Oxygen control in Rhizobium.

Rhizobia are gram-negative bacteria with two distinct habitats: the soil rhizosphere in which they have a saprophytic and, usually, aerobic life and a plant ecological niche, the legume nodule, which constitutes a microoxic environment compatible with the operation of the nitrogen reducing enzyme nitrogenase. The purpose of this review is to summarize the present knowledge of the changes induced in these bacteria when shifting to a microoxic environment. Oxygen concentration regulates the expression of two major metabolic pathways: energy conservation by respiratory chains and nitrogen fixation. After reviewing the genetic data on these metabolic pathways and their response to oxygen we will put special emphasis on the regulatory molecules which are involved in the control of gene expression. We will show that, although homologous regulatory molecules allow response to oxygen in different species, they are assembled in various combinations resulting in a variable regulatory coupling between genes for microaerobic respiration and nitrogen fixation genes. The significance of coordinated regulation of genes not essential for nitrogen fixation with nitrogen fixation genes will also be discussed.