No abstract

In Phormidium laminosum cells, nitrogen starvation caused a decrease in the intracellular levels of all amino acids, except glutamate, and an increase in the total level of the analyzed organic acids. The addition of nitrate or ammonium to N-starved cells resulted in substantial increases in the pool size of most amino acids. Upon addition of ammonium the total level of organic acids diminished, whereas it increased upon addition of nitrate, after a transient decay during the first minutes. Nitrogen resupply stimulated amino acid synthesis, the effect being faster and higher when ammonium was assimilated. The data indicate that nitrate and ammonium assimilation induced an enhancement of carbon flow through the glycolytic and the tricarboxylic-acid pathways to amino acid biosynthesis, with a concurrent decrease in the carbohydrate reserves. The results suggest that the availability of carbon skeletons limited the rate of ammonium assimilation, whereas the availability of reducing equivalents limited the rate of nitrate assimilation.

MalF is one of the two integral inner membrane proteins of the maltose-maltodextrin transport system. To identify functional regions in this protein, we characterized a collection of malFmutants obtained by random mutagenesis. We analyzed their growth on maltose and maltodextrins, the steady-state levels and subcellular localization of the mutant proteins, and the subcellular localization of MalK. Only 2 of the 21 MalF mutant proteins allowed growth on maltose and maltodextrins. Most mutations resulting in immunodetectable proteins mapped to hydrophilic domains, indicating that insertions affecting transmembrane segments gave rise to unstable or lethal proteins. All MalF mutant proteins, even those C-terminally truncated or with large N-terminal deletions, were inserted into the cytoplasmic membrane. Having identified mutations leading to reduced steady-state level, to partial mislocation, and/or to misfolding, we were able to assign to some regions of MalF a role in the assembly of the MalFGK2 complex and/or in the transport mechanism.

In the non‐N2‐fixing cyanobacterium Phormidium laminosum (Agardh) Gomont (strain OH‐I‐pCl1), N starvation induced an increase in the rate of respiration and a decrease in the rate of O2 evolution. When NO3− was added to illuminated N‐starved cells, O2 evolution immediately increased to levels shown by NO3− grown cells, even though N‐starved cells had lost most of their in vitro photosynthetic activities. Stimulation of noncyclic electron flow was maximal under light‐saturating conditions and after 2–3 days of N starvation. The respiratory rate of N‐starved cells was stimulated by the addition of NO3− or NH4+ and partially inhibited at very low irradiances, even in the presence of DCMU (3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea). Results indicate that N‐starved cells obtain the energy supply for N assimilation through a process different from that used by N‐sufficient cells. N‐starved cells were able to take up NO3− in the dark and when illuminated in the presence of DCMU under anaerobiosis. Following NO3− addition, the photosynthetic yield of the in vivo noncyclic electron transport slightly increased, whereas it decreased after NH4+ addition. Addition of NO3− or NH4+ favored photoinhibition of photosystem II, the effect being faster after NH4+ addition.