Physiology of Ex Planta Nitrogenase Activity in Rhizobium japonicum

Abstract

Thirty-nine wild-type strains of Rhizobium japonicum have been studied for their ability to synthesize nitrogenase ex planta in defined liquid media under microaerobic conditions. Twenty-one produced more than trace amounts of acetylene reduction activity, but only a few of these yielded high activity. The oxygen response curves were similar for most of the nitrogenase-positive strains. The strains derepressible for activity had several phenotypic characteristics different from non-derepressible strains. These included slower growth and lower oxygen consumption under microaerobic conditions and lower extracellular polysaccharide production. Extracellular polysaccharide production during growth on gluconate in every nitrogenase-positive strain assayed was lower under both aerobic and microaerobic conditions than the non-derepressible strains. These phenotypic characteristics may be representative of a genotype of a subspecies of R. japonicum. These studies were done in part to enlarge the base number of strains available for studies on the physiology, biochemistry, and genetics of nitrogen fixation. MATERIALS AND METHODS M/icroorganisms. R. japonicum strains were obtained from Deane Weber or Harold H.

Role of bacterial polysaccharides in the derepression of ex-planta nitrogenase activity with rhizobia

“…A direct positive correlation between EPS production and ex-planta nitrogenase activity in rhizobia was also suggested [1,3]. Production of EPS material allows derepression of nitrogenase activity by lowering 02 tension surrounding the cells, but it may also limit the availability of oxygen to the cells and thus impede N 2 fixation [4]. This paper further investigates the role of EPS in the development of ex-planta nitrogenase activity in rhizobial strains.…”

“…typically produce less EPS when grown on dicarboxylic adds than when grown on sugars. EPS produced by rhizobia in the presence of sugars may regulate O 2 diffusion to the ceils [4] and hence affect nitrugenase activity. In our experiment, the development of nitrogenase activity in an arctic rhizobial strain was correlated with the synthesis of EPS from mannitol.…”

“…Analyses of ex-planta nitrogenase activity revealed that the non-slimy, small colonies were significantly more active than slimy, large colonies. Agarwai and Keister [4] evaluated strains of B. japonicum for their N 2 fixation ability and EPS production. They observed a quantitative reciprocal correlation between rhizobial EPS production and ~,.itrogenase activity.…”

“…This activity was attributed to bacteria within the colony, where the O 2 tension would be reduced to well below atmospheric tension by surrounding cells. Others reported acetylene reduction in liquid cultures [4][5][6]. In stationary liquid cultures nitrogenas, activity was observed at normal atmospheric 02 tension, because the dissolved O 2 concentration adjacent to the cells was low [7].…”

“…In stationary liquid cultures nitrogenas, activity was observed at normal atmospheric 02 tension, because the dissolved O 2 concentration adjacent to the cells was low [7]. By shald q liquid cultures, low but defined or undefined O 2 concentrations were maintained for N 2 fixation [4,6,[8][9][10].…”