Nitrite accumulation by Synechococcus 6301 as a consequence of carbon- or sulfur-deficiency

S-deficiency responsive accumulation of amino acids is mainly due to hydrolysis of the previously synthesized proteins - not to de novo synthesis in Brassica napus

To characterize the mechanisms of amino acid accumulation under sulphur (S)-deficiency and its physiological significance in Brassica napus, stable isotopes (15) N and (34) S were employed. The plants were exposed for 9 days to S-deficient conditions (0.05 mM vs 1.5 mM sulphate). After 9 days of S-deficiency, leaf-osmotic potential and total chlorophyll content significantly decreased. S uptake decreased by 94%, whereas N uptake and biomass were not significantly changed. Using (15) N and (34) S labelling, de novo synthesis of amino acids and proteins derived from newly absorbed NO(3) (-) and SO(4) (2) (-) and the content of N and S in the previously synthesized amino acids and proteins were quantified. At the whole plant level, S-deficiency increased the pool of amino acids but resulted in strong decrease of incorporation of newly absorbed NO(3) (-) and SO(4) (2) (-) into amino acids by 22.2 and 76.6%, respectively, compared to the controls. Total amount of N and S incorporated into proteins also decreased by 28.8 and 62.1%, respectively. The levels of (14) N- and (32) S-proteins (previously synthesized proteins) strongly decreased, mainly in mature leaves. The data thus indicate that amino acid accumulation under short-term S-deficiency results from the degradation of previously synthesized proteins rather than from de novo synthesis.

Interaction of sulfate assimilation with carbon and nitrogen metabolism in Lemna minor

Cysteine synthesis from sulfide and O-acetyl-L-serine (OAS) is a reaction interconnecting sulfate, nitrogen, and carbon assimilation. Using Lemna minor, we analyzed the effects of omission of CO(2) from the atmosphere and simultaneous application of alternative carbon sources on adenosine 5'-phosphosulfate reductase (APR) and nitrate reductase (NR), the key enzymes of sulfate and nitrate assimilation, respectively. Incubation in air without CO(2) led to severe decrease in APR and NR activities and mRNA levels, but ribulose-1,5-bisphosphate carboxylase/oxygenase was not considerably affected. Simultaneous addition of sucrose (Suc) prevented the reduction in enzyme activities, but not in mRNA levels. OAS, a known regulator of sulfate assimilation, could also attenuate the effect of missing CO(2) on APR, but did not affect NR. When the plants were subjected to normal air after a 24-h pretreatment in air without CO(2), APR and NR activities and mRNA levels recovered within the next 24 h. The addition of Suc and glucose in air without CO(2) also recovered both enzyme activities, with OAS again influenced only APR. (35)SO(4)(2-) feeding showed that treatment in air without CO(2) severely inhibited sulfate uptake and the flux through sulfate assimilation. After a resupply of normal air or the addition of Suc, incorporation of (35)S into proteins and glutathione greatly increased. OAS treatment resulted in high labeling of cysteine; the incorporation of (35)S in proteins and glutathione was much less increased compared with treatment with normal air or Suc. These results corroborate the tight interconnection of sulfate, nitrate, and carbon assimilation.

Sulphate-limited growth in the N2 -fixing unicellular cyanobacterium Gloeothece (Nägeli) sp. PCC 6909

The unicellular diazotrophic cyanobacterium Gloeothece PCC 6909 was grown in batch and continuous culture, It was found that the organism scavenged sulphate from the medium and incorporated it in the polysaccharide sheath. As a result batch cultures became sulphur-depleted whilst continuous cultures were sulphur-limited. This was observed regardless of the nitrogen source of the culture. Sulphur-depletion was the reason for the low nitrogenase activities usually observed in batch cultures of Gloeothece. Sulphate-amended cultures showed sustained high nitrogenase activity in the light. Sulphate uptake from the medium was light-dependent. Earlier work on N₂ fixation in Gloeothece is discussed in the light of possible sulphur limitation.