Most Sinorhizobium meliloti Extracytoplasmic Function Sigma Factors Control Accessory Functions

Area of Science:

• Microbiology
• Molecular Biology
• Plant-Bacterial Interactions

Background:

• Bacteria utilize extracytoplasmic function sigma factors (ECF σs) to regulate gene expression in response to environmental cues.
• Sinorhizobium meliloti, a nitrogen-fixing symbiont, possesses 11 ECF σ genes, but their role in symbiosis and stress response is largely unknown.
• Understanding these regulatory mechanisms is crucial for optimizing symbiotic nitrogen fixation, a key factor in soil fertility and agriculture.

Purpose of the Study:

• To investigate the role of the 11 ECF σ factors in Sinorhizobium meliloti during symbiosis with Medicago sativa and Medicago truncatula.
• To determine if ECF σs are involved in stress responses, particularly cell envelope stress.
• To characterize the targets and regulatory motifs of S. meliloti ECF σs.

Main Methods:

• Genetic manipulation to create precise deletions of individual ECF σ and anti-σ genes, as well as a strain lacking all 11 ECF σ and anti-σ genes.
• Transcriptomic analysis to identify ECF σ targets.
• Phenotypic analysis including free-living growth, Biolog Phenotype MicroArray assays, and symbiotic performance assays on host plants.

Main Results:

• All 11 ECF σ and anti-σ genes could be deleted without affecting S. meliloti's free-living growth or its ability to form nitrogen-fixing nodules on Medicago plants.
• The all-ECF σ deletion strain exhibited normal responses to various stresses, including cell envelope stress.
• New targets for individual ECF σs were identified, and promoter motifs were confirmed for nine of them, suggesting accessory roles.

Conclusions:

• ECF σ factors are not essential for symbiotic nitrogen fixation in Sinorhizobium meliloti.
• These ECF σs primarily regulate accessory functions rather than core symbiotic processes or stress survival.
• The absence of ECF σs does not compromise the symbiotic relationship or the bacterium's ability to withstand environmental challenges.