Academic journal article Genetics

Genetic Interactions between Regulators of Chlamydomonas Phosphorus and Sulfur Deprivation Responses

Academic journal article Genetics

Genetic Interactions between Regulators of Chlamydomonas Phosphorus and Sulfur Deprivation Responses

Article excerpt

ABSTRACT

The Chlamydomonas reinhardtii PSR1 gene is required for proper acclimation of the cells to phosphorus (P) deficiency. P-starved psr1 mutants show signs of secondary sulfur (S) starvation, exemplified by the synthesis of extracellular arylsulfatase and the accumulation of transcripts encoding proteins involved in S scavenging and assimilation. Epistasis analysis reveals that induction of the S-starvation responses in P-limited psr1 cells requires the regulatory protein kinase SNRK2.1, but bypasses the membrane-targeted activator, SAC1. The inhibitory kinase SNRK2.2 is necessary for repression of S-starvation responses during both nutrient-replete growth and P limitation; arylsulfatase activity and S deficiency-responsive genes are partially induced in the P-deficient snrk2.2 mutants and become fully activated in the P-deficient psr1snrk2.2 double mutant. During P starvation, the sac1snrk2.2 double mutants or the psr1sac1snrk2.2 triple mutants exhibit reduced arylsulfatase activity compared to snrk2.2 or psr1snrk2.2, respectively, but the sac1 mutation has little effect on the abundance of S deficiency-responsive transcripts in these strains, suggesting a post-transcriptional role for SAC1 in elicitation of S-starvation responses. Interestingly, P-starved psr1snrk2.2 cells bleach and die more rapidly than wild-type or psr1 strains, suggesting that activation of S-starvation responses during P deprivation is deleterious to the cell. From these results we infer that (i) P-deficient growth causes some internal S limitation, but the S-deficiency responses are normally inhibited during acclimation to P deprivation; (ii) the S-deficiency responses are not completely suppressed in P-deficient psr1 cells and consequently these cells synthesize some arylsulfatase and exhibit elevated levels of transcripts for S-deprivation genes; and (iii) this increased expression is controlled by regulators that modulate transcription of S-responsive genes during S-deprivation conditions. Overall, the work strongly suggests integration of the different circuits that control nutrient-deprivation responses in Chlamydomonas.

THE elements phosphorus (P) and sulfur (S) are essential macronutrients for sustaining life. P is a structural component of nucleic acids and phospholipids, and is a ubiquitous modifier of carbohydrates and proteins, while S is incorporated into sulfolipids, polysaccharides, proteins, cofactors, and a wide variety of important metabolites including S-adenosyl-methionine, glutathione, and phytochelatins. The preferred forms of P and S that are assimilated by plants and microbes are the orthophosphate ion, PO^sub 4^^sup 3-^ (Pi), and the sulfate ion, SO^sub 4^^sup 2-^. The available pools of these anions can vary significantly as environmental conditions change. Most organisms have a limited capacity to store S, and thus require a continual supply of S-containing nutrients for survival. In contrast, cells often have considerable reserves of P, which are bound in polymers of DNA, RNA, and polyphosphate. The ability of microbes to acclimate to periods of nutrient insufficiency is essential to their survival in the natural environment (reviewed by Grossman and Takahashi 2001).

The green, unicellular alga Chlamydomonas reinhardtii (Chlamydomonas throughout) exhibits both specific and general responses when experiencing P or S deprivation. The general responses are common to a number of different stress conditions, while the specific responses enable processes that are advantageous during particular nutrient deficiencies, often allowing for better scavenging of the limiting nutrient from internal and external stores. P and S limitation elicit qualitatively similar effects on growth and photosynthesis, differing only in that the responses to S starvation occur more quickly following exposure of cells to medium devoid of S. General responses to nutrient limitation that have been analyzed include the cessation of growth at low cell densities (Shimogawara et al. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.