NtcA-Mediated Nitrogen Control in Cyanobacteria

Abstract

Sources of nitrogen commonly assimilated by cyanobacteria include ammonium, nitrate, and dinitrogen (Flores, Herrero 1994). Ammonium prevents the expression of proteins involved in the assimilation of other nitrogen sources and abolishes the process of differentiation of heterocysts, cells specialized in fixation of dinitrogen that are formed in some filamentous cyanobacteria like Anabaena sp. (Wolk 1996). Structural genes whose expression is hampered or significantly reduced in ammonium-grown cultures of cyanobacteria (see Flores, Herrero 1994) include: glnA encoding glutamine synthetase, the nir-nrtABCD-narB operon for nitrate assimilation (studied in Synechococcus sp. PCC 7942 and Anabaena sp. PCC 7120), amt1 encoding an ammonium/methylammonium permease (studied in Synechocystis sp. PCC 6803; M. L. Montesinos, unpublished), icd encoding isocitrate dehydrogenase (studied in strains PCC 6803 and PCC 7120; Muro-Pastor et al. 1996), and nitrogenase-encoding operons like strain PCC 7120 nifHDK that is expressed in heterocysts (see Wolk 1996) or Anabaena sp. PCC 7937 nifHDK2 that is expressed not only in heterocysts but also in vegetative cells (Thiel et al. 1995). The hetR regulatory gene required for heterocyst development is also subjected to repression by ammonium (Wolk 1996). Some of these genes (e. g., amt, nif) are not only repressed by ammonium but also by nitrate, but regulation by nitrate requires its reduction to the term of ammonium (Flores, Herrero 1994). A common feature of all the ammonium repressive effects in cyanobacteria is their requirement for ammonium metabolism through glutamine synthetase to be manifest (Flores, Herrero 1994).KeywordsGlutamine SynthetaseFilamentous CyanobacteriumNitrate AssimilationAmmonium MetabolismTranscription Start PointThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.