Metabolic control of acclimation to nutrient deprivation dependent on polyphosphate synthesis

Polyphosphate, an energy-rich polymer conserved in all kingdoms of life, is integral to many cellular stress responses, including nutrient deprivation, and yet, the mechanisms that underlie its biological roles are not well understood. In this work, we elucidate the physiological function of this polymer in the acclimation of the model alga Chlamydomonas reinhardtii to nutrient deprivation. Our data reveal that polyphosphate synthesis is vital to control cellular adenosine 5'-triphosphate homeostasis and maintain both respiratory and photosynthetic electron transport upon sulfur deprivation. Using both genetic and pharmacological approaches, we show that electron flow in the energy-generating organelles is essential to induce and sustain acclimation to sulfur deprivation at the transcriptional level. These previously unidentified links among polyphosphate synthesis, photosynthetic and respiratory electron flow, and the acclimation of cells to nutrient deprivation could unveil the mechanism by which polyphosphate helps organisms cope with a myriad of stress conditions in a fluctuating environment.

Biosynthesis of carbonic anhydrase in Chlamydomonas reinhardtii during adaptation to low CO(2)

The unicellular green alga Chlamydomonas reinhardtii synthesizes carbonic anhydrase in response to low levels of CO(2) (i.e., air levels of CO(2)). This enzyme, localized predominantly in the periplasmic space of the alga (or associated with the cell wall), is an important component of the machinery required for the active accumulation of inorganic carbon by C. reinhardtii and the saturation of ribulose-1,5-bisphosphate carboxylase at low extracellular carbon concentrations. We have begun to examine the synthesis and compartmentalization of carbonic anhydrase in C. reinhardtii. The monomeric species associated with carbonic anhydrase activity is synthesized as a precursor on 80S cytoplasmic ribosomes. This precursor can be detected immunologically in the profiles of translation products when a reticulocyte lysate, cell-free system is primed with poly(A)-RNA from either air-grown C. reinhardtii or cells shifted from growth on 5% CO(2) to air for 12 hr. It is not synthesized when the in vitro system is primed with poly(A)-RNA from CO(2)-grown algae. Since translatable RNA for the polypeptide responsible for carbonic anhydrase activity was only present in cells that experienced low levels of CO(2), the adaptation process either involves the regulation of transcription of the carbonic anhydrase gene (and perhaps other genes involved in adaptation) or the post-transcriptional processing of the messenger RNA. Furthermore, the appearance of the mature polypeptide associated with carbonic anhydrase activity in the periplasmic space of C. reinhardtii is inhibited by tunicamycin, an antibiotic that prevents core glycosylation of polypeptides on the endoplasmic reticulum. Together, these results suggest that the biosynthesis of this extracellular algal enzyme involves the translation of mRNA for the carbonic anhydrase monomer on ribosomes bound to the endoplasmic reticulum, the cleavage of a signal sequence during transport of the nascent polypeptide into the lumen of the endoplasmic reticulum, and subsequent glycosylation events prior to export across the plasmalemma.

Isolation and characterization of calmodulin from spinach leaves and in vitro translation mixtures

Calmodulin, a multifunctional calcium-modulated protein, has been isolated from spinach leaf tissue and from spinach leaf messenger RNA translation products. The translation protein and the spinach leaf protein have been partially characterized and compared to vertebrate calmodulins. Spinach leaf calmodulin will quantitatively activate bovine brain phosphodiesterase and will undergo a calcium-dependent shift in electrophoretic mobility similar to that of bovine brain calmodulin. In the presence of Ca(2+) the spinach and brain proteins comigrate, but in the presence of chelators they do not. A polyadenylylated RNA fraction has been isolated from spinach leaf tissue and translated in a wheat germ cell-free translation system. The calmodulin synthesized in vitro has been isolated by using calcium-dependent affinity chromatography on phenothiazine-Sepharose conjugates. The translation protein comigrates with spinach calmodulin during polyacrylamide gel electrophoresis whether in the presence or the absence of Ca(2+). The translation protein also undergoes a calcium-dependent mobility shift identical to that of spinach calmodulin. Amino acid analysis of the translation calmodulin indicates that it does not contain N(epsilon)-trimethyllysine, an amino acid residue that is characteristic of all calmodulins previously examined. These studies suggest that N(epsilon)-trimethyllysine is not required for the calcium-dependent interaction of calmodulin with phenothiazines and indicate the potential utility of phenothiazine-Sepharose conjugates as affinity-based adsorbents in biological and biochemical investigations.

A genome?s-eye view of the light-harvesting polypeptides of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a valuable model system for defining the structure and function of polypeptides of the photosynthetic apparatus and the dynamic aspects of photosynthesis. Recently, a genome-wide analysis of cDNAs and a draft genome sequence that covers approximately 90% of the genome were made available, providing a clear picture of the composition of specific gene families, the relationships among the gene family members, and the location of each member on the genome. We used the available sequence information to analyze the extensive family of light-harvesting genes in C. reinhardtii. There are nine genes encoding polypeptides of the major light-harvesting complex of photosystem II, two genes encoding the minor light-harvesting polypeptides of photosystem II, and nine genes encoding polypeptides predicted to comprise the photosystem I light-harvesting complex. Furthermore, there are five genes encoding early light-induced proteins and two genes encoding LI818 polypeptides. A candidate for the PsbS gene has also been found in the raw genome sequence data (Niyogi, personal communication), although no genes encoding homologues of the Sep, or Hli polypeptides have been identified. In this manuscript, we identify and classify the family of light-harvesting polypeptides encoded on the C. reinhardtii genome. This is an important first step in designing specific genetic, biochemical, and physiological studies aimed at characterizing the composition, function, and regulation of the light-harvesting complexes.

