Growth, Respiration, and Polypeptide Patterns of Bradyrhizobium sp. (Arachis) Strain 3G4b20 from Succinate- or Oxygen-Limited Continuous Cultures †
Applied and Environmental Microbiology
Succinateor oxygen-limited continuous cultures were used to study the influences of different concentrations of dissolved oxygen and ammonia on the growth, respiration, and polypeptide patterns of Bradyrhizobium sp. (Arachis) strain 3G4b20. During succinate-limited growth, molar growth yields on succinate (Yucc) ranged from 38.9 to 44.4 g (dry weight) of cells mol of succinate' and were not greatly influenced by changes in dilution rates or changes in the oxygen concentrations that we tested.
... s that we tested. Succinate, malate, and fumarate induced the highest rates of oxygen uptake in all of the steady states in which the supply rates of (NH4)2SO4 ranged between 322 and 976 ,umol h-1. However, the amino acids aspartate, asparagine, and glutamate could also be used as respiratory substrates, especially when the (NH4)2SO4 supply rate was decreased to 29 ,imol h-. Glutamine-dependent respiration was seen only when the (NH4)2SO4 supply rate was 29 ,imol h-1 and thus appears to be under tight ammonia control. Nitrogenase activity was detected only when the culture was switched from a succinate-limited steady state to an oxygen-limited steady state. Comparison of major silver-stained proteins from three steady states by two-dimensional gel electrophoresis revealed that nearly 60% were affected by oxygen and 24% were affected by ammonia. These data are consistent with reports that oxygen has a major regulatory role over developmental processes in Rhizobium sp. and Bradyrhizobium sp. The nodule is a highly specialized plant structure in which differentiated bacteria (bacteroids) function symbiotically with the host plant. The bacteroid provides fixed nitrogen, which then is assimilated by the host plant. The host plant provides carbon, energy, and shelter. For the symbiosis to develop, it is necessary for there to be a coordinated communication between plant and bacterium. Presumably, this is the result of a signaling process; however, the physiology of the process is poorly understood. One possible approach to understanding symbiotic communication is to develop a system in which rhizobia can be studied in the absence of the plant (ex planta) under a variety of controlled environmental conditions that are similar to those found in the legume nodule. By varying one condition at a time, it may eventually be possible to study how specific environmental stimuli control the process of bacteroid development and subsequent nitrogen fixation. When free-living rhizobia differentiate into symbiotic bacteroids, there is a marked alteration in the types of carbon and energy sources used. Many substrates that are typically used by the free-living bacteria, such as glucose or arabinose, are only passively transported by isolated bacteroids and do not support acetylene reduction (33). The C4 dicarboxylic acids, on the other hand, appear to play a central role in nodule metabolism. Bacteroids of both Bradyrhizobium sp. and Rhizobium sp. actively transport C4 dicarboxylic acids (33, 37). The importance of this is illustrated by reports that mutants of Rhizobium sp. defective in C4-* Corresponding author. t Paper no.