CONTROL OF THE EXPRESSION OF BACTERIAL GENES INVOLVED IN SYMBIOTIC NITROGEN-FIXATION

Several genera of N2-fixing bacteria establish symbiotic associations with plants. Among these, the genus Rhizobium has the most significant contribution, in terms of yield, in many important crop plants. The e stablishment of the Rhizobium-legume symbiosis is a very complex proce ss involving many genes which need to be co-ordinately regulated. In t he first instance, plant signal molecules, known to be flavonoids, tri gger the expression of host-specific genes in the bacterial partner th rough the action of the regulatory NodD protein. In response to these signals, Rhizobium bacteria synthesize lipo-oligosaccharide molecules which in turn cause cell differentiation and nodule development. Once the nodule has formed, Rhizobium cells differentiate into bacteroids a nd another set of genes is activated. These genes, designated nif and fix, are responsible for N2 fixation. In this system, several regulato ry proteins are involved in a complex manner, the most important being NifA and a two component (FixK and FixL) regulatory system. Our knowl edge about the establishment of these symbioses has advanced recently, although there are many questions yet to be solved.