Sulfur economy and cell wall biosynthesis during sulfur limitation of Chlamydomonas reinhardtii

We have identified two novel periplasmic/cell wall polypeptides that specifically accumulate during sulfur limitation of Chlamydomonas reinhardtii. These polypeptides, present at high levels in the extracellular polypeptide fraction from a sulfur-deprived, cell wall-minus C. reinhardtii strain, have apparent molecular masses of 76 and 88 kD and are designated Ecp76 and Ecp88. N-terminal sequences of these polypeptides facilitated the isolation of full-length Ecp76 and Ecp88 cDNAs. Ecp76 and Ecp88 polypeptides are deduced to be 583 and 595 amino acids, respectively. Their amino acid sequences are similar to each other, with features characteristic of cell wall-localized hydroxyproline-rich glycoproteins; the N terminus of each polypeptide contains a predicted signal sequence, whereas the C terminus is rich in proline, alanine, and serine. Ecp76 and Ecp88 have either no (Ecp88) or one (Ecp76) sulfur-containing amino acid and transcripts encoding these polypeptides are not detected in cultures maintained on complete medium, but accumulate when cells are deprived of sulfur. This accumulation is temporally delayed relative to the accumulation of sulfur stress-induced arylsulfatase and ATP sulfurylase transcripts. The addition of sulfate back to sulfur-starved cultures caused a rapid decline in Ecp76 and Ecp88 mRNAs (half lives < 10 min). Furthermore, the C. reinhardtii sac1 mutant, which lacks a regulatory protein critical for acclimation to sulfur limitation, does not accumulate Ecp76 or Ecp88 transcripts. These results suggest that the Ecp76 and Ecp88 genes are under SacI control, and that restructuring of the C. reinhardtii cell wall during sulfur limitation may be important for redistribution of internal and efficient utilization of environmental sulfur-containing molecules.

Sulfur economy and cell wall biosynthesis during sulfur limitation of Chlamydomonas reinhardtii

We have identified two novel periplasmic/cell wall polypeptides that specifically accumulate during sulfur limitation of Chlamydomonas reinhardtii. These polypeptides, present at high levels in the extracellular polypeptide fraction from a sulfur-deprived, cell wall-minus C. reinhardtii strain, have apparent molecular masses of 76 and 88 kD and are designated Ecp76 and Ecp88. N-terminal sequences of these polypeptides facilitated the isolation of full-length Ecp76 and Ecp88 cDNAs. Ecp76 and Ecp88 polypeptides are deduced to be 583 and 595 amino acids, respectively. Their amino acid sequences are similar to each other, with features characteristic of cell wall-localized hydroxyproline-rich glycoproteins; the N terminus of each polypeptide contains a predicted signal sequence, whereas the C terminus is rich in proline, alanine, and serine. Ecp76 and Ecp88 have either no (Ecp88) or one (Ecp76) sulfur-containing amino acid and transcripts encoding these polypeptides are not detected in cultures maintained on complete medium, but accumulate when cells are deprived of sulfur. This accumulation is temporally delayed relative to the accumulation of sulfur stress-induced arylsulfatase and ATP sulfurylase transcripts. The addition of sulfate back to sulfur-starved cultures caused a rapid decline in Ecp76 and Ecp88 mRNAs (half lives < 10 min). Furthermore, the C. reinhardtii sac1 mutant, which lacks a regulatory protein critical for acclimation to sulfur limitation, does not accumulate Ecp76 or Ecp88 transcripts. These results suggest that the Ecp76 and Ecp88 genes are under SacI control, and that restructuring of the C. reinhardtii cell wall during sulfur limitation may be important for redistribution of internal and efficient utilization of environmental sulfur-containing molecules.

Novel motility mutants of synechocystis strain PCC 6803 generated by in vitro transposon mutagenesis

We screened for transposon-generated mutants of Synechocystis sp. strain PCC 6803 that exhibited aberrant phototactic movement. Of the 300 mutants generated, about 50 have been partially characterized; several contained transposons in genes encoding chemotaxis-related proteins, while others mapped to novel genes. These novel genes and their possible roles in motility are discussed.

Light regulation of type IV pilus-dependent motility by chemosensor-like elements in Synechocystis PCC6803

To optimize photosynthesis, cyanobacteria move toward or away from a light source by a process known as phototaxis. Phototactic movement of the cyanobacterium Synechocystis PCC6803 is a surface-dependent phenomenon that requires type IV pili, cellular appendages implicated in twitching and social motility in a range of bacteria. To elucidate regulation of cyanobacterial motility, we generated transposon-tagged mutants with aberrant phototaxis; mutants were either nonmotile or exhibited an "inverted motility response" (negative phototaxis) relative to wild-type cells. Several mutants contained transposons in genes similar to those involved in bacterial chemotaxis. Synechocystis PCC6803 has three loci with chemotaxis-like genes, of which two, Tax1 and Tax3, are involved in phototaxis. Transposons interrupting the Tax1 locus yielded mutants that exhibited an inverted motility response, suggesting that this locus is involved in controlling positive phototaxis. However, a strain null for taxAY1 was nonmotile and hyperpiliated. Interestingly, whereas the C-terminal region of the TaxD1 polypeptide is similar to the signaling domain of enteric methyl-accepting chemoreceptor proteins, the N terminus has two domains resembling chromophore-binding domains of phytochrome, a photoreceptor in plants. Hence, TaxD1 may play a role in perceiving the light stimulus. Mutants in the Tax3 locus are nonmotile and do not make type IV pili. These findings establish links between chemotaxis-like regulatory elements and type IV pilus-mediated phototaxis.

Trophic conversion of an obligate photoautotrophic organism through metabolic engineering

Most microalgae are obligate photoautotrophs and their growth is strictly dependent on the generation of photosynthetically derived energy. We show that the microalga Phaeodactylum tricornutum can be genetically engineered to thrive on exogenous glucose in the absence of light through the introduction of a gene encoding a glucose transporter (glut1 or hup1). This demonstrates that a fundamental change in the metabolism of an organism can be accomplished through the introduction of a single gene. This also represents progress toward the use of fermentation technology for large-scale commercial exploitation of algae by reducing limitations associated with light-dependent growth.

Genes essential to iron transport in the cyanobacterium Synechocystis sp. strain PCC 6803

Genes encoding polypeptides of an ATP binding cassette (ABC)-type ferric iron transporter that plays a major role in iron acquisition in Synechocystis sp. strain PCC 6803 were identified. These genes are slr1295, slr0513, slr0327, and recently reported sll1878 (Katoh et al., J. Bacteriol. 182:6523-6524, 2000) and were designated futA1, futA2, futB, and futC, respectively, for their involvement in ferric iron uptake. Inactivation of these genes individually or futA1 and futA2 together greatly reduced the activity of ferric iron uptake in cells grown in complete medium or iron-deprived medium. All the fut genes are expressed in cells grown in complete medium, and expression was enhanced by iron starvation. The futA1 and futA2 genes appear to encode periplasmic proteins that play a redundant role in iron binding. The deduced products of futB and futC genes contain nucleotide-binding motifs and belong to the ABC transporter family of inner-membrane-bound and membrane-associated proteins, respectively. These results and sequence similarities among the four genes suggest that the Fut system is related to the Sfu/Fbp family of iron transporters. Inactivation of slr1392, a homologue of feoB in Escherichia coli, greatly reduced the activity of ferrous iron transport. This system is induced by intracellular low iron concentrations that are achieved in cells exposed to iron-free medium or in the fut-less mutants grown in complete medium.

