A mechanistic study of the influence of nitrogen and energy availability on the NH₄+ sensitivity of nitrogen assimilation in Synechococcus

In most algae, NO3- assimilation is tightly controlled and is often inhibited by the presence of NH4+. In the marine, non-colonial, non-diazotrophic cyanobacterium Synechococcus UTEX 2380, NO3- assimilation is sensitive to NH4+ only when N does not limit growth. We sequenced the genome of Synechococcus UTEX 2380, studied the genetic organization of the Nitrogen Assimilation Related (NAR) genes, and investigated expression and kinetics of the main NAR enzymes, under N or light limitation. We found that Synechococcus UTEX 2380 is a β-cyanobacterium with a full complement of N uptake and assimilation genes and NAR regulatory elements. The nitrate reductase (NR) of our strain showed biphasic kinetics, previously observed only in freshwater or soil diazotrophic Synechococcus strains. Nitrite reductase and glutamine synthetase showed little response to our growth treatments, and their activity was usually much higher than that of nitrate reductase. NH4+ insensitivity of NAR genes may be associated with the stimulation of the binding of the regulator NtcA to NAR genes promoters by the high 2-oxoglutarate concentrations produced under N-limitation. NH4+-sensitivity in energy-limited cells fits with the fact that, under these conditions, the use of NH4+ rather than NO3- decreases N-assimilation cost, whereas it would exacerbate N-shortage under N-limitation.