Cadmium exposure and phosphorus limitation increases metal content in the freshwater alga Chlamydomonas reinhardtii

The characteristics of metal accumulation in freshwater microalgae are important to elucidate for a full understanding of metal cycling and toxicity in a freshwater system. This study has utilized an elemental profiling approach to investigate the impacts of Cd exposure and phosphorus (P) availability on metal accumulation after 7 days in batch culture-grown Chlamydomonas reinhardtii. Multivariate statistical analysis of the elemental data demonstrated distinct responses between both stresses. Sublethal concentrations of Cd (up to 15 μM) caused increased accumulation of Co. Cu, Fe, and Zn content also increased in response to enhanced Cd concentrations but only when P availability was low. While Cd exposure effected the accumulation of a few specific metals, P limitation increased the accumulation of all essential trace metals and macronutrients analyzed including Co, Fe, K, Na, and Zn but not Mn. The accumulation of Cd also markedly increased in response to P limitation. The impact of P availability on essential metal accumulation was the same when either inorganic P or an organic P source (glycerophosphate) was used. These results highlight the potential risks of metal toxicity for freshwater microalgae and aquatic food chains when P availability is limiting and which can be exacerbated by Cd pollution.

Photosynthetic energy storage efficiency in Chlamydomonas reinhardtii, based on microsecond photoacoustics

Using a novel, pulsed micro-second time-resolved photoacoustic (PA) instrument, we measured thermal dissipation and energy storage (ES) in the intact cells of wild type (WT) Chlamydomonas reinhardtii, and mutants lacking either PSI or PSII reaction centers (RCs). On this time scale, the kinetic contributions of the thermal expansion component due to heat dissipation of absorbed energy and the negative volume change due to electrostriction induced by charge separation in each of the photosystems could be readily distinguished. Kinetic analysis revealed that PSI and PSII RCs exhibit strikingly different PA signals where PSI is characterized by a strong electrostriction signal and a weak thermal expansion component while PSII has a small electrostriction component and large thermal expansion. The calculated ES efficiencies at ~10 μs were estimated to be 80 ± 5 and 50 ± 13% for PSII-deficient mutants and PSI-deficient mutants, respectively, and 67 ± 2% for WT. The overall ES efficiency was positively correlated with the ratio of PSI to PSI + PSII. Our results suggest that the shallow excitonic trap in PSII limits the efficiency of ES as a result of an evolutionary frozen metabolic framework of two photosystems in all oxygenic photoautotrophs.

Chlamydomonas reinhardtii: duration of its cell cycle and phases at growth rates affected by temperature

Synchronized cultures of the green alga Chlamydomonas reinhardtii were grown photoautotrophically under a wide range of environmental conditions including temperature (15-37 °C), different mean light intensities (132, 150, 264 μmol m⁻² s⁻¹), different illumination regimes (continuous illumination or alternation of light/dark periods of different durations), and culture methods (batch or continuous culture regimes). These variable experimental approaches were chosen in order to assess the role of temperature in the timing of cell division, the length of the cell cycle and its pre- and post-commitment phases. Analysis of the effect of temperature, from 15 to 37 °C, on synchronized cultures showed that the length of the cell cycle varied markedly from times as short as 14 h to as long as 36 h. We have shown that the length of the cell cycle was proportional to growth rate under any given combination of growth conditions. These findings were supported by the determination of the temperature coefficient (Q₁₀), whose values were above the level expected for temperature-compensated processes. The data presented here show that cell cycle duration in C. reinhardtii is a function of growth rate and is not controlled by a temperature independent endogenous timer or oscillator, including a circadian one.

