, translocations, and duplications. These rearrangements can alter gene order and change replichore length. Replichores are defined as the halves of the chromosome between the origin of replication and the terminus region in the vicinity of the dif site (4, 6, 10, 15). In most bacteria, both replichores are approximately the same length, with each comprising 180°around the circular chromosome. In this state, the replichores and DNA replication are balanced. Imbalance is introduced when one replichore becomes longer than the other. For example, if the replichores comprise 200°and 160°around the circular chromosome, the replichores are 20°imbalanced. Various studies over the years have investigated the effect of imbalanced DNA replication on fitness in Escherichia coli (8,9,16,17). In a recent analysis, E. coli strains that contained asymmetrical interreplichore inversions were found to have growth defects when the imbalance was at least 50°(8). While these studies have demonstrated that imbalanced replichores can affect fitness, the approaches used to introduce the imbalance either do not occur or are extremely rare in nature.Duplications play important evolutionary roles because the increase in gene copy number can facilitate adaptation to certain growth conditions (20), as well as being a source for the evolution of new genes (3). Typically duplications occur at frequencies between 10 Ϫ3 and 10 Ϫ5 in unselected cultures (2) but are lost at rates of up to 1,000-fold more often if they do not provide a selective advantage (19). Duplications also result in replichore imbalance. However, the effect of duplications on fitness in relation to replichore balance has not been investigated.A major hurdle in studying the effects of duplications on fitness is that if the duplication is detrimental, haploid revertants will outgrow the parental strain containing the duplication. To circumvent this problem, we used a set of 11 isogenic Salmonella enterica strains with transposon-held duplications (5) ( Fig. 1 and Table 1). Culturing these strains in the presence of chloramphenicol selects for the duplication because if the duplication collapses, the transposon is lost and the cells become chloramphenicol sensitive. As the size of the duplications in these strains varies, so does the amount of introduced replichore imbalance, ranging from 5°to 23°. In addition, fitness effects due to the location of the duplication versus the size of the duplication ...

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