Highly expressed and alien genes of the Synechocystis genome

Comparisons of codon frequencies of genes to several gene classes are used to characterize highly expressed and alien genes on the SYNECHOCYSTIS: PCC6803 genome. The primary gene classes include the ensemble of all genes (average gene), ribosomal protein (RP) genes, translation processing factors (TF) and genes encoding chaperone/degradation proteins (CH). A gene is predicted highly expressed (PHX) if its codon usage is close to that of the RP/TF/CH standards but strongly deviant from the average gene. Putative alien (PA) genes are those for which codon usage is significantly different from all four classes of gene standards. In SYNECHOCYSTIS:, 380 genes were identified as PHX. The genes with the highest predicted expression levels include many that encode proteins vital for photosynthesis. Nearly all of the genes of the RP/TF/CH gene classes are PHX. The principal glycolysis enzymes, which may also function in CO(2) fixation, are PHX, while none of the genes encoding TCA cycle enzymes are PHX. The PA genes are mostly of unknown function or encode transposases. Several PA genes encode polypeptides that function in lipopolysaccharide biosynthesis. Both PHX and PA genes often form significant clusters (operons). The proteins encoded by PHX and PA genes are described with respect to functional classifications, their organization in the genome and their stoichiometry in multi-subunit complexes.

The high light-inducible polypeptides in Synechocystis PCC6803. Expression and function in high light

There are five Synechocystis PCC6803 genes encoding polypeptides with similarity to the Lhc polypeptides of plants. Four of the polypeptides, designated HliA-D (Dolganov, N. A. M., Bhaya, D., and Grossman, A. R. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 636-640) (corresponding to ScpC, ScpD, ScpB, and ScpE in Funk, C., and Vermaas, W. (1999) Biochemistry 38, 9397-9404) contain a single transmembrane domain. The fifth polypeptide (HemH) represents a fusion between a ferrochelatase and an Hli-like polypeptide. By using an epitope tag to identify specifically the different Hli polypeptides, the accumulation of each (excluding HemH) was examined under various environmental conditions. The levels of all of the Hli polypeptides were elevated in high light and during nitrogen limitation, whereas HliA, HliB, and HliC also accumulated to high levels following exposure to sulfur deprivation and low temperature. The temporal pattern of accumulation was significantly different among the different Hli polypeptides. HliC rapidly accumulated in high light, and its level remained high for at least 24 h. HliA and HliB also accumulated rapidly, but their levels began to decline 9-12 h following the imposition of high light. HliD was transiently expressed in high light and was not detected 24 h after the initiation of high light exposure. These results demonstrate that there is specificity to the accumulation of the Hli polypeptides under a diverse range of environmental conditions. Furthermore, mutants for the individual and combinations of the hli genes were evaluated for their fitness to grow in high light. Although all of the mutants grew as fast as wild-type cells in low light, strains inactivated for hliA or hliC/hliD were unable to compete with wild-type cells during co-cultivation in high light. A mutant lacking all four hli genes gradually lost its photosynthesis capacity and died in high light. Hence, the Hli polypeptides are critical for survival when Synechocystis PCC6803 is absorbing excess excitation energy and may allow the cells to cope more effectively with the production of reactive oxygen species.

A gene of Synechocystis sp. Strain PCC 6803 encoding a novel iron transporter

A mutant of Synechocystis sp. strain PCC 6803 disrupted for sll1878 exhibited greatly reduced Fe(3+) transport activity. The K(m) value of sll1878-dependent Fe(3+) transport in cells grown in iron-replete medium was 0.5 microM. Both the maximal rate and K(m) value were increased in iron-starved cells.

Isolation of regulated genes of the cyanobacterium Synechocystis sp. strain PCC 6803 by differential display

Global identification of differentially regulated genes in prokaryotes is constrained because the mRNA does not have a 3' polyadenylation extension; this precludes specific separation of mRNA from rRNA and tRNA and synthesis of cDNAs from the entire mRNA population. Knowledge of the entire genome sequence of Synechocystis sp. strain PCC 6803 has enabled us to develop a differential display procedure that takes advantage of a short palindromic sequence that is dispersed throughout the Synechocystis sp. strain PCC 6803 genome. This sequence, designated the HIP (highly iterated palindrome) element, occurs in approximately half of the Synechocystis sp. strain PCC 6803 genes but is absent in rRNA and tRNA genes. To determine the feasibility of exploiting the HIP element, alone or in combination with specific primer subsets, for analyzing differential gene expression, we used HIP-based primers to identify light intensity-regulated genes. Several gene fragments, including those encoding ribosomal proteins and phycobiliprotein subunits, were differentially amplified from RNA templates derived from cells grown in low light or exposed to high light for 3 h. One novel finding was that expression of certain genes of the pho regulon, which are under the control of environmental phosphate levels, were markedly elevated in high light. High-light activation of pho regulon genes correlated with elevated growth rates that occur when the cells are transferred from low to high light. These results suggest that in high light, the rate of growth of Synechocystis sp. strain PCC 6803 exceeds its capacity to assimilate phosphate, which, in turn, may trigger a phosphate starvation response and activation of the pho regulon.

Chlamydomonas reinhardtii and photosynthesis: genetics to genomics

Genetic and physiological features of the green alga Chlamydomonas reinhardtii have provided a useful model for elucidating the function, biogenesis and regulation of the photosynthetic apparatus. Combining these characteristics with newly developed molecular technologies for engineering Chlamydomonas and the promise of global analyses of nuclear and chloroplast gene expression will add a new perspective to views on photosynthetic function and regulation.

A pigment-binding protein essential for regulation of photosynthetic light harvesting

Photosynthetic light harvesting in plants is regulated in response to changes in incident light intensity. Absorption of light that exceeds a plant's capacity for fixation of CO2 results in thermal dissipation of excitation energy in the pigment antenna of photosystem II by a poorly understood mechanism. This regulatory process, termed nonphotochemical quenching, maintains the balance between dissipation and utilization of light energy to minimize generation of oxidizing molecules, thereby protecting the plant against photo-oxidative damage. To identify specific proteins that are involved in nonphotochemical quenching, we have isolated mutants of Arabidopsis thaliana that cannot dissipate excess absorbed light energy. Here we show that the gene encoding PsbS, an intrinsic chlorophyll-binding protein of photosystem II, is necessary for nonphotochemical quenching but not for efficient light harvesting and photosynthesis. These results indicate that PsbS may be the site for nonphotochemical quenching, a finding that has implications for the functional evolution of pigment-binding proteins.