Linoleic acid-induced ultra-weak photon emission from Chlamydomonas reinhardtii as a tool for monitoring of lipid peroxidation in the cell membranes

Reactive oxygen species formed as a response to various abiotic and biotic stresses cause an oxidative damage of cellular component such are lipids, proteins and nucleic acids. Lipid peroxidation is considered as one of the major processes responsible for the oxidative damage of the polyunsaturated fatty acid in the cell membranes. Various methods such as a loss of polyunsaturated fatty acids, amount of the primary and the secondary products are used to monitor the level of lipid peroxidation. To investigate the use of ultra-weak photon emission as a non-invasive tool for monitoring of lipid peroxidation, the involvement of lipid peroxidation in ultra-weak photon emission was studied in the unicellular green alga Chlamydomonas reinhardtii. Lipid peroxidation initiated by addition of exogenous linoleic acid to the cells was monitored by ultra-weak photon emission measured with the employment of highly sensitive charged couple device camera and photomultiplier tube. It was found that the addition of linoleic acid to the cells significantly increased the ultra-weak photon emission that correlates with the accumulation of lipid peroxidation product as measured using thiobarbituric acid assay. Scavenging of hydroxyl radical by mannitol, inhibition of intrinsic lipoxygenase by catechol and removal of molecular oxygen considerably suppressed ultra-weak photon emission measured after the addition of linoleic acid. The photon emission dominated at the red region of the spectrum with emission maximum at 680 nm. These observations reveal that the oxidation of linoleic acid by hydroxyl radical and intrinsic lipoxygenase results in the ultra-weak photon emission. Electronically excited species such as excited triplet carbonyls are the likely candidates for the primary excited species formed during the lipid peroxidation, whereas chlorophylls are the final emitters of photons. We propose here that the ultra-weak photon emission can be used as a non-invasive tool for the detection of lipid peroxidation in the cell membranes.

Control of hydrogen photoproduction by the proton gradient generated by cyclic electron flow in Chlamydomonas reinhardtii

Hydrogen photoproduction by eukaryotic microalgae results from a connection between the photosynthetic electron transport chain and a plastidial hydrogenase. Algal H₂ production is a transitory phenomenon under most natural conditions, often viewed as a safety valve protecting the photosynthetic electron transport chain from overreduction. From the colony screening of an insertion mutant library of the unicellular green alga Chlamydomonas reinhardtii based on the analysis of dark-light chlorophyll fluorescence transients, we isolated a mutant impaired in cyclic electron flow around photosystem I (CEF) due to a defect in the Proton Gradient Regulation Like1 (PGRL1) protein. Under aerobiosis, nonphotochemical quenching of fluorescence (NPQ) is strongly decreased in pgrl1. Under anaerobiosis, H₂ photoproduction is strongly enhanced in the pgrl1 mutant, both during short-term and long-term measurements (in conditions of sulfur deprivation). Based on the light dependence of NPQ and hydrogen production, as well as on the enhanced hydrogen production observed in the wild-type strain in the presence of the uncoupling agent carbonyl cyanide p-trifluoromethoxyphenylhydrazone, we conclude that the proton gradient generated by CEF provokes a strong inhibition of electron supply to the hydrogenase in the wild-type strain, which is released in the pgrl1 mutant. Regulation of the trans-thylakoidal proton gradient by monitoring pgrl1 expression opens new perspectives toward reprogramming the cellular metabolism of microalgae for enhanced H₂ production.

Multiple facets of anoxic metabolism and hydrogen production in the unicellular green alga Chlamydomonas reinhardtii

Many microbes in the soil environment experience micro-oxic or anoxic conditions for much of the late afternoon and night, which inhibit or prevent respiratory metabolism. To sustain the production of energy and maintain vital cellular processes during the night, organisms have developed numerous pathways for fermentative metabolism. This review discusses fermentation pathways identified for the soil-dwelling model alga Chlamydomonas reinhardtii, its ability to produce molecular hydrogen under anoxic conditions through the activity of hydrogenases, and the molecular flexibility associated with fermentative metabolism that has only recently been revealed through the analysis of specific mutant strains.

