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1.  Independent Localization of Plasma Membrane and Chloroplast Components during Eyespot Assembly 
Eukaryotic Cell  2013;12(9):1258-1270.
Like many algae, Chlamydomonas reinhardtii is phototactic, using two anterior flagella to swim toward light optimal for photosynthesis. The flagella are responsive to signals initiated at the photosensory eyespot, which comprises photoreceptors in the plasma membrane and layers of pigment granules in the chloroplast. Phototaxis depends on placement of the eyespot at a specific asymmetric location relative to the flagella, basal bodies, and bundles of two or four highly acetylated microtubules, termed rootlets, which extend from the basal bodies toward the posterior of the cell. Previous work has shown that the eyespot is disassembled prior to cell division, and new eyespots are assembled in daughter cells adjacent to the nascent four-membered rootlet associated with the daughter basal body (D4), but the chronology of these assembly events has not been determined. Here we use immunofluorescence microscopy to follow assembly and acetylation of the D4 rootlet, localization of individual eyespot components in the plasma membrane or chloroplast envelope, and flagellar emergence during and immediately following cell division. We find that the D4 rootlet is assembled before the initiation of eyespot assembly, which occurs within the same time frame as rootlet acetylation and flagellar outgrowth. Photoreceptors in the plasma membrane are correctly localized in eyespot mutant cells lacking pigment granule layers, and chloroplast components of the eyespot assemble in mutant cells in which photoreceptor localization is retarded. The data suggest that plasma membrane and chloroplast components of the eyespot are independently responsive to a cytoskeletal positioning cue.
PMCID: PMC3811559  PMID: 23873865
2.  Defects in Mitochondrial Fatty Acid Synthesis Result in Failure of Multiple Aspects of Mitochondrial Biogenesis in Saccharomyces cerevisiae 
Molecular microbiology  2013;90(4):824-840.
Mitochondrial fatty acid synthesis (mtFAS) shares acetyl-CoA with the Krebs cycle as a common substrate and is required for the production of octanoic acid (C8) precursors of lipoic acid (LA) in mitochondria. MtFAS is a conserved pathway essential for respiration. In a genetic screen in Saccharomyces cerevisiae designed to further elucidate the physiological role of mtFAS, we isolated mutants with defects in mitochondrial post-translational gene expression processes, indicating a novel link to mitochondrial gene expression and respiratory chain biogenesis. In our ensuing analysis, we show that mtFAS, but not lipoylation per se, is required for respiratory competence. We demonstrate that mtFAS is required for mRNA splicing, mitochondrial translation and respiratory complex assembly, and provide evidence that not LA per se, but fatty acids longer than C8 play a role in these processes. We also show that mtFAS- and LA-deficient strains suffer from a mild heme deficiency that may contribute to the respiratory complex assembly defect. Based on our data and previously published information, we propose a model implicating mtFAS as a sensor for mitochondrial acetyl-CoA availability and a coordinator of nuclear and mitochondrial gene expression by adapting the mitochondrial compartment to changes in the metabolic status of the cell.
PMCID: PMC4153884  PMID: 24102902
mitochondrial-nuclear coordination; mitochondrial gene expression; heme; lipoic acid; mitochondrial fatty acid synthesis
3.  The Daughter Four-Membered Microtubule Rootlet Determines Anterior-Posterior Positioning of the Eyespot in Chlamydomonas reinhardtii 
Cytoskeleton (Hoboken, N.J.)  2011;68(8):459-469.
The characteristic geometry of the unicellular chlorophyte Chlamydomonas reinhardtii has contributed to its adoption as a model system for cellular asymmetry and organelle positioning. The eyespot, a photosensitive organelle, is localized asymmetrically in the cell at a precisely-defined position relative to the flagella and cytoskeletal microtubule rootlets. We have isolated a mutant, named pey1 for posterior eyespot, with variable microtubule rootlet lengths. The length of the acetylated daughter four-membered microtubule rootlet correlates with the position of the eyespot, which appears in a posterior position in the majority of cells. The correlation of rootlet length with eyespot positioning was also observed in the cmu1 mutant, which has longer acetylated microtubules, and the mlt1 mutant, in which the rootlet microtubules are shorter. Observation of eyespot positioning after depolymerization of rootlet microtubules indicated that eyespot position is fixed early in eyespot development and becomes independent of the rootlet. Our data demonstrate that the length of the daughter four-membered rootlet is the major determinant of eyespot positioning on the anterior-posterior axis and are suggestive that the gene product of the PEY1 locus is a novel regulator of acetylated microtubule length.
PMCID: PMC3201734  PMID: 21766471
Chlamydomonas; eyespot; microtubule rootlet; organelle positioning; pey1
4.  New insights into eyespot placement and assembly in Chlamydomonas 
Bioarchitecture  2011;1(4):196-199.
Aspects of cellular architecture, such as cytoskeletal asymmetry cues, play critical roles in directing the placement of organelles and establishing the sites of their formation. In the model green alga Chlamydomonas, the photosensory eyespot occupies a defined position in relation to the flagella and microtubule cytoskeleton. Investigations into the cellular mechanisms of eyespot placement and assembly have aided our understanding of the interplay between cytoskeletal and plastid components of the cell. The eyespot, which must be assembled anew after each cell division, is a multi-layered organelle consisting of stacks of carotenoid-filled pigment granules in the chloroplast and rhodopsin photoreceptors in the plasma membrane. Placement of the eyespot is determined on both the latitudinal and longitudinal axes of the cell by the daughter four-membered (D4) microtubule rootlet. Recent findings have contributed to the hypothesis that the eyespot photoreceptor molecules are directed from the Golgi to the daughter hemisphere of the cell and trafficked along the D4 microtubule rootlet. EYE2, a chloroplast-envelope protein, forms an elliptical patch together with the photoreceptors and establishes the site for assembly of the pigment granule arrays in the chloroplast, connecting the positioning information of the cytoskeleton to assembly of the pigment granule arrays in the chloroplast.
PMCID: PMC3210518  PMID: 22069514
Chlamydomonas; eyespot; organelle placement; organelle assembly; microtubule rootlet; asymmetry; photoreception
5.  Asymmetric properties of the Chlamydomonas reinhardtii cytoskeleton direct rhodopsin photoreceptor localization 
The Journal of Cell Biology  2011;193(4):741-753.
Daughter four-membered rootlet microtubules direct eyespot positioning and assembly.
The eyespot of the unicellular green alga Chlamydomonas reinhardtii is a photoreceptive organelle required for