Psr1, a nuclear localized protein that regulates phosphorus metabolism in Chlamydomonas

Understanding the ways in which phosphorus metabolism is regulated in photosynthetic eukaryotes is critical for optimizing crop productivity and managing aquatic ecosystems in which phosphorus can be a major source of pollution. Here we describe a gene encoding a regulator of phosphorus metabolism, designated Psr1 (phosphorus starvation response), from a photosynthetic eukaryote. The Psr1 protein is critical for acclimation of the unicellular green alga Chlamydomonas reinhardtii to phosphorus starvation. The N-terminal half of Psr1 contains a region similar to myb DNA-binding domains and the C-terminal half possesses glutamine-rich sequences characteristic of transcriptional activators. The level of Psr1 increases at least 10-fold upon phosphate starvation, and immunocytochemical studies demonstrate that this protein is nuclear-localized under both nutrient-replete and phosphorus-starvation conditions. Finally, Psr1 and angiosperm proteins have domains that are similar, suggesting a possible role for Psr1 homologs in the control of phosphorus metabolism in vascular plants. With the identification of regulators such as Psr1 it may become possible to engineer photosynthetic organisms for more efficient utilization of phosphorus and to establish better practices for the management of agricultural lands and natural ecosystems.

Psr1, a nuclear localized protein that regulates phosphorus metabolism in Chlamydomonas

Understanding the ways in which phosphorus metabolism is regulated in photosynthetic eukaryotes is critical for optimizing crop productivity and managing aquatic ecosystems in which phosphorus can be a major source of pollution. Here we describe a gene encoding a regulator of phosphorus metabolism, designated Psr1 (phosphorus starvation response), from a photosynthetic eukaryote. The Psr1 protein is critical for acclimation of the unicellular green alga Chlamydomonas reinhardtii to phosphorus starvation. The N-terminal half of Psr1 contains a region similar to myb DNA-binding domains and the C-terminal half possesses glutamine-rich sequences characteristic of transcriptional activators. The level of Psr1 increases at least 10-fold upon phosphate starvation, and immunocytochemical studies demonstrate that this protein is nuclear-localized under both nutrient-replete and phosphorus-starvation conditions. Finally, Psr1 and angiosperm proteins have domains that are similar, suggesting a possible role for Psr1 homologs in the control of phosphorus metabolism in vascular plants. With the identification of regulators such as Psr1 it may become possible to engineer photosynthetic organisms for more efficient utilization of phosphorus and to establish better practices for the management of agricultural lands and natural ecosystems.

Chlamydomonas reinhardtii mutants abnormal in their responses to phosphorus deprivation

P-starved plants scavenge inorganic phosphate (Pi) by developing elevated rates of Pi uptake, synthesizing extracellular phosphatases, and secreting organic acids. To elucidate mechanisms controlling these acclimation responses in photosynthetic organisms, we characterized the responses of the green alga Chlamydomonas reinhardtii to P starvation and developed screens for isolating mutants (designated psr [phosphorus-stress response]) abnormal in their responses to environmental levels of Pi. The psr1-1 mutant was identified in a selection for cells that survived exposure to high concentrations of radioactive Pi. psr1-2 and psr2 were isolated as strains with aberrant levels of extracellular phosphatase activity during P-deficient or nutrient-replete growth. The psr1-1 and psr1-2 mutants were phenotypically similar, and the lesions in these strains were recessive and allelic. They exhibited no increase in extracellular phosphatase activity or Pi uptake upon starvation. Furthermore, when placed in medium devoid of P, the psr1 strains lost photosynthetic O2 evolution and stopped growing more rapidly than wild-type cells; they may not be as efficient as wild-type cells at scavenging/accessing P stores. In contrast, psr2 showed elevated extracellular phosphatase activity during growth in nutrient-replete medium, and the mutation was dominant. The mutant phenotypes and the roles of Psr1 and Psr2 in P-limitation responses are discussed.

Sac3, an Snf1-like serine/threonine kinase that positively and negatively regulates the responses of Chlamydomonas to sulfur limitation

The Sac3 gene product of Chlamydomonas positively and negatively regulates the responses of the cell to sulfur limitation. In wild-type cells, arylsulfatase activity is detected only during sulfur limitation. The sac3 mutant expresses arylsulfatase activity even when grown in nutrient-replete medium, which suggests that the Sac3 protein has a negative effect on the induction of arylsulfatase activity. In contrast to its effect on arylsulfatase activity, Sac3 positively regulates the high-affinity sulfate transport system-the sac3 mutant is unable to fully induce high-affinity sulfate transport during sulfur limitation. We have complemented the sac3 mutant and cloned a cDNA copy of the Sac3 gene. The deduced amino acid sequence of the Sac3 gene product is similar to the catalytic domain of the yeast Snf1 family of serine/threonine kinases and is therefore classified as a Snf1-related kinase (SnRK). Specifically, Sac3 falls within the SnRK2 subfamily of kinases from vascular plants. In addition to the 11 subdomains common to Snf1-like serine/threonine kinases, Sac3 and the plant kinases have two additional subdomains and a highly acidic C-terminal region. The role of Sac3 in the signal transduction system that regulates the responses of Chlamydomonas to sulfur limitation is discussed.

The role of an alternative sigma factor in motility and pilus formation in the cyanobacterium Synechocystis sp. strain PCC6803

Disruption of a gene for an alternative sigma factor, designated sigF, in the freshwater, unicellular cyanobacterium Synechocystis sp. strain PCC6803 resulted in a pleiotropic phenotype. Most notably, this mutant lost phototactic movement with a concomitant loss of pili, which are abundant on the surface of wild-type cells. The sigF mutant also secreted both high levels of yellow-brown and UV-absorbing pigments and a polypeptide that is similar to a large family of extracellular proteins that includes the hemolysins. Furthermore, the sigF mutant had a dramatically reduced level of the transcript from two tandemly arranged pilA genes (sll1694 and sll1695), which encode major structural components of type IV pili. Inactivation of these pilA genes eliminated phototactic movement, though some pili were still present in this strain. Together, these results demonstrate that SigF plays a critical role in motility via the control of pili formation and is also likely to regulate other features of the cell surface. Furthermore, the data provide evidence that type IV pili are required for phototactic movement in certain cyanobacteria and suggest that different populations of pili present on the Synechocystis cell surface may perform different functions.

