Negative control of cell size in the cyanobacterium Synechococcus elongatus PCC 7942 by the essential response regulator RpaB

► S. elongatus strains with either constitutively increased or decreased expression of the essential rpaB gene are viable. ► The intracellular levels of RpaB affect cell size. ► Phosphorylated and non-phosphorylated RpaB play different cellular roles.