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

## TL;DR

The protein RebA is crucial for nitrogen fixation in the cyanobacterium Anabaena PCC 7120 by regulating RNA metabolism during heterocyst development.

## Contribution

This study reveals a novel posttranscriptional regulatory mechanism linking RNase E activity to heterocyst development and nitrogen transport in cyanobacteria.

## Key findings

- The rebA mutant (ΔrebA) shows severe growth defects under nitrogen starvation despite increased heterocyst formation.
- RebA is transiently upregulated during heterocyst differentiation and is required for normal thylakoid structure and pigment synthesis.
- The ΔrebA phenotype is rescued only by wild-type RebA, confirming its function depends on binding to RNase E.

## Abstract

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.

Cyanobacteria play a vital role in global nitrogen cycling and hold great potential for applications in sustainable agriculture and biotechnology. This study uncovers a previously unrecognized regulatory mechanism in cyanobacterial nitrogen fixation, demonstrating that RebA, an inhibitor of ribonuclease E (RNase E), is essential for heterocyst differentiation and function in Anabaena PCC 7120. By regulating RNase E activity, RebA influences heterocyst frequency, thylakoid integrity, pigment synthesis, and fixed nitrogen transport. These findings provide novel insights into the role of RNA metabolism in diazotrophic growth, advancing our understanding of cyanobacterial physiology and its broader ecological and biotechnological implications.