AI Article Synopsis
- Rhizobia must adjust their regulatory networks and activate key genes for effective symbiosis with legumes, focusing on nodulation and nitrogen fixation.
- The ancestral regulator MucR1 is crucial for nitrogen fixation in Sinorhizobium fredii CCBAU45436 in soybean nodules, while its paralog MucR2, with a mutation, is located on a symbiosis plasmid and has horizontal transfer potential.
- RNA-seq analysis shows MucR1 is necessary for the expression of transporters needed for nitrogenase activity but not for the transcription of key nitrogen fixation genes, highlighting its evolutionary importance in supporting nitrogen fixation.
Article Abstract
To achieve effective symbiosis with legume, rhizobia should fine-tune their background regulation network in addition to activating key genes involved in nodulation (nod) and nitrogen fixation (nif). Here, we report that an ancestral zinc finger regulator, MucR1, other than its paralog, MucR2, carrying a frameshift mutation, is essential for supporting nitrogen fixation of Sinorhizobium fredii CCBAU45436 within soybean nodules. In contrast to the chromosomal mucR1, mucR2 is located on symbiosis plasmid, indicating its horizontal transfer potential. A MucR2 homolog lacking the frameshift mutation, such as the one from S. fredii NGR234, can complement phenotypic defects of the mucR1 mutant of CCBAU45436. RNA-seq analysis revealed that the MucR1 regulon of CCBAU45436 within nodules exhibits significant difference compared with that of free-living cells. MucR1 is required for active expression of transporters for phosphate, zinc, and elements essential for nitrogenase activity (iron, molybdenum, and sulfur) in nodules but is dispensable for transcription of key genes (nif/fix) involved in nitrogen fixation. Further reverse genetics suggests that S. fredii uses high-affinity transporters to meet the demand for zinc and phosphate within nodules. These findings, together with the horizontal transfer potential of the mucR homolog, imply an intriguing evolutionary role of this ancestral regulator in supporting nitrogen fixation.
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| http://dx.doi.org/10.1094/MPMI-01-16-0019-R | DOI Listing |
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