Functional Analysis Of A Bacteroid Development Gene BacA In Mesorhizobium Huakuii

Mesorhizobium huakuii is capable of fixing atmospheric nitrogen in symbiosis with the legume Astragalus sinicus,which is an economically important forage and green manure growing in winter throughout Eastern Asia.So far,little is known about the molecular basis of events after the bacteria are released from infection threads into the nodule cells and before nitrogen fixation begins.In this study,a bacA gene homologue of S.meliloti was isolated from M.huakuii. Alignment of BacA amino acid sequences and topological prediction for cytoplasmic membrane-spanning region both demonstrated that the BacA protein of M.huakuii showed high homology to S.meliloti.Furthermore,phylogenetic analysis illustrated that BacA of M.huakuii was much closer to BacA of S.meliloti rather than SbmA of E.coli, which is functionally interchangeable with BacA of S.meliloti.Inactivation of the bacA gene abolished the ability of M.huakuii to establish a successful symbiosis with its host.Compared with wild-type M.huakuii,the bacA mutant M1 was more sensitive to cell envelope-disrupting agents(acid pH,ethanol,SDS,and crystal violet) and showed increased resistance to bleomycin,a glycolpeptide antibiotic.Mass spectrometry analysis revealed that BacA affected the very-long-chain fatty acids(27-OHC-28:0 and 29-OHC-30:0) contents of lipid A in M.huakuii.The evidences from this study suggest that the cell envelope was altered in the bacA mutant of M.huakuii.This alteration of cell envelope may also frustrate bacterial attempts to survive the environmental chang encountered within host cells,while there is no evidence whether this change of cell envelope is related to the unusual lipid A modification.