Alcohol dehydrogenase and hydrogenase transcript fluctuations during a day-night cycle in Chlamydomonas reinhardtii: the role of anoxia

• The unicellular green alga Chlamydomonas reinhardtii contains two iron (Fe)-hydrogenases which are responsible for hydrogen production under anoxia. In the present work the patterns of expression of alcohol dehydrogenase, a typical anaerobic gene in plants, of the hydrogenases genes (HYD1, HYD2) and of the genes responsible for their maturation (HYDEF, HYDG), were analysed. • The expression patterns were analysed by real-time reverse-transcription polymerase chain reaction in Chlamydomonas cultures during the day-night cycle, as well as in response to oxygen availability. • The results indicated that ADH1, HYD1, HYD2, HYDEF and HYDG were expressed following precise day-night fluctuations. ADH1 and HYD2 were modulated by the day-night cycle. Low oxygen plays an important role for the induction of HYD1, HYDEF and HYDG, while ADH1 and HYD2 expression was relatively insensitive to oxygen availability. • The regulation of the anaerobic gene expression in Chlamydomonas is only partly explained by responses to anoxia. The cell cycle and light-dark cycles are equally important elements in the regulatory network modulating the anaerobic response in Chlamydomonas.

Random walk of a swimmer in a low-Reynolds-number medium

Swimming at a micrometer scale demands particular strategies. When inertia is negligible compared to viscous forces, hydrodynamics equations are reversible in time. To achieve propulsion, microswimmers must therefore deform in a way that is not invariant under time reversal. Here, we investigate dispersal properties of the microalga Chlamydomonas reinhardtii by means of microscopy and cell tracking. We show that tracked trajectories are well modeled by a correlated random walk. This process is based on short time correlations in the direction of movement called persistence. At longer times, correlation is lost and a standard random walk characterizes the trajectories. Moreover, high-speed imaging enables us to show how the back-and-forth motion of flagella at very short times affects the statistical description of the dynamics. Finally, we show how drag forces modify the characteristics of this particular random walk.

Chlamydomonas reinhardtii: duration of its cell cycle and phases at growth rates affected by light intensity

In the cultures of the alga Chlamydomonas reinhardtii, division rhythms of any length from 12 to 75 h were found at a range of different growth rates that were set by the intensity of light as the sole source of energy. The responses to light intensity differed in terms of altered duration of the phase from the beginning of the cell cycle to the commitment to divide, and of the phase after commitment to cell division. The duration of the pre-commitment phase was determined by the time required to attain critical cell size and sufficient energy reserves (starch), and thus was inversely proportional to growth rate. If growth was stopped by interposing a period of darkness, the pre-commitment phase was prolonged corresponding to the duration of the dark interval. The duration of the post-commitment phase, during which the processes leading to cell division occurred, was constant and independent of growth rate (light intensity) in the cells of the same division number, or prolonged with increasing division number. It appeared that different regulatory mechanisms operated through these two phases, both of which were inconsistent with gating of cell division at any constant time interval. No evidence was found to support any hypothetical timer, suggested to be triggered at the time of daughter cell release.

A simple method for chloroplast transformation in Chlamydomonas reinhardtii

Photosystem I (PSI) is a multisubunit pigment-protein complex that uses light energy to transfer electrons from plastocyanin to ferredoxin. Application of genetic engineering to photosynthetic reaction center proteins has led to a significant advancement in our understanding of primary electron transfer events and the role of the protein environment in modulating these processes. Chlamydomonas reinhardtii provides a system particularly amenable to analyze the structure-function relationship of Photosystem I. C. reinhardtii is also a particularly favorable organism for chloroplast transformation because it contains only a single chloroplast and grows heterotrophically when supplemented with acetate. Chlamydomonas has, therefore, served as a model organism for the development of chloroplast transformation procedures and the study of photosynthetic mutants generated using this method. Exogenous cloned cpDNA can be introduced into the chloroplast by using this biolistic gene gun method. DNA-coated tungsten or gold particles are bombarded onto cells. Upon its entry into chloroplasts, the transforming DNA is released from the particles and integrated into the chloroplast genome through homologous recombination. The most versatile chloroplast selectable marker is aminoglycoside adenyl transferase (aadA), which can be expressed in the chloroplast to confer resistance to spectinomycin or streptomycin. This article describes the procedures for chloroplast transformation.

