Specific features of rol-gene polymorphism of Rhizobium rhizogenes

Bacterium Rhizobium rhizogenes has the unique ability to cause hairy root disease. Symptoms of this disease occur when the bacterial genes rolA, rolB, rolC, and rolD, contained in plasmid T-DNA, are expressed after integration into the plant genome. A biotechnological method for obtaining fast-growing in vitro cultures of hairy roots capable of synthesizing secondary metabolites is based on this feature. Also, the ability of the bacterium to mediate horizontal gene transfer may have led to the appearance of rol-gene homologs in plant genomes, which by now have been found in a wide range of species. The variability of rol-genes may play an important role in the evolution of the whole mechanism of natural transformation, as it can potentially affect the physiological properties of transformed plants. The sequence analysis revealed a significant overall degree of variability in rol-genes between bacterial strains. But at the same time, a part of rol-gene sequences remained conserved in both bacterial and plant genomes. A detailed study demonstrated that in all of the considered bacterial rol-genes, as well as in plant genes potentially capable of expression in full-length form, the variability was represented by either nucleotide substitutions or insertions and deletions of multiples of three, which did not permit reading frame displacement.