Homologous chromosome number (ploidy) has diversified among bacteria, archaea, and eukaryotes over evolution. In bacteria, model organisms such as Escherichia coli possess a single chromosome. In contrast, other bacteria, including cyanobacteria, maintain multiple copies of individual chromosomes (polyploid). Although a correlation between ploidy level and cell size has been observed in bacteria and eukaryotes, it is poorly understood how replication of multi-copy chromosomes is regulated and how ploidy level is adjusted to cell size. Here we show that only one or a few multi-copy chromosomes are replicated at once in the cyanobacteria Synechococcus elongatus and that this restriction depends on regulation of DnaA activity. When cell growth rate was increased or decreased, DnaA level, DnaA activity, and the number of replicating chromosomes also increased or decreased in parallel, resulting in nearly constant chromosome copy number per unit cell volume. Thus, it is suggested that the stepwise replication of the genome enables cyanobacteria to maintain nearly constant gene copy number per unit cell volume.

Figure. 1: Microscopic images of SYBR Green-stained nucleoids, SSB-GFP (replicating chromosomes), and RpoC2-GFP (transcribed chromosomes). All copies of chromosomes are transcribed while chromosomes are replicated one by one.

Figure. 2: Schematic diagrams showing changes in cell volume (upper), gene copy number per cell (middle), and mRNA level per unit volume (bottom) during cell growth in the case of stepwise (as observed in this study) replication of multi-copy chromosomes.