Proteomic analysis of thylakoid membranes

Chlamydomonas is a model organism to study photosynthesis. Thylakoid membranes comprise several proteins belonging to photosystems I and II. In this chapter, we show the accurate proteomic measurements in thylakoid membranes. The chlorophyll-containing membrane protein complexes were precipitated using chloroform/methanol solution. These complexes were separated using two-dimensional gel electrophoresis, and the resolved spots were exercised from the gel matrix and digested with trypsin. These peptide fragments were separated by MALDI-TOF, and the isotopic masses were blasted to a MASCOT server to obtain the protein sequence. Matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF). The method discussed here would be a useful method for the separation and identification of thylakoid membrane proteins.

Microbial population dynamics by digital in-line holographic microscopy

Measurements of population dynamics are ubiquitous in experiments with microorganisms. Studies with microbes elucidating adaptation, selection, and competition rely on measurements of changing populations in time. Despite this importance, quantitative methods for measuring population dynamics microscopically, with high time resolution, across many replicates remain limited. Here we present a new noninvasive method to precisely measure microbial spatiotemporal population dynamics based on digital in-line holographic (DIH) microscopy. Our inexpensive, replicate DIH microscopes imaged hundreds of swimming algae in three dimensions within a volume of several microliters on a time scale of minutes over periods of weeks.

[Study on the removal of nitrogen and phosphorus from wastewater by Chlamydomonas reinhardtii]

Chlamydomonas reinhardtii was employed to remove nitrogen and phosphorus from wastewater. The effects of initial NH4(+) -N and TP concentrations, N/P ratios, Light/Darkness ratios, pH and immobilization on the removal of NH4(+) -N and TP were evaluated. The results showed that C. reinhardtii could almost 100% remove NH4(+) -N and TP as initial concentrations of NH4(+) -N and TP were no more than 55 mg x L(-1) and 7 mg x L(-1), respectively, whereas the removal ratio of NH4(+) -N decreased drastically to 50% with initial NH4(+) -N concentration coming up to 75 mg x L(-1). Under N/P ratios of 5:1 and 10:1, C. reinhardtii could completely remove NH4(+) -N within 3d, while 6 d was needed with N/P ratio being 25: 1. Different from NH4(+) -N removal, the removal ratio of TP could reach almost 100% within 4 d under 3 N/P ratios. With L/D ratios of 24 h: 0 h and 12 h: 12 h, 100% removal of NH4(+) -N and TP could be achieved by C. reinhardtii, but the removal rate under L/D ratio of 24 h: 0 h was relatively faster. The optimal pH range for C. reinhardtii to remove NH4(+) -N and TP was 6-7. After immobilization, the ability of C. reinhardtii to remove NH4(+) -N was significantly enhanced as the removal ratio of NH4(+) -N came up to 100% with initial NH4(+) -N concentration being 75 mg x L(-1). The ability of immobilized C. reinhardtii to remove TP kept stable, whereas the removal rate was slowed slightly.

Using FTIR spectroscopy for rapid determination of lipid accumulation in response to nitrogen limitation in freshwater microalgae

In this study Fourier transform infrared micro-spectroscopy (FTIR) was used to determine lipid and carbohydrate content over time in the freshwater microalgae Chlamydomonas reinhardtii and Scenedesmus subspicatus grown in batch culture in limiting concentrations of nitrogen (N). Both algae exhibited restricted cell division and increased cell size following N-limitation. FTIR spectra of cells in N-limited media showed increasing lipid:amide I and carbohydrate:amide I ratios over time. The use of lipid- and starch-staining dyes confirmed that the observed ratio changes were due to increased lipid and carbohydrate synthesis. These results demonstrate rapid metabolic responses of C. reinhardtii and S. subspicatus to changing nutrient availability, and indicate the efficiency of FTIR as a reliable method for high-throughput determination of lipid induction.