A polypeptide with similarity to phycocyanin alpha-subunit phycocyanobilin lyase involved in degradation of phycobilisomes

To optimize the utilization of photosynthate and avoid damage that can result from the absorption of excess excitation energy, photosynthetic organisms must rapidly modify the synthesis and activities of components of the photosynthetic apparatus in response to environmental cues. During nutrient-limited growth, cyanobacteria degrade their light-harvesting complex, the phycobilisome, and dramatically reduce the rate of photosynthetic electron transport. In this report, we describe the isolation and characterization of a cyanobacterial mutant that does not degrade its phycobilisomes during either sulfur or nitrogen limitation and exhibits an increased ratio of phycocyanin to chlorophyll during nutrient-replete growth. The mutant phenotype was complemented by a gene encoding a polypeptide with similarities to polypeptides that catalyze covalent bond formation between linear tetrapyrrole chromophores and subunits of apophycobiliproteins. The complementing gene, designated nblB, is expressed at approximately the same level in cells grown in nutrient-replete medium and medium devoid of either sulfur or nitrogen. These results suggest that the NblB polypeptide may be a constitutive part of the machinery that coordinates phycobilisome degradation with environmental conditions.

Bilin deletions and subunit stability in cyanobacterial light-harvesting proteins

Light-harvesting in cyanobacteria and red algae is a function of the biliproteins, which have covalently bound bilin chromophores. The biliproteins are assembled with linker proteins into the phycobilisome, a large complex that resides on the surface of the photosynthetic membranes. Early steps in the phycobilisome assembly pathway include the folding of biliprotein alpha- and beta-subunits, covalent modification of subunits by bilin attachment and formation of the primary assembly unit, the alphabeta heterodimer. The potential role of bilins in subunit structure and assembly is examined in this study by site mutagenesis of biliprotein genes. Phycocyanin subunits from Synechocystis sp. 6701 that were unable to bind chromophores at specific sites were generated by changing the codons for bilin-binding cysteines to alanine residues. The altered genes were then expressed in a phycocyanin-minus mutant of the transformable Synechocystis sp. strain 6803. Single and multiple chromophore deletions cause specific and reproducible variations in phycobilisome-associated phycocyanin that do not correlate with transcript levels. Sedimentation equilibrium studies with purified proteins showed that bilin absence reduces the strength of alphabeta interaction in the heterodimer. These results suggest that phycocyanin instability in bilin-deletion mutants is a consequence of diversion of unassembled alpha- and beta-subunits to a degradation pathway. Attachment of the central bilin, which is common to all biliprotein subunits, may facilitate alphabeta interaction by completing the final stage of subunit folding and stabilizing the contact domains of binding partners in the heterodimer.

A response regulator of cyanobacteria integrates diverse environmental signals and is critical for survival under extreme conditions

Microorganisms must sense their environment and rapidly tune their metabolism to ambient conditions to efficiently use available resources. We have identified a gene encoding a response regulator, NblR, that complements a cyanobacterial mutant unable to degrade its light-harvesting complex (phycobilisome), in response to nutrient deprivation. Cells of the nblR mutant (i) have more phycobilisomes than wild-type cells during nutrient-replete growth, (ii) do not degrade phycobilisomes during sulfur, nitrogen, or phosphorus limitation, (iii) cannot properly modulate the phycobilisome level during exposure to high light, and (iv) die rapidly when starved for either sulfur or nitrogen, or when exposed to high light. Apart from regulation of phycobilisome degradation, NblR modulates additional functions critical for cell survival during nutrient-limited and high-light conditions. NblR does not appear to be involved in acclimation responses that occur only during a specific nutrient limitation. In contrast, it controls at least some of the general acclimation responses; those that occur during any of a number of different stress conditions. NblR plays a pivotal role in integrating different environmental signals that link the metabolism of the cell to light harvesting capabilities and the activities of the photosynthetic apparatus; this modulation is critical for cell survival.

Thermal injuries as a result of CO2 laser resurfacing

CO2 laser resurfacing of the face for fine wrinkles has gained great popularity over a short period of time. The use of the CO2 laser has proven to be effective in reducing or eliminating fine wrinkles. This tool in the surgeon's armamentarium has been added to those of dermabrasion and chemical peel. The theoretical advantage of the use of the CO2 laser for resurfacing has been better accuracy and reportedly more control of the depth of penetration. The use of the CO2 laser has been welcomed by many cosmetic surgeons. Until now, there have been few reported cases of complications with the use of the CO2 laser. To many, this would sound too good to be true; unfortunately, that is the case. The CO2 laser is a high-energy machine that can indeed cause thermal injury. This thermal injury can result in deep burns to the skin and hypertrophic scarring. We feel this is more common than is currently being reported, and we share our experience as a burn and wound care referral service. During an 18-month period, 20 consecutive patients were referred to our practice who had received injuries from the CO2 laser resurfacing laser. We present here in this review a summary of those injuries. The CO2 resurfacing laser is a very effective tool for the treatment of fine wrinkles, but it is not without the potential for serious complications. We urge caution with the use of the laser and prompt recognition and treatment of thermal injury to the skin.

Arabidopsis mutants define a central role for the xanthophyll cycle in the regulation of photosynthetic energy conversion

A conserved regulatory mechanism protects plants against the potentially damaging effects of excessive light. Nearly all photosynthetic eukaryotes are able to dissipate excess absorbed light energy in a process that involves xanthophyll pigments. To dissect the role of xanthophylls in photoprotective energy dissipation in vivo, we isolated Arabidopsis xanthophyll cycle mutants by screening for altered nonphotochemical quenching of chlorophyll fluorescence. The npq1 mutants are unable to convert violaxanthin to zeaxanthin in excessive light, whereas the npq2 mutants accumulate zeaxanthin constitutively. The npq2 mutants are new alleles of aba1, the zeaxanthin epoxidase gene. The high levels of zeaxanthin in npq2 affected the kinetics of induction and relaxation but not the extent of nonphotochemical quenching. Genetic mapping, DNA sequencing, and complementation of npq1 demonstrated that this mutation affects the structural gene encoding violaxanthin deepoxidase. The npq1 mutant exhibited greatly reduced nonphotochemical quenching, demonstrating that violaxanthin deepoxidation is required for the bulk of rapidly reversible nonphotochemical quenching in Arabidopsis. Altered regulation of photosynthetic energy conversion in npq1 was associated with increased sensitivity to photoinhibition. These results, in conjunction with the analysis of npq mutants of Chlamydomonas, suggest that the role of the xanthophyll cycle in nonphotochemical quenching has been conserved, although different photosynthetic eukaryotes rely on the xanthophyll cycle to different extents for the dissipation of excess absorbed light energy.

