Composition and structure of assimilatory nitrate reductase from Ankistrodesmus braunii

Coevolution of metal availability and nitrogen assimilation in cyanobacteria and algae

Marine primary producers adapted over eons to the changing chemistry of the oceans. Because a number of metalloenzymes are necessary for N assimilation, changes in the availability of transition metals posed a particular challenge to the supply of this critical nutrient that regulates marine biomass and productivity. Integrating recently developed geochemical, biochemical, and genetic evidence, we infer that the use of metals in N assimilation - particularly Fe and Mo - can be understood in terms of the history of metal availability through time. Anoxic, Fe-rich Archean oceans were conducive to the evolution of Fe-using enzymes that assimilate abiogenic NH(4)(+) and NO(2)(-). The N demands of an expanding biosphere were satisfied by the evolution of biological N(2) fixation, possibly utilizing only Fe. Trace O(2) in late Archean environments, and the eventual 'Great Oxidation Event' c. 2.3 Ga, mobilized metals such as Mo, enabling the evolution of Mo (or V)-based N(2) fixation and the Mo-dependent enzymes for NO(3)(-) assimilation and denitrification by prokaryotes. However, the subsequent onset of deep-sea euxinia, an increasingly-accepted idea, may have kept ocean Mo inventories low and depressed Fe, limiting the rate of N(2) fixation and the supply of fixed N. Eukaryotic ecosystems may have been particularly disadvantaged by N scarcity and the high Mo requirement of eukaryotic NO(3)(-) assimilation. Thorough ocean oxygenation in the Neoproterozoic led to Mo-rich oceans, possibly contributing to the proliferation of eukaryotes and thus the Cambrian explosion of metazoan life. These ideas can be tested by more intensive study of the metal requirements in N assimilation and the biological strategies for metal uptake, regulation, and storage.

Characterization and daily variation of nitrate reductase in Gracilaria tenuistipitata (Rhodophyta)

A daily rhythm in the activity of nitrate reductase (NR: EC 1.6.6.1) isolated from the marine red algae Gracilaria tenuistipitata is shown to be attributable to changes in amounts of the protein. The enzyme was purified in four steps: ion exchange Q-Sepharose separation, ammonium sulfate precipitation, gel filtration on Sephacryl S-300, and affinity chromatography on Affigel-blue resin. This purification procedure yielded an active purified NR of about 500-fold with a recovery of 85%. The SDS-PAGE silver staining of purified NR revealed a 110 kDa single band. Non-denaturated protein showed a molecular mass of 440 kDa on gel filtration comparing with SDS-PAGE, the enzyme is apparently composed of four identical subunits. In extracts of algae grown under either constant dim light or a light-dark cycle, the activity of NR exhibited a daily rhythm, peaking at midday phase as does photosynthesis. Staining with monoclonal antibodies, raised against NR from Porphyra yezoensis, showed that the amount of protein changes by a factor of about 12, with a maximum occurring in the midday phase.

Synechocystis 6803 plastocyanin isolated from both the cyanobacterium and E. coli transformed cells are identical

Native plastocyanin from Synechocystis 6803 has been isolated and purified to electrophoretic homogeneity. The corresponding gene (petE) has been cloned and expressed in E. coli, thus leading to a protein completely identical to plastocyanin purified from the cyanobacterial cells. The petE gene product is correctly processed in E. coli as deduced from the N-terminal amino acid sequences. These results, along with the identical physicochemical and kinetic properties of the two protein preparations, confirm that expression of petE in E. coli is an adequate tool to address the study of Synechocystis plastocyanin by site-directed mutagenesis.