Direct extraction of photosynthetic electrons from single algal cells by nanoprobing system

There are numerous sources of bioenergy that are generated by photosynthetic processes, for example, lipids, alcohols, hydrogen, and polysaccharides. However, generally only a small fraction of solar energy absorbed by photosynthetic organisms is converted to a form of energy that can be readily exploited. To more efficiently use the solar energy harvested by photosynthetic organisms, we evaluated the feasibility of generating bioelectricity by directly extracting electrons from the photosynthetic electron transport chain before they are used to fix CO(2) into sugars and polysaccharides. From a living algal cell, Chlamydomonas reinhardtii, photosynthetic electrons (1.2 pA at 6000 mA/m(2)) were directly extracted without a mediator electron carrier by inserting a nanoelectrode into the algal chloroplast and applying an overvoltage. This result may represent an initial step in generating "high efficiency" bioelectricity by directly harvesting high energy photosynthetic electrons.

Recharacterization of Chlamydomonas reinhardtii and its relatives with new isolates from Japan

Chlamydomonas reinhardtii P. A. Dang. (Volvocales, Chlorophyceae) is one of the most intensely studied algae, and its whole genome was sequenced. Although this species was originally described based on materials from France and is often referred to as a cosmopolitan species, all culture strains available today have been isolated from eastern North America. The distinctions with similar and/or closely related species, such as Chlamydomonas globosa J. Snow and Chlamydomonas orbicularis E. G. Pringsh., are also contentious. In this study, new strains of C. reinhardtii and C. globosa were isolated from Japan and compared with several strains similar to C. reinhardtii. Based on the morphological, genealogical, phylogenetical, and mating studies including the new Japanese strains, the circumscription of C. reinhardtii was clarified. C. reinhardtii was most closely related to C. globosa, and they were shown to be different species. Although C. reinhardtii was similar to C. orbicularis, the authentic strain of C. orbicularis was morphologically distinguishable and phylogenetically distant from C. reinhardtii. Discovery of the Japanese strains of C. reinhardtii supports the cosmopolitan distribution of this species. Based on Japanese strains and/or strains from other countries, emended descriptions of C. reinhardtii, C. globosa, and C. orbicularis are given.

Total internal reflection fluorescence (TIRF) microscopy of Chlamydomonas flagella

The eukaryotic flagellum is host to a variety of dynamic behaviors, including flagellar beating, the motility of glycoproteins in the flagellar membrane, and intraflagellar transport (IFT), the bidirectional traffic of protein particles between the flagellar base and tip. IFT is of particular interest, as it plays integral roles in flagellar length control, cell signaling, development, and human disease. However, our ability to understand dynamic flagellar processes such as IFT is limited in large part by the fidelity with which we can image these behaviors in living cells. This chapter introduces the application of total internal reflection fluorescence (TIRF) microscopy to visualize the flagella of Chlamydomonas reinhardtii. The advantages and challenges of TIRF are discussed in comparison to confocal and differential interference contrast techniques. This chapter also reviews current IFT insights gleaned from TIRF microscopy of Chlamydomonas and provides an outlook on the future of the technique, with particular emphasis on combining TIRF with other emerging imaging technologies.

Purification of IFT particle proteins and preparation of recombinant proteins for structural and functional analysis

Intraflagellar transport (IFT) is characterized by a robust bidirectional movement of large proteinaceous particles along the length of eukaryotic cilia and flagella. Essential for the assembly and function of the organelle, IFT is believed to transport a large array of ciliary components in and out of the organelle. Biochemical analysis of the proteins involved with this transport has been largely dependent on the ability to isolate suitable quantities of intact cilia or flagella. One model organism, Chlamydomonas reinhardtii, has proven to be especially well-suited for such endeavors. Indeed, many of the IFT particle proteins were initially identified through biochemical analysis of green algae. This chapter describes some of the most effective methods for the purification of IFT particle proteins from Chlamydomonas flagella. This chapter also describes complementary approaches where recombinant IFT proteins are generated with affinity tags that allow rapid and specific purification. The recombinant proteins can be used to analyze protein-protein interactions and can be directly delivered to mutant cells to analyze functional domains. Although the techniques described here are focused entirely on Chlamydomonas IFT proteins, the approaches, especially regarding recombinant proteins, should be applicable to the study of IFT machinery in other model organisms.