Arabidopsis mutants define a central role for the xanthophyll cycle in the regulation of photosynthetic energy conversion

A conserved regulatory mechanism protects plants against the potentially damaging effects of excessive light. Nearly all photosynthetic eukaryotes are able to dissipate excess absorbed light energy in a process that involves xanthophyll pigments. To dissect the role of xanthophylls in photoprotective energy dissipation in vivo, we isolated Arabidopsis xanthophyll cycle mutants by screening for altered nonphotochemical quenching of chlorophyll fluorescence. The npq1 mutants are unable to convert violaxanthin to zeaxanthin in excessive light, whereas the npq2 mutants accumulate zeaxanthin constitutively. The npq2 mutants are new alleles of aba1, the zeaxanthin epoxidase gene. The high levels of zeaxanthin in npq2 affected the kinetics of induction and relaxation but not the extent of nonphotochemical quenching. Genetic mapping, DNA sequencing, and complementation of npq1 demonstrated that this mutation affects the structural gene encoding violaxanthin deepoxidase. The npq1 mutant exhibited greatly reduced nonphotochemical quenching, demonstrating that violaxanthin deepoxidation is required for the bulk of rapidly reversible nonphotochemical quenching in Arabidopsis. Altered regulation of photosynthetic energy conversion in npq1 was associated with increased sensitivity to photoinhibition. These results, in conjunction with the analysis of npq mutants of Chlamydomonas, suggest that the role of the xanthophyll cycle in nonphotochemical quenching has been conserved, although different photosynthetic eukaryotes rely on the xanthophyll cycle to different extents for the dissipation of excess absorbed light energy.

The regulation of photosynthetic electron transport during nutrient deprivation in Chlamydomonas reinhardtii

The light-saturated rate of photosynthetic O2 evolution in Chlamydomonas reinhardtii declined by approximately 75% on a per-cell basis after 4 d of P starvation or 1 d of S starvation. Quantitation of the partial reactions of photosynthetic electron transport demonstrated that the light-saturated rate of photosystem (PS) I activity was unaffected by P or S limitation, whereas light-saturated PSII activity was reduced by more than 50%. This decline in PSII activity correlated with a decline in both the maximal quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers (PSII centers capable of performing a charge separation but unable to reduce the plastoquinone pool). In addition to a decline in the light-saturated rate of O2 evolution, there was reduced efficiency of excitation energy transfer to the reaction centers of PSII (because of dissipation of absorbed light energy as heat and because of a transition to state 2). These findings establish a common suite of alterations in photosynthetic electron transport that results in decreased linear electron flow when C. reinhardtii is limited for either P or S. It was interesting that the decline in the maximum quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers were regulated specifically during S-limited growth by the SacI gene product, which was previously shown to be critical for the acclimation of C. reinhardtii to S limitation (J.P. Davies, F.H. Yildiz, and A.R. Grossman [1996] EMBO J 15: 2150-2159).

The roles of specific xanthophylls in photoprotection

Xanthophyll pigments have critical structural and functional roles in the photosynthetic light-harvesting complexes of algae and vascular plants. Genetic dissection of xanthophyll metabolism in the green alga Chlamydomonas reinhardtii revealed functions for specific xanthophylls in the nonradiative dissipation of excess absorbed light energy, measured as nonphotochemical quenching of chlorophyll fluorescence. Mutants with a defect in either the alpha- or beta-branch of carotenoid biosynthesis exhibited less nonphotochemical quenching but were still able to tolerate high light. In contrast, a double mutant that was defective in the synthesis of lutein, loroxanthin (alpha-carotene branch), zeaxanthin, and antheraxanthin (beta-carotene branch) had almost no nonphotochemical quenching and was extremely sensitive to high light. These results strongly suggest that in addition to the xanthophyll cycle pigments (zeaxanthin and antheraxanthin), alpha-carotene-derived xanthophylls such as lutein, which are structural components of the subunits of the light-harvesting complexes, contribute to the dissipation of excess absorbed light energy and the protection of plants from photo-oxidative damage.

Chlamydomonas Xanthophyll Cycle Mutants Identified by Video Imaging of Chlorophyll Fluorescence Quenching

The photosynthetic apparatus in plants is protected against oxidative damage by processes that dissipate excess absorbed light energy as heat within the light-harvesting complexes. This dissipation of excitation energy is measured as nonphotochemical quenching of chlorophyll fluorescence. Nonphotochemical quenching depends primarily on the [delta]pH that is generated by photosynthetic electron transport, and it is also correlated with the amounts of zeaxanthin and antheraxanthin that are formed from violaxanthin by the operation of the xanthophyll cycle. To perform a genetic dissection of nonphotochemical quenching, we have isolated npq mutants of Chlamydomonas by using a digital video-imaging system. In excessive light, the npq1 mutant is unable to convert violaxanthin to antheraxanthin and zeaxanthin; this reaction is catalyzed by violaxanthin de-epoxidase. The npq2 mutant appears to be defective in zeaxanthin epoxidase activity, because it accumulates zeaxanthin and completely lacks antheraxanthin and violaxanthin under all light conditions. Characterization of these mutants demonstrates that a component of nonphotochemical quenching that develops in vivo in Chlamydomonas depends on the accumulation of zeaxanthin and antheraxanthin via the xanthophyll cycle. However, observation of substantial, rapid, [delta]pH-dependent nonphotochemical quenching in the npq1 mutant demonstrates that the formation of zeaxanthin and antheraxanthin via violaxanthin de-epoxidase activity is not required for all [delta]pH-dependent nonphotochemical quenching in this alga. Furthermore, the xanthophyll cycle is not required for survival of Chlamydomonas in excessive light.

