Osmoregulation in Rhizobium Meliloti: Control of Glycine Betaine Biosynthesis and Catabolism

Abstract

In the root nodule bacterium Rhizobium meliloti, glycine betaine and choline can support growth, and they are also effective osmoprotectants. A kinetic approach has been used to investigate the possibility of multiple transport systems. Three separate transport activities for choline have been distinguished: two high-affinity transport activities, one induced by choline itself, and one low-affinity transport activity. Only the induced high- and low-affinity activities are important for osmoregulation. In the case of glycine betaine a high-affinity transport activity strongly stimulated by high osmolarity has been characterized. A periplasmic glycine betaine-binding protein has been found. Cells never accumulate choline which is rapidly converted into glycine betaine. Very high intracellular levels of glycine betaine have been observed in cells maintained under salt stress whereas a strong catabolism exists in low-salt-grown cells. High osmolarity in the medium decreased the activities of enzymes involved in the degradation of glycine betaine but not those of enzymes that lead to its biosynthesis from choline.