Analysis of the ciliary/flagellar beating of Chlamydomonas

Eukaryotic flagella and cilia are alternative names, for the slender cylindrical protrusions of a cell (240nm diameter, approximately 12,800nm-long in Chlamydomonas reinhardtii) that propel a cell or move fluid. Cilia are extraordinarily successful complex organelles abundantly found in animals performing many tasks. They play a direct or developmental role in the sensors of fluid flow, light, sound, gravity, smells, touch, temperature, and taste in mammals. The failure of cilia can lead to hydrocephalus, infertility, and blindness. However, in spite of their large role in human function and pathology, there is as yet no consensus on how cilia beat and perform their many functions, such as moving fluids in brain ventricles and lungs and propelling and steering sperm, larvae, and many microorganisms. One needs to understand and analyze ciliary beating and its hydrodynamic interactions. This chapter provides a guide for measuring, analyzing, and interpreting ciliary behavior in various contexts studied in the model system of Chlamydomonas. It describes: (1) how cilia work as self-organized beating structures (SOBSs), (2) the overlaid control in the cilia that optimizes the SOBS to achieve cell dispersal, phototaxis steering, and avoidance of obstacles, (3) the assay of a model intracellular signal processing system that responds to multiple external and internal inputs, choosing mode of behavior and then controlling the cilia, (4) how cilia sense their environment, and (5) potentially an assay of ciliary performance for toxicology or medical assessment.

Dynamics of enhanced tracer diffusion in suspensions of swimming eukaryotic microorganisms

In contexts such as suspension feeding in marine ecologies there is an interplay between brownian motion of nonmotile particles and their advection by flows from swimming microorganisms. As a laboratory realization, we study passive tracers in suspensions of eukaryotic swimmers, the alga Chlamydomonas reinhardtii. While the cells behave ballistically over short intervals, the tracers behave diffusively, with a time-dependent but self-similar probability distribution function of displacements consisting of a gaussian core and robust exponential tails. We emphasize the role of flagellar beating in creating oscillatory flows that exceed brownian motion far from each swimmer.

A novel in situ tool for the exposure and analysis of microorganisms in natural aquatic systems

To evaluate the effects of contaminants or nutrient limitation in natural waters, it is often desirable to perform controlled exposures of organisms. While in situ exposures are routine for caged organisms or macrophytes, they are extremely difficult to perform for microorganisms, mainly due to difficulties in designing an exposure device that isolates the cells while allowing rapid equilibration with the external media. In this paper, a stirred underwater biouptake system (SUBS) based on the diffusion of chemicals across a semipermeable membrane housing a controlled population of microorganisms is reported. Cd diffusion through the semipermeable membrane was evaluated by voltammetry using a microelectrode. Comparison of stirred and unstirred solutions demonstrated a significantly increased diffusive flux in the presence of stirring. Lab tests using Chlamydomonas reinhardtii showed that diffusion across the semipermeable membrane was not limiting with respect to the biouptake of Cd. The SUBS device was field tested and the results of viability studies and trace metal biouptake by C. reinhardtii are reported. No diffusion limitation due to the SUBS was observed for Cd under the tested field conditions. The SUBS device was also shown to be useful for field exposures and subsequent measurements of trace metal uptake and viability. The results support the future use of the SUBS for the in situ measurement of phytochelatin/metallothionein production, photosynthetic efficiency, or reporter gene induction of controlled organisms.

