The ribonuclease E regulator RebA is essential for diazotrophic growth in the cyanobacterium <i>Anabaena</i> 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 <i>rebA</i> is essential for diazotrophic growth of <i>Anabaena</i> PCC 7120, a filamentous cyanobacterium capable of forming heterocysts-specialized nitrogen-fixing cells-upon nitrogen starvation. The <i>rebA</i> mutant strain (Δ<i>rebA</i>) 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 Δ<i>rebA</i> 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 Δ<i>rebA</i> 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 Δ<i>rebA</i> 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.