Chlamydomonas Xanthophyll Cycle Mutants Identified by Video Imaging of Chlorophyll Fluorescence Quenching

The photosynthetic apparatus in plants is protected against oxidative damage by processes that dissipate excess absorbed light energy as heat within the light-harvesting complexes. This dissipation of excitation energy is measured as nonphotochemical quenching of chlorophyll fluorescence. Nonphotochemical quenching depends primarily on the [delta]pH that is generated by photosynthetic electron transport, and it is also correlated with the amounts of zeaxanthin and antheraxanthin that are formed from violaxanthin by the operation of the xanthophyll cycle. To perform a genetic dissection of nonphotochemical quenching, we have isolated npq mutants of Chlamydomonas by using a digital video-imaging system. In excessive light, the npq1 mutant is unable to convert violaxanthin to antheraxanthin and zeaxanthin; this reaction is catalyzed by violaxanthin de-epoxidase. The npq2 mutant appears to be defective in zeaxanthin epoxidase activity, because it accumulates zeaxanthin and completely lacks antheraxanthin and violaxanthin under all light conditions. Characterization of these mutants demonstrates that a component of nonphotochemical quenching that develops in vivo in Chlamydomonas depends on the accumulation of zeaxanthin and antheraxanthin via the xanthophyll cycle. However, observation of substantial, rapid, [delta]pH-dependent nonphotochemical quenching in the npq1 mutant demonstrates that the formation of zeaxanthin and antheraxanthin via violaxanthin de-epoxidase activity is not required for all [delta]pH-dependent nonphotochemical quenching in this alga. Furthermore, the xanthophyll cycle is not required for survival of Chlamydomonas in excessive light.

Suppression of mutants aberrant in light intensity responses of complementary chromatic adaptation

Complementary chromatic adaptation is a process in which cyanobacteria alter the pigment protein (phycocyanin and phycoerythrin) composition of their light-harvesting complexes, the phycobilisomes, to help optimize the absorbance of prevalent wavelengths of light in the environment. Several classes of mutants that display aberrant complementary chromatic adaptation have been isolated. One of the mutant classes, designated "blue" or FdB, accumulates high levels of the blue chromoprotein phycocyanin in low-intensity green light, a condition that normally suppresses phycocyanin synthesis. We demonstrate here that the synthesis of the phycocyanin protein and mRNA in the FdB mutants can be suppressed by increasing the intensity of green light. Hence, these mutants have a decreased sensitivity to green light with respect to suppression of phycocyanin synthesis. Although we were unable to complement the blue mutants, we did isolate genes that could suppress the mutant phenotype. These genes, which have been identified previously, encode a histidine kinase sensor and response regulator protein that play key roles in controlling complementary chromatic adaptation. These findings are discussed with respect to the mechanism by which light quality and quantity control the biosynthesis of the phycobilisome.

New classes of mutants in complementary chromatic adaptation provide evidence for a novel four-step phosphorelay system

Complementary chromatic adaptation appears to be controlled by a complex regulatory system with similarity to four-step phosphorelays. Such pathways utilize two histidine and two aspartate residues for signal transduction. Previous studies of the signaling system controlling complementary chromatic adaptation have uncovered two elements of this pathway, a putative sensor, RcaE, and a response regulator, RcaC. In this work, we describe a second response regulator controlling complementary chromatic adaptation, RcaF, and identify putative DNA binding and histidine phosphoacceptor domains within RcaC. RcaF is a small response regulator with similarity to SpoOF of Bacillus subtilis; the latter functions in the four-step phosphorelay system controlling sporulation. We have also determined that within this phosphorelay pathway, RcaE precedes RcaF, and RcaC is probably downstream of RcaE and RcaF. This signal transduction pathway is novel because it appears to use at least five, instead of four, phosphoacceptor domains in the phosphorelay circuit.

Stable nuclear transformation of the diatom Phaeodactylum tricornutum

A nuclear transformation system has been developed for the diatom Phaeodactylum tricornutum using microparticle bombardment to introduce the sh ble gene from Streptoalloteichus hindustanus into cells. The sh ble gene encodes a protein that confers resistance to the antibiotics Zeocin and phleomycin. Chimeric genes containing promoter and terminator sequences from the P. tricornutum fcp genes were used to drive expression of sh ble. Between 10-100 transformants were recovered/10(8) cells. Transformants were able to grow on at least 500 micrograms/ml of Zeocin, which is 10 times the amount necessary to kill wild-type cells. Based on Southern hybridizations the sh ble gene was present in 1-3 copies/transformant. Relative levels of correctly processed transcripts were correlated with the abundance of the Sh ble protein (present at 0.1-2.0 micrograms/mg total protein). The cat reporter gene fused to a fcp promoter could also be introduced by microparticle bombardment and was found to be highly expressed (average of 7.1 U/mg total protein). This work demonstrates that heterologous genes can be readily expressed in P. tricornutum. The development of selectable marker and reporter gene constructs provides the tools necessary for dissecting gene structure and regulation, and introducing novel functions into diatoms.

Similarity of a chromatic adaptation sensor to phytochrome and ethylene receptors

Complementary chromatic adaptation in cyanobacteria acts through photoreceptors to control the biosynthesis of light-harvesting complexes. The mutant FdBk, which appears black, cannot chromatically adapt and may contain a lesion in the apparatus that senses light quality. The complementing gene identified here, rcaE, encodes a deduced protein in which the amino-terminal region resembles the chromophore attachment domain of phytochrome photoreceptors and regions of plant ethylene receptors; the carboxyl- terminal half is similar to the histidine kinase domain of two-component sensor kinases.

Biochemical characterization of the extracellular phosphatases produced by phosphorus-deprived Chlamydomonas reinhardtii

We have examined the extracellular phosphatases produced by the terrestrial green alga Chlamydomonas reinhardtii in response to phosphorus deprivation. Phosphorus-deprived cells increase extra-cellular alkaline phosphatase activity 300-fold relative to unstarved cells. The alkaline phosphatases are released into the medium by cell-wall-deficient strains and by wild-type cells after treatment with autolysin, indicating that they are localized to the periplasm. Anion-exchange chromatography and analysis by nondenaturing polyacrylamide gel electrophoresis revealed that there are two major inducible alkaline phosphatases. A calcium-dependent enzyme composed of 190-kD glycoprotein subunits accounts for 85 to 95% of the Alkaline phosphatase activity. This phosphatase has optimal activity at pH 9.5 and a Km of 120 to 262 microns for all physiological substrates tested, with the exception of phytic acid, which it cleaved with a 50-fold lower efficiency. An enzyme with optimal activity at pH 9 and no requirement for divalent cations accounts for 2 to 10% of the alkaline phosphatase activity. This phosphatase was only able to efficiently hydrolyze arylphosphates. The information reported here, in conjunction with the results of previous studies, defines the complement of extracellular phosphatases produced by phosphorus-deprived Chlamydomonas cells.

