Vegetative cells may perform nitrogen fixation function under nitrogen deprivation in Anabaena sp. strain PCC 7120 based on genome-wide differential expression analysis.

Lilong Huang,Hongli He,Runyu Miao,Yunqing Cheng,Hongshan Jiang

doi:10.1371/journal.pone.0248155

Abstract

Nitrogen assimilation is strictly regulated in cyanobacteria. In an inorganic nitrogen-deficient environment, some vegetative cells of the cyanobacterium Anabaena differentiate into heterocysts. We assessed the photosynthesis and nitrogen-fixing capacities of heterocysts and vegetative cells, respectively, at the transcriptome level. RNA extracted from nitrogen-replete vegetative cells (NVs), nitrogen-deprived vegetative cells (NDVs), and nitrogen-deprived heterocysts (NDHs) in Anabaena sp. strain PCC 7120 was evaluated by transcriptome sequencing. Paired comparisons of NVs vs. NDHs, NVs vs. NDVs, and NDVs vs. NDHs revealed 2,044 differentially expressed genes (DEGs). Kyoto Encyclopedia of Genes and Genomes enrichment analysis of the DEGs showed that carbon fixation in photosynthetic organisms and several nitrogen metabolism-related pathways were significantly enriched. Synthesis of Gvp (Gas vesicle synthesis protein gene) in NVs was blocked by nitrogen deprivation, which may cause Anabaena cells to sink and promote nitrogen fixation under anaerobic conditions; in contrast, heterocysts may perform photosynthesis under nitrogen deprivation conditions, whereas the nitrogen fixation capability of vegetative cells was promoted by nitrogen deprivation. Immunofluorescence analysis of nitrogenase iron protein suggested that the nitrogen fixation capability of vegetative cells was promoted by nitrogen deprivation. Our findings provide insight into the molecular mechanisms underlying nitrogen fixation and photosynthesis in vegetative cells and heterocysts at the transcriptome level. This study provides a foundation for further functional verification of heterocyst growth, differentiation, and water bloom control.

Highlights

Cyanobacteria evolved approximately 3.5 billion years ago
These results suggest that most differentially expressed genes (DEGs) were involved in metabolism, carbon fixation, and the synthesis of important nitrogenous biomacromolecules including, but not limited to, arginine, DNA, and purine, providing important insight into screening of candidate genes involved in regulating vegetative cells and heterocysts in response to nitrogen deprivation conditions
Under conditions of nitrogenreplete conditions, cyanobacterial filaments consisted of only vegetative cells (Fig 1A); under nitrogen deprivation conditions, these filaments consisted of both vegetative cells and heterocysts (Fig 1B)

Summary

Introduction

Cyanobacteria evolved approximately 3.5 billion years ago. As the first photosynthetic prokaryotes on earth, cyanobacteria can efficiently fix atmospheric nitrogen via a process catalyzed by nitrogenase [1]. Fragments per kilobase of transcript per million mapped reads (FPKM) values were used to calculate gene expression levels; the results suggested that the average FPKM values of NDHs were lower than those of NVs and NDVs (Fig 2A). KEGG functional enrichment analysis of all DEGs was performed to determine the pathways involved in regulating the responses of vegetative cells and heterocysts to nitrogen deprivation conditions. These results suggest that most DEGs were involved in metabolism, carbon fixation, and the synthesis of important nitrogenous biomacromolecules including, but not limited to, arginine, DNA, and purine, providing important insight into screening of candidate genes involved in regulating vegetative cells and heterocysts in response to nitrogen deprivation conditions.

Discussion

Findings

Conclusion

Materials and methods