Biological Impact of a Large-Scale Genomic Inversion That Grossly Disrupts the Relative Positions of the Origin and Terminus Loci of the Streptococcus pyogenes Chromosome

Abstract

A large-scale genomic inversion encompassing 0.79 Mb of the 1.816-Mb-long Streptococcus pyogenes serotype M49 strain NZ131 chromosome spontaneously occurs in a minor subpopulation of cells, and in this report genetic selection was used to obtain a stable lineage with this chromosomal rearrangement. This inversion, which drastically displaces the ori site relative to the terminus, changes the relative length of the replication arms so that one replichore is approximately 0.41 Mb while the other is about 1.40 Mb in length. Genomic reversion to the original chromosome constellation is not observed in PCR-monitored analyses after 180 generations of growth in rich medium. Compared to the parental strain, the inversion surprisingly demonstrates a nearly identical growth pattern in the first phase of the exponential phase, but differences do occur when resources in the medium become limited. When cultured separately in rich medium during prolonged stationary phase or in an experimental acute infection animal model (Galleria mellonella), the parental strain and the invertant have equivalent survival rates. However, when they are coincubated together, both in vitro and in vivo, the survival of the invertant declines relative to the level for the parental strain. The accompanying aspect of the study suggests that inversions taking place near oriC always happen to secure the linkage of oriC to DNA sequences responsible for chromosome partition. The biological relevance of large-scale inversions is also discussed.IMPORTANCE Based on our previous work, we created to our knowledge the largest asymmetric inversion, covering 43.5% of the S. pyogenes genome. In spite of a drastic replacement of origin of replication and the unbalanced size of replichores (1.4 Mb versus 0.41 Mb), the invertant, when not challenged with its progenitor, showed impressive vitality for growth in vitro and in pathogenesis assays. The mutant supports the existing idea that slightly deleterious mutations can provide the setting for secondary adaptive changes. Furthermore, comparative analysis of the mutant with previously published data strongly indicates that even large genomic rearrangements survive provided that the integrity of the oriC and the chromosome partition cluster is preserved.

Keywords: Streptococcus pyogenes; asymmetric DNA inversions; chromosomal rearrangements; population competitions; survival.