Abstract
BackgroundA wide variety of bacterial adaptative responses to environmental conditions are mediated by signal transduction pathways. Two-component signal transduction systems are one of the predominant means used by bacteria to sense the signals of the host plant and adjust their interaction behaviour. A total of seven open reading frames have been identified as putative two-component response regulators in the gram-negative nitrogen-fixing bacteria Azorhizobium caulinodans ORS571. However, the biological functions of these response regulators in the symbiotic interactions between A. caulinodans ORS571 and the host plant Sesbania rostrata have not been elucidated to date.ResultsIn this study, we identified and investigated a two-component response regulator, AcfR, with a phosphorylatable N-terminal REC (receiver) domain and a C-terminal HTH (helix-turn-helix) LuxR DNA-binding domain in A. caulinodans ORS571. Phylogenetic analysis showed that AcfR possessed close evolutionary relationships with NarL/FixJ family regulators. In addition, six histidine kinases containing HATPase_c and HisKA domains were predicted to interact with AcfR. Furthermore, the biological function of AcfR in free-living and symbiotic conditions was elucidated by comparing the wild-type strain and the ΔacfR mutant strain. In the free-living state, the cell motility behaviour and exopolysaccharide production of the ΔacfR mutant were significantly reduced compared to those of the wild-type strain. In the symbiotic state, the ΔacfR mutant showed a competitive nodule defect on the stems and roots of the host plant, suggesting that AcfR can provide A. caulinodans with an effective competitive ability for symbiotic nodulation.ConclusionsOur results showed that AcfR, as a response regulator, regulates numerous phenotypes of A. caulinodans under the free-living conditions and in symbiosis with the host plant. The results of this study help to elucidate the involvement of a REC + HTH_LuxR two-component response regulator in the Rhizobium-host plant interaction.
Highlights
A wide variety of bacterial adaptative responses to environmental conditions are mediated by signal transduction pathways
The protein sequence was entered in SMART, which indicated that it contains an N-terminal signal receiver domain Receiver domain (REC) (SM00448) and an HTH_LuxR DNA-binding domain (SM00421) (Supplemental Fig. 1a)
The evolutionary relationship between AcfR and several proteins that contain REC + HTH_LuxR domains was analysed. These proteins were aligned with the ClustalW program and the phylogenetic tree was created with MEGA-X software by the neighbor-joining method
Summary
A wide variety of bacterial adaptative responses to environmental conditions are mediated by signal transduction pathways. A total of seven open reading frames have been identified as putative two-component response regulators in the gram-negative nitrogen-fixing bacteria Azorhizobium caulinodans ORS571. The biological functions of these response regulators in the symbiotic interactions between A. caulinodans ORS571 and the host plant Sesbania rostrata have not been elucidated to date. The alphaproteobacterium Azorhizobium caulinodans ORS571, as a gram-negative nitrogen-fixing bacterium, has the dual ability to fix nitrogen both under free-living conditions and in a symbiotic interaction with the tropical legume Sesbania rostrata, which forms both stem nodules and root nodules [1]. Host plants signal compounds, which are released by induced rhizobial cells, induce the curling of root hairs. The interaction between rhizobia and the host is initiated by a complex molecular dialogue
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