Pathways of assimilation of [13N]N2 and 13NH4+ by cyanobacteria with and without heterocysts

Abstract

The principal initial product of metabolism of ['3N]N2 and '3NH4' by five diverse cyanobacteria is glutamine. Methionine sulfoximine inhibits formation of [l:JN]glutamine except in the case of Gloeothece sp., an organism with a thick sheath through which the inhibitor may not penetrate. Thus, glutamine synthetase appears to catalyze the initial step in the assimilation of N2-derived or exogenous NH4' by these organisms. ['3N]Glutamate is, in all cases, the second major product of assimilation of "IN-labeled N2 and NH4+. In all of the N2-fixing cyanobacteria studied, the fraction of "N in glutamine declines and that in glutamate increases with increasing times of assimilation of ['3N]N2 and l NH4', and (Gloeothece again excepted) methionine sulfoximine reduces incorporation of`N into glutamate as well as into glutamine. Glutamate synthase therefore appears to catalyze the formation of glutamate in a wide range of N2-flxing cyanobacteria. However, the major fraction of [I:JN]glutamate formed by Anacystis nidulans incubated with "NH4' may be formed by glutamic acid dehydrogenase. The formation of ['3N]alanine from ":3NH4+ appears to be catalyzed principally either by alanine dehydrogenase (as in Cylindrospermum licheniforme) or by a transaminase (as in Anabaena variabilis). Experiments using the radioisotope '3N haveshown that in the heterocyst-forming cyanobacterium Anabaena cylindrica the major enzymatic pathway for the asimilation of NH4', whether derived from N2 or supplied exogenously, consists of glutamine synthetase (L-glutamate:ammonia ligase [ADP forming], EC 6.3.1.2) and glutamate synthase (L-glutamate:ferredoxin oxidoreductase [transaminating], EC 1.4.7.1) (11,19). Glutamic acid dehydrogenase and alanine dehydrogenase also function in the assimilation ofexogenously supplied NH41 in this cyanobacterium, but at a much lower rate even in the presence of relatively high levels of NH4+ (11). In this respect, A. cylindrica differs from some heterotrophic, dinitrogen-fixing bacteria in which N2-derived NH4+ is assimilated principally by the glutamine synthetase/ glutamate synthase pathway, whereas, during growth in the presence of high exogenous concentrations of NHEV, the NHE is assimilated via glutamic acid dehydrogenase (6).When A. cylindrica is grown aerobically, t Preent address:

Coevolution of metal availability and nitrogen assimilation in cyanobacteria and algae

“…The eight electrons required for each catalytic cycle are supplied by ferredoxin, a small [2Fe-2S]-cluster protein that accepts electrons from the photosynthetic electron transport chain (Schrautemeier & Bohme, 1985). The product NH 3 gas is rapidly converted into the soluble and directed into glutamine and glutamate biosynthesis (Wolk et al, 1976;Meeks et al, 1978;Carpenter et al, 1992) (see Ammonium Assimilation section).…”

Regulation of nitrate reductase levels in the cyanobacteria Anacystis nidulans, Anabaena sp. strain 7119, and Nostoc sp. strain 6719

“…strain 6719 (both nitrogen fixers). In this type of cyanobacteria, ammonium assimilation is also effectively inhibited by MSX (12). Table 3 shows the results obtained with Nostoc sp.…”

Adaptation of cyanobacteria to environmental stimuli: new steps towards molecular mechanisms

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“…As a consequence of the incomplete tricarboxylic acid cycle (lacking both 2-oxoglutarate dehydrogenase and succinyl-CoA synthetase activities [13]), 2-oxoglutarate is, in cyanobacteria, a final-step metabolite that provides the carbon skeleton required for ammonium assimilation through the GS/GOGAT system [275]. During CO2-fixation, 2-oxoglutarate is produced by NADP-isocitrate dehydrogenase.…”