title: Regulation of nitrate assimilation in cyanobacteria.
abstract: Nitrate assimilation by cyanobacteria is inhibited by the presence of ammonium in the growth medium. Both nitrate uptake and transcription of the nitrate assimilatory genes are regulated. The major intracellular signal for the regulation is, however, not ammonium or glutamine, but 2-oxoglutarate (2-OG), whose concentration changes according to the change in cellular C/N balance. When nitrogen is limiting growth, accumulation of 2-OG activates the transcription factor NtcA to induce transcription of the nitrate assimilation genes. Ammonium inhibits transcription by quickly depleting the 2-OG pool through its metabolism via the glutamine synthetase/glutamate synthase cycle. The P(II) protein inhibits the ABC-type nitrate transporter, and also nitrate reductase in some strains, by an unknown mechanism(s) when the cellular 2-OG level is low. Upon nitrogen limitation, 2-OG binds to P(II) to prevent the protein from inhibiting nitrate assimilation. A pathway-specific transcriptional regulator NtcB activates the nitrate assimilation genes in response to nitrite, either added to the medium or generated intracellularly by nitrate reduction. It plays an important role in selective activation of the nitrate assimilation pathway during growth under a limited supply of nitrate. P(II) was recently shown to regulate the activity of NtcA negatively by binding to PipX, a small coactivator protein of NtcA. On the basis of accumulating genome information from a variety of cyanobacteria and the molecular genetic data obtained from the representative strains, common features and group- or species-specific characteristics of the response of cyanobacteria to nitrogen is summarized and discussed in terms of ecophysiological significance.
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan.
authors.forename: Yoshitake
authors.initials: Y
authors.lastname: Ohashi
authors.forename: Wei
authors.initials: W
authors.lastname: Shi
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Takatani
authors.forename: Makiko
authors.initials: M
authors.lastname: Aichi
authors.forename: Shin-ichi
authors.initials: S
authors.lastname: Maeda
authors.forename: Satoru
authors.initials: S
authors.lastname: Watanabe
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Yoshikawa
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Omata
mesh_terms: D027321:Anion Transport Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D000458:Cyanobacteria / Q000235:genetics / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D007656:Ketoglutaric Acids / Q000378:metabolism; D008957:Models, Genetic; D008969:Molecular Sequence Data; D000091285:Nitrate Transporters; D009566:Nitrates / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D016415:Sequence Alignment; D015398:Signal Transduction; D014157:Transcription Factors / Q000378:metabolism
chemical_list: D027321:Anion Transport Proteins; D001426:Bacterial Proteins; D007656:Ketoglutaric Acids; D000091285:Nitrate Transporters; D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D014157:Transcription Factors