The ribonuclease E regulator RebA is essential for diazotrophic growth in the cyanobacterium Anabaena PCC 7120.

Ribonuclease E (RNase E) is central to bacterial RNA metabolism. In cyanobacteria, its activity is inhibited by RebA, a key mechanism for controlling cell morphology. Here, we demonstrate that rebA is essential for diazotrophic growth of Anabaena PCC 7120, a filamentous cyanobacterium capable of forming heterocysts-specialized nitrogen-fixing cells-upon nitrogen starvation. The rebA mutant strain (ΔrebA) showed severe growth defects in nitrogen-deprived conditions, despite forming more heterocysts than the wild type. With a GFP fusion strain, we show that RebA is transiently upregulated during heterocyst differentiation. Microscopic and ultrastructural analyses revealed that ΔrebA heterocysts accumulated abnormally large cyanophycin granules, while vegetative cells showed reduced pigment levels and disorganized thylakoid membranes, phenotypes indicative of a severe nitrogen deficiency response. However, esculin tracer diffusion and SepJ-GFP localization in ΔrebA were comparable to the wild type, suggesting that cell-cell communication via septal junctions remains functional. Thus, the growth defect likely results from impaired degradation or mobilization of fixed nitrogen. Notably, the ΔrebA phenotype could be rescued only by wild-type RebA, but not by variants unable to bind RNase E, indicating that RebA's function depends on its modulation of RNase E activity. Together, these findings reveal a key posttranscriptional mechanism linking RNase E regulation to heterocyst development and intercellular nutrient transfer, highlighting the importance of regulated RNA metabolism for diazotrophic growth.