The sulfur acclimation SAC3 kinase is required for chloroplast transcriptional repression under sulfur limitation in Chlamydomonas reinhardtii

Abstract

Sulfur (S) deprivation responses have been studied extensively in algae and land plants; however, little is known of the signals that link perception of S status to chloroplast gene expression. Here, we have compared the chloroplast S limitation response in WT vs. sac1 and sac3 sulfur acclimation mutants of the green alga Chlamydomonas reinhardtii. We provide evidence that in the WT, chloroplast transcriptional activity rapidly decreases after removal of S from the medium, leading to reduced transcript accumulation. This decrease correlates with reduced abundance of a 70 -like factor, Sig1, which is most likely the unique chloroplast transcription specificity factor. We further show that reduced transcription activity and diminished Sig1 accumulation are mediated by the SAC3 gene product, a putative Snf1-type Ser͞Thr kinase previously shown to have both positive and negative effects on nuclear gene expression. Inclusion of the protein kinase inhibitor 6-dimethylaminopurine during S limitation yielded a pattern of expression that was largely similar to that seen in the sac3 mutant, lending support to the hypothesis that Sac3 kinase activation leads to transcriptional repression and Sig1 proteolysis. The finding that Sac3 regulates chloroplast gene expression suggests that it has a previously unknown role in integrating the S limitation response in multiple subcellular compartments. sigma factor ͉ dimethylaminopurine ͉ RNA polymerase S ulfur (S) is a required macronutrient; it is a constituent of proteins, lipids, electron transport components, and many cell metabolites (1, 2). Most photosynthetic organisms have the capacity to assimilate S as a sulfate anion (SO 4 2Ϫ ) and translocate it to the plastid, where primary S metabolism takes place. In chloroplasts, S is first activated by ATP sulfurylase to form APS (5Ј-adenylyl sulfate), which is a branch-point intermediate. APS can be phosphorylated by APS kinase, and the product can be used in sulfation reactions or to synthesize cysteine and methionine (1). Chloroplasts are thus crucial in the assimilation of S, because ATP and photosynthetic reductant are needed for the first activation and reduction steps.S insufficiency constitutes a serious stress situation for plants and algae, which respond with a series of metabolic adjustments (3, 4). Studies of S deprivation in the green alga Chlamydomonas reinhardtii have shown that limiting S leads to altered photosynthetic performance, reduced levels of photosynthetic proteins, and induction of genes encoding high-affinity S transporters (5, 6). S deprivation also leads to increased accumulation of arylsulfatase (ARS), which releases sulfate anion from esterified organic sulfates, allowing cells to access new sulfur stores (7). These acclimation responses are thought to prevent photodamage under conditions where the cell has limited capacity for metabolism of S-containing compounds, particularly proteins.Several Chlamydomonas mutants that do not respond correctly to S deprivation have been identified based on their inab...

SNF1-Related Protein Kinases Type 2 Are Involved in Plant Responses to Cadmium Stress

“…Since SnRK2s are activated in plants exposed to cadmium ions and because there are strong indications that in the lower plant C. reinhardtii SnRK2s are involved in sulfur metabolism (Davies et al, 1999;Irihimovitch and Stern, 2006;González-Ballester et al, 2008), which is closely connected with regulation of heavy metal tolerance, we addressed the question of the role of SnRK2.4, a putative NtOSAK functional homolog in Arabidopsis, in the plant response to cadmium stress. For this purpose we applied a reverse genetics approach.…”

“…There is growing evidence that SnRK2s (for SNF1-related protein kinase2) play a role in the regulation of sulfur metabolism. Most information showing a connection between SnRK2s and sulfur metabolism comes from experiments on the lower plant Chlamydomonas reinhardtii (Davies et al, 1999;Irihimovitch and Stern, 2006;González-Ballester et al, 2008. SNRK2.1 is considered a general regulator of S-responsive gene expression in C. reinhardtii (González-Ballester et al, 2008).…”

Interference Expression at Levels of the Transcript and Protein among Group 1, 2, and 3 Sigma Factor Genes in a Cyanobacterium

“…In another case, abundant transcripts of group 3 σ factor genes existed whereas low levels of proteins were observed 21) , indicating that there is potentially negative regulatory effect on later steps after transcription in the wild-type strains. Inconsistencies in the amounts of transcript and protein have often been observed in yeast and photosynthesizing organisms, Chramydomonas and Arabidopsis 7,15,22) . In addition, global control involves ribonuclease activities for mRNA stability in bacteria and chloroplasts 6,16) .…”

RNA-Seq Analysis of Sulfur-Deprived Chlamydomonas Cells Reveals Aspects of Acclimation Critical for Cell Survival

“…SNRK2.2 is responsible for repression of S-inducible genes when cells are replete for S (Davies et al, 1999;Ravina et al, 2002;Gonzá lez-Ballester et al, 2008). It may also be involved in repression of chloroplast transcription during S deprivation (Irihimovitch and Stern, 2006). The SNRK2.1 kinase is required for most S-responsive gene expression and for maintaining cell viability during S deprivation.…”