Light-harvesting complexes in oxygenic photosynthesis: diversity, control, and evolution

This article focuses on light-harvesting complexes (LHCs) in oxygen evolving photosynthetic organisms. These organisms include cyanobacteria, red algae, plants, green algae, brown algae, diatoms, chrysophytes, and dinoflagellates. We highlight the diversity of pigment-protein complexes that fuel the conversion of radiant energy to chemical bond energy in land plants and the diverse groups of the algae, detail the ways in which environmental parameters (i.e. light quantity and quality, nutrients) modulate the synthesis of these complexes, and discuss the evolutionary relationships among the LHC structural polypeptides.

Disruption of a gene encoding a novel thioredoxin-like protein alters the cyanobacterial photosynthetic apparatus

A gene that may encode a novel protein disulfide oxidoreductase, designated txlA (thioredoxin-like), was isolated from the cyanobacterium Synechococcus sp. strain PCC7942. Interruption of txlA near the putative thioredoxin-like active site yielded cells that grew too poorly to be analyzed. In contrast, a disruption of txlA near the C terminus that left the thioredoxin-like domain intact yielded two different mutant phenotypes. One type, designated txlXb, exhibited a slightly reduced growth rate and an increased cellular content of apparently normal phycobilisomes. The cellular content of phycobilisomes also increased in in the other mutant strain, designated txlXg. However, txlXg also exhibited a proportionate increase in chlorophyll and other components of the photosynthetic apparatus and grew as fast as wild-type cells. Both the txlXb and txlXg phenotypes were stable. The differences between the two strains may result from a genetic polymorphism extant in the original cell population. Further investigation of txlA may provide new insights into mechanisms that regulate the structure and function of the cyanobacterial photosynthetic apparatus.

Evolution of the phycobiliproteins

Amino acid sequence alignments and phylogenetic analyses have been used to examine the relationships among 100 phycobiliprotein sequences. The alignments revealed a number of highly conserved amino acid residues that are involved in chromophore attachment and conformation, alpha-beta interactions and phycobilisome assembly. The phylogenetic analysis confirmed that the phycobiliprotein subfamilies, previously classified by their biochemical and spectroscopic properties, also formed coherent evolutionary groups. The alpha and beta subunits formed two distinct evolutionary lines that originate from a common ancestor. The pattern of divergence among the alpha subfamilies was identical to that of the beta subfamilies, strongly suggesting that the alpha and beta subunits of each phycobiliprotein type have coevolved. The phylogenetic data support a monophyletic separation of the eukaryotic sequences from the extant cyanobacterial sequences. The eukaryotic phycoerythrins appeared more closely related to the marine Synechococcus phycoerythrins than to the other cyanobacterial phycoerythrins. The cryptophyte phycobiliproteins formed a monophyletic group within the rhodophyte lineage. In conjunction with other phylogenetic markers, the analysis of additional phycobiliprotein sequences may help to further resolve the relationships among phycobiliprotein-containing organisms.

The gene family encoding the fucoxanthin chlorophyll proteins from the brown alga Macrocystis pyrifera

Six members of a multigene family encoding polypeptide constituents of the fucoxanthin, chlorophyll a/c protein complex from female gametophytes of the brown alga Macrocystis pyrifera have been cloned and characterized. The deduced amino acid sequences are very similar to those of fucoxanthin chlorophyll binding proteins (Fcp) from the diatom Phaeodactylum tricornutum and exhibit limited homology to chlorophyll a/b binding (Cab) polypeptides from higher plants. The primary translation products from the M. pyrifera fcp genes are synthesized as higher molecular weight precursors that are processed prior to their assembly into the Fcp complex. The presumed N-terminal 40-amino acid presequence of the Fcp precursor polypeptide has features resembling that of a signal sequence. This presequence may be required for the protein to transverse the endoplasmic reticulum that surrounds the plastid in brown algae. A subsequent targeting step would be required for the protein to cross the double membrane of the plastid envelope. M. pyrifera fcp transcripts are of two sizes, 1.2 and 1.6 kb. The size difference is accounted for by the length of the 3' untranslated region, which can be up to 1000 bases. Transcript abundance's of members of the fcp gene family are dependent on light quantity, light quality, or both. Transcript levels of one gene increased approximately five- to tenfold in thalli grown in low intensity relative to high intensity white or blue light. Transcripts from this gene also significantly increase in red light relative to blue light at equivalent light intensities.

Cyanobacterial protein with similarity to the chlorophyll a/b binding proteins of higher plants: evolution and regulation

We have isolated, from the prokaryotic cyanobacterium Synechococcus sp. strain PCC 7942, a gene encoding a protein of 72 amino acids [designated high light inducible protein (HLIP)] with similarity to the extended family of eukaryotic chlorophyll a/b binding proteins (CABs). HLIP has a single membrane-spanning alpha-helix, whereas both the CABs and the related early light inducible proteins have three membrane-spanning helices. Hence, HLIP may represent an evolutionary progenitor of the eukaryotic members of the CAB extended family. We also show that the gene encoding HLIP is induced by high light and blue/UV-A radiation. The evolution, regulation, and potential function of HLIP are discussed.

Changes in the cyanobacterial photosynthetic apparatus during acclimation to macronutrient deprivation

When the cyanobacterium Synechococcus sp. Strain PCC 7942 is deprived of an essential macronutrient such as nitrogen, sulfur or phosphorus, cellular phycobiliprotein and chlorophyll contents decline. The level of β-carotene declines proportionately to chlorophyll, but the level of zeaxanthin increases relative to chlorophyll. In nitrogen- or sulfur-deprived cells there is a net degradation of phycobiliproteins. Otherwise, the declines in cellular pigmentation are due largely to the diluting effect of continued cell division after new pigment synthesis ceases and not to net pigment degradation. There was also a rapid decrease in O2 evolution when Synechococcus sp. Strain PCC 7942 was deprived of macronutrients. The rate of O2 evolution declined by more than 90% in nitrogen- or sulfur-deprived cells, and by approximately 40% in phosphorus-deprived cells. In addition, in all three cases the fluorescence emissions from Photosystem II and its antennae were reduced relative to that of Photosystem I and the remaining phycobilisomes. Furthermore, state transitions were not observed in cells deprived of sulfur or nitrogen and were greatly reduced in cells deprived of phosphorus. Photoacoustic measurements of the energy storage capacity of photosynthesis also showed that Photosystem II activity declined in nutrient-deprived cells. In contrast, energy storage by Photosystem I was unaffected, suggesting that Photosystem I-driven cyclic electron flow persisted in nutrient-deprived cells. These results indicate that in the modified photosynthetic apparatus of nutrient-deprived cells, a much larger fraction of the photosynthetic activity is driven by Photosystem I than in nutrient-replete cells.