Julie V. Cullimore

Bio: Julie V. Cullimore is an academic researcher from University of East Anglia. The author has contributed to research in topics: Chlamydomonas & Glutamine synthetase. The author has an hindex of 5, co-authored 5 publications receiving 313 citations.

An association between photorespiration and protein catabolism: Studies with Chlamydomonas

Abstract: Work demonstrating the operation of a photorespiratory N cycle in Chlamydomonas is described. NH3 release by this process is light dependent, sensitive to changes in pO2 and pCO2, and abolished by a photosystem II inhibitor. Evidence is presented which shows that this NH3 derives its N from protein rather than from freshly synthesised glutamate. Protein turnover is shown to provide amino-N at a rate sufficient to account for the highest photorespiratory N excretion observed suggesting that changes in excretion can be accounted for by increased catabolism of normally recirculating amino acids. It is equally possible however that a direct link between photorespiration and protein turnover exists, increased NH3 excretion resulting from enhanced protein turnover. The data suggest that if similar mechanisms operate in higher plants, previous estimates of the amount of N recycled in photorespiration may have been too high.

Glutamine synthetase of Chlamydomonas: its role in the control of nitrate assimilation.

Abstract: Work is described which suggests that glutamine synthetase (GS) could play an important and direct regulatory role in the control of NO3 assimilation by the alga. In both steady-state cells and ones disturbed physiologically by changes in light or nitrogen supply the assimilation of NO3 appears to be limited by the activity of GS. Moreover although in normal cells NH3 can completely inhibit NO3 uptake, promote the deactivation of nitrate reductase (NR) and repress the synthesis of NR and nitrite reductase (NIR), these controls are relaxed in cells in which GS is deactivated by treatment with L-methionine-DL-sulfoximine (MSO). It is proposed that the reversible deactivation of GS may play an important part in the regulation of NO3 assimilation although it is still not clear whether the enzyme itself or products of its metabolism are responsible.

Glutamine synthetase of Chlamydomonas: Rapid reversible deactivation.

Abstract: A 70% reduction in glutamine synthetase (GS) activity was observed within 5 min when 5 mM NH3 and darkness was applied to steady-state cells of Chlamydomonas utilising NO3. The enzyme was reactivated in vivo by reillumination of the culture and in vitro by treatment with thiol reagents. The activity modulations affected the synthetase and transferase activities similarly and were not influenced by protein synthesis inhibitors. Deactivation of GS was also observed when steady-state cells were treated with an uncoupler of phosphorylation, carbonylcyanide m-chlorophenylhydrazone (CCCP) or inhibitors of the electron transport chain but under these conditions the activity modulation affected over 90% of the activity and was irreversible. The mechanism of the physiological deactivation of GS is discussed in relation to both the in vivo and in vitro findings.

Effects of inorganic n availability on algal photosynthesis and carbon metabolism

Abstract: The purpose of this contribution is to assess the effects of N-limitation on algal photosynthesis and address the key physiological and biochemical interactions that occur between the assimilation of inorganic nitrogen and algal photosynthesis and carbon metabolism

Nitrogen-Sparing Mechanisms in Chlamydomonas Affect the Transcriptome, the Proteome, and Photosynthetic Metabolism

Abstract: Nitrogen (N) is a key nutrient that limits global primary productivity; hence, N-use efficiency is of compelling interest in agriculture and aquaculture. We used Chlamydomonas reinhardtii as a reference organism for a multicomponent analysis of the N starvation response. In the presence of acetate, respiratory metabolism is prioritized over photosynthesis; consequently, the N-sparing response targets proteins, pigments, and RNAs involved in photosynthesis and chloroplast function over those involved in respiration. Transcripts and proteins of the Calvin-Benson cycle are reduced in N-deficient cells, resulting in the accumulation of cycle metabolic intermediates. Both cytosolic and chloroplast ribosomes are reduced, but via different mechanisms, reflected by rapid changes in abundance of RNAs encoding chloroplast ribosomal proteins but not cytosolic ones. RNAs encoding transporters and enzymes for metabolizing alternative N sources increase in abundance, as is appropriate for the soil environmental niche of C. reinhardtii. Comparison of the N-replete versus N-deplete proteome indicated that abundant proteins with a high N content are reduced in N-starved cells, while the proteins that are increased have lower than average N contents. This sparing mechanism contributes to a lower cellular N/C ratio and suggests an approach for engineering increased N-use efficiency.

Understanding nitrate assimilation and its regulation in microalgae.

Abstract: Nitrate assimilation is a key process for nitrogen (N) acquisition in green microalgae. Among Chlorophyte algae, Chlamydomonas reinhardtii has resulted to be a good model system to unravel important facts of this process, and has provided important insights for agriculturally relevant plants. In this work, the recent findings on nitrate transport, nitrate reduction and the regulation of nitrate assimilation are presented in this and several other algae. Latest data have shown nitric oxide (NO) as an important signal molecule in the transcriptional and posttranslational regulation of nitrate reductase and inorganic N transport. Participation of regulatory genes and proteins in positive and negative signaling of the pathway and the mechanisms involved in the regulation of nitrate assimilation, as well as those involved in Molybdenum cofactor synthesis required to nitrate assimilation, are critically reviewed.