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. The essential NblS–RpaB pathway for photosynthesis regulation and acclimatization to a variety of environmental conditions is the most conserved two-component system in cyanobacteria. To get insights into the RpaB implication in cell homeostasis we investigated the phenotypic impact of altering expression of the essential rpaB gene of Synechococcus elongatus PCC 7942 and determined the in vivo levels of the RpaB and RpaB∼P polypeptides. Our results implicate non-phosphorylated RpaB in controlling cell length and shape and suggest that intrinsic regulation may be important to prevent drastic variations in RpaB levels and activity.