Even if one considers the large size of the B. japonicum genome (9.1 Mbp), the number of 24 putative RND-type transporters it encodes is notably high. Yet, none of the defects in the ΔbdeAB strain could apparently be rescued by its paralogs, suggesting a rather specific substrate profile of the BdeAB exporter. This is in line with the fact that BdeB clusters phylogenetically with its orthologs from other bacteria rather than with its paralogs. Based on
14 aa involved in E. coli AcrB ligand binding, Hernandez-Mendoza et al. (2007) defined 16 groups of different ligand preferences among 47 RND proteins. While the phylogenetic similarity check details between BdeB and the four RND transporters MexD, AmrB, MexY, and MtrD is well reflected at the critical amino acid positions (two to four identical positions among the five most informative amino acids), a correlation with substrate preferences is less obvious because individual BdeB orthologs have relatively broad substrate ranges that may (MexD,
AmrB, and MexY) or may not (MtrD) include aminoglycosides. In any case, it appears that apart from extruding antimicrobial compounds, efflux systems may also contribute to the export of intrinsic, potentially harmful molecules, for example those generated under oxidative or membrane stress conditions (Poole, 2007). Inner membrane export systems require the presence of an outer membrane factor (OMF) in order to Everolimus channel the substrate to the extracellular space (Koronakis et al., 2004). The OMF structural gene often maps in close vicinity to the genes for the transport system (Poole et al., 1993). Yet, inspection of the genomic region around bdeAB did not reveal a gene that might code for an OMF. According to the Transport Database (Ren et al., 2007), the B. japonicum genome encodes nine putative OMFs. It is conceivable that any one of these OMFs may form a tripartite efflux pump together
with BdeAB. Intriguingly, however, none of them seems to belong to the RegR regulon (Lindemann et al., 2007). Originally, the symbiotic defect of the ΔregR mutant was solely explained selleck products by the fact that RegR in the wild type activates the essential nitrogen-fixation regulatory gene nifA (Bauer et al., 1998). However, subsequent microarray experiments revealed that the RegR regulon comprises numerous NifA-independent genes, which, in principle, may contribute to symbiosis (Lindemann et al., 2007). The key finding of this study, i.e., mutation of the RegR-controlled bdeAB genes causes a symbiotic defect, demonstrates that such genes indeed exist. Moreover, the defect is observed primarily on soybean and much less so on the other hosts tested (cowpea, mungbean, and siratro). Interestingly, based on recent proteomics data, RegR and, although marginally, also BdeA are more abundant in soybean than in cowpea and siratro.