Cytochromes of Rhizobium

Genes involved in the formation and assembly of rhizobial cytochromes and their role in symbiotic nitrogen fixation

Rhizobia fix nitrogen in a symbiotic association with leguminous plants and this occurs in nodules. A low-oxygen environment is needed for nitrogen fixation, which paradoxically has a requirement for rapid respiration to produce ATP. These conflicting demands are met by control of oxygen flux and production of leghaemoglobin (an oxygen carrier) by the plant, coupled with the expression of a high-affinity oxidase by the nodule bacteria (bacteroids). Many of the bacterial genes encoding cytochrome synthesis and assembly have been identified in a variety of rhizobial strains. Nitrogen-fixing bacteroids use a cytochrome cbb3-type oxidase encoded by the fixNOQP operon; electron transfer to this high-affinity oxidase is via the cytochrome bc1 complex. During free-living growth, electron transport from the cytochrome bc1 complex to cytochrome aa3 occurs via a transmembrane cytochrome c (CycM). In some rhizobia (such as Bradyrhizobium japonicum) there is a second cytochrome oxidase that also requires electron transport via the cytochrome bc1 complex. In parallel with these cytochrome c oxidases there are quinol oxidases that are expressed during free-living growth. A cytochrome bb3 quinol oxidase is thought to be present in B. japonicum; in Rhizobium leguminosarum, Rhizobium etli and Azorhizobium caulinodans cytochrome d-type oxidases have been identified. Spectroscopic data suggest the presence of a cytochrome o-type oxidase in several rhizobia, although the absence of haem O in B. japonicum may indicate that the absorption attributed to cytochrome o could be due to a high-spin cytochrome b in a cytochrome bb3-type oxidase. In some rhizobia, mutation of genes involved in cytochrome c assembly does not strongly affect growth, presumably because the bacteria utilize the cytochrome c-independent quinol oxidases. In this review, we outline the work on various rhizobial mutants affected in different components of the electron transport pathways, and the effects of these mutations on symbiotic nitrogen fixation and free-living growth.

A high-affinity cbb3-type cytochrome oxidase terminates the symbiosis-specific respiratory chain of Bradyrhizobium japonicum

It has been a long-standing hypothesis that the endosymbiotic rhizobia (bacteroids) cope with a concentration of 10 to 20 nM free O2 in legume root nodules by the use of a specialized respiratory electron transport chain terminating with an oxidase that ought to have a high affinity for O2. Previously, we suggested that the microaerobically and anaerobically induced fixNOQP operon of Bradyrhizobium japonicum might code for such a special oxidase. Here we report the biochemical characteristics of this terminal oxidase after a 27-fold enrichment from membranes of anaerobically grown B. japonicum wild-type cells. The purified oxidase has TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) oxidase activity as well as cytochrome c oxidase activity. N-terminal amino acid sequencing of its major constituent subunits confirmed that presence of the fixN,fixO, and fixP gene products. FixN is a highly hydrophobic, heme B-binding protein. FixO and FixP are membrane-anchored c-type cytochromes (apparent Mrs of 29,000 and 31,000, respectively), as shown by their peroxidase activities in sodium dodecyl sulfate-polyacrylamide gels. All oxidase properties are diagnostic for it to be a member of the cbb3-type subfamily of heme-copper oxidases. The FixP protein was immunologically detectable in membranes isolated from root nodule bacteroids, and 85% of the total cytochrome c oxidase activity in bacteroid membranes was contributed by the cbb3-type oxidase. The Km values for O2 of the purified enzyme and of membranes from different B. japonicum wild-type and mutant strains were determined by a spectrophotometric method with oxygenated soybean leghemoglobin as the sole O2 delivery system. The derived Km value for O2 of the cbb3-type oxidase in membranes was 7 nM, which is six- to eightfold lower than that determined for the aerobic aa3-type cytochrome c oxidase. We conclude that the cbb3-type oxidase supports microaerobic respiration in endosymbiotic bacteroids.

Nitrogenase activity and respiration of cultures of Rhizobium spp. with special reference to concentrations of dissolved oxygen

Studies of nitrogenase in cultures of the cowpea rhizobia (Rhizobium spp.) strains 32H1 and CB756 are reported. Preliminary experiments established that, even when agar cultures were grown in air, suspensions of bacteria prepared anaerobically from them were most active at low concentrations of free dissolved O2. Consequently, assays for activity used low concentrations of O2, stabilized by adding the nodule pigment leghaemoglobin. In continuous, glutamine-limited cultures of 32H1, nitrogenase activity appeared only when the concentration of dissolved O2 in the cultures approached 1 muM. Lowering the glutamine concentration in the medium supplied to the culture from 2 to 1 mM halved the cell yield and nitrogenase activity was also diminished. Omitting succinate from the medium caused the concentration of dissolved O2 to rise and nitrogenase activity was lost. Upon restoration of the succinate supply, the O2 concentration immediately fell and nitrogenase was restored. The activity doubled in about 8 h, whereas the doubling time of this culture was 14 h. Sonic extracts of 32H1 cells from continuous cultures with active nitrogenase contained components reacting with antiserum against nitrogenase Mo-Fe protein from soybean bacteroids. Continuous cultures grown at higher O2 concentration, with only a trace of active nitrogenase, contained less of these antigens and they were not detected in highly aerobic cultures. Nitrogenase activity of a continuous culture was repressed by NH+4; the apparent half-life was about 90 min. Cells of 32H1 from a continuous culture growing at between 30 and 100 muM dissolved O2 possessed a protective mechanism which permitted respiration to increase following exposure to a rapid increase in O2 concentration from low levels (O2 shock). This effect disappeared as the O2 concentration for growth was reduced towards 1 muM.

Ultrastructure of free-living and nitrogen-fixing forms of Rhizobium meliloti as revealed by freeze-etching

Freeze-etching of Rhizobium meliloti provided considerable insight into the ultrastructure of this bacterium and into the changes accompanying the transformation from the free-living rod forms to the nitrogen-fixing bacteroid forms. In the small rods, one cleavage plane was revealed at the level of the cell wall and a second at the level of the plasma membrane. Very little structure was evident at the cell wall level, but distinctly different convex and concave fracture faces were exposed at the cell membrane cleavage plane. During the transformation into the bacteroidal state the wall decreased in thickness, became less rigid, and developed a particulate surface. In addition, changes in particle density were observed in the plasma membrane. The fine structure of the plant membranes, the infection threads, and the arrangement of the bacteroids within the plant cells also were revealed.