Amino Acid Transport Systems in Biotechnologically Relevant Bacteria

Abstract

Rhizobia are soil bacteria that are able to fix nitrogen in symbiosis with plants from the family Leguminosae. Rhizobia populate both soil and nodule niches. The rhizobial genome can be divided into a core of housekeeping genes and an accessory pool of non-essential genes. The accessory genome, together with factors acting within plants, confers symbiotic interaction between rhizobia and plant hosts. Rhizobia are distributed within the α - and β subdivisions of the Proteobacteria and intermingled with non-symbiotic photosynthetic and pathogenic relatives in the following genera: Allorhizobium , Azorhizobium , Bradyrhizobium , Mesorhizobium , Rhizobium , Sinorhizobium and Methylobacterium ( α -rhizobia), as well as Burkholderia and Cupriavidus ( β -rhizobia). Recently α -proteobacterial nitrogen-fixing isolates have been identified in the genera Ochrobactrum, Devosia and Blastobacter . Lateral gene transfer of the mobile accessory genome is the most likely explanation for the occurrence of rhizobial symbiotic loci in distantly related genera of proteobacteria. Consequently, similar symbiotic types can be found in different chromosomal backgrounds, and the same chromosomal background can harbour different symbiotic genotypes. Thus, comparisons of genes from core loci and the accessory gene pool reflect their separate evolutionary histories. Similarly, phylogenies from ribosomal sequences of rhizobia do not show parallel divergence with plant taxonomy. Phylogenies of closely evolving organisms might develop in parallel. In the case of rhizobia, only phylogenetic trees based on symbiotic genes show some correlation with host plant range, indicating that evolution of nodulation genes could develop under the functional constraint of the plant.