Monodisperse semi-permeable microcapsules for continuous observation of cells

We present a method for forming monodisperse semi-permeable microcapsules composed of an alginate-poly-L-lysine (PLL) membrane for the observation of encapsulated cells. These microcapsules were prepared with a monolithic three-dimensional microfluidic axisymmetric flow-focusing device by an internal gelation method using glucono-1,5-lactone in order to provide mild conditions for the cells. The microcapsules were sufficiently monodisperse and robust to be trapped in a bead-based microfluidic array system for easy observation. We also confirmed that (i) the alginate-PLL membrane is semi-permeable so that cells and microorganisms cannot pass through it but nutrients and wastes can, (ii) cells are able to move freely inside the semi-permeable microcapsules, and (iii) cells can be successfully proliferated in the microcapsules.

An outer arm dynein light chain acts in a conformational switch for flagellar motility

A system distinct from the central pair-radial spoke complex was proposed to control outer arm dynein function in response to alterations in the mechanical state of the flagellum. In this study, we examine the role of a Chlamydomonas reinhardtii outer arm dynein light chain that associates with the motor domain of the gamma heavy chain (HC). We demonstrate that expression of mutant forms of LC1 yield dominant-negative effects on swimming velocity, as the flagella continually beat out of phase and stall near or at the power/recovery stroke switchpoint. Furthermore, we observed that LC1 interacts directly with tubulin in a nucleotide-independent manner and tethers this motor unit to the A-tubule of the outer doublet microtubules within the axoneme. Therefore, this dynein HC is attached to the same microtubule by two sites: via both the N-terminal region and the motor domain. We propose that this gamma HC-LC1-microtubule ternary complex functions as a conformational switch to control outer arm activity.

Two Chlamydomonas CTR copper transporters with a novel cys-met motif are localized to the plasma membrane and function in copper assimilation

Inducible high-affinity copper uptake is key to copper homeostasis in Chlamydomonas reinhardtii. We generated cDNAs and updated gene models for four genes, CTR1, CTR2, CTR3, and COPT1, encoding CTR-type copper transporters in Chlamydomonas. The expression of CTR1, CTR2, and CTR3 increases in copper deficient cells and in response to hypoxia or Ni(2+) supplementation; this response depends on the transcriptional activator CRR1. A copper response element was identified by mutational analysis of the 5' upstream region of CTR1. Functional analyses identify CTR1 and CTR2 as the assimilatory transporters of Chlamydomonas based on localization to the plasma membrane and ability to rescue a Saccharomyces cerevisiae mutant defective in high-affinity copper transport. The Chlamydomonas CTRs contain a novel Cys-Met motif (CxxMxxMxxC-x(5/6)-C), which occurs also in homologous proteins in other green algae, amoebae, and pathogenic fungi. CTR3 appears to have arisen by duplication of CTR2, but CTR3 lacks the characteristic transmembrane domains found in the transporters, suggesting that it may be a soluble protein. Thus, Chlamydomonas CTR genes encode a distinct subset of the classical CTR family of Cu(I) transporters and represent new targets of CRR1-dependent signaling.

The Chlamydomonas hatching enzyme, sporangin, is expressed in specific phases of the cell cycle and is localized to the flagella of daughter cells within the sporangial cell wall

The timely breakdown of the extracellular matrix by proteolytic enzymes is essential for development, morphogenesis and cell proliferation in plant and animal cells. Sporangin of the unicellular green alga Chlamydomonas reinhardtii that mediates breakdown of the sporangial cell wall to liberate the daughter cells after cell division is characterized as a subtilase-like serine protease. The sporangin gene is specifically transcribed during S/M phase in a synchronized vegetative cell cycle. In immunoblot analyses using a polyclonal antibody raised against the sporangin polypeptide, the enzyme is synthesized after mitotic cell division and accumulated in the daughter cells before hatching. Immunofluorescence analyses showed that sporangin is localized to the flagella of the daughter cells within the sporangial cell wall, and released into the culture medium. The data suggest that sporangin is released from flagella concurrently with the digestion of sporangial cell wall, and then the daughter cells are hatched from the sporangia in the Chlamydomonas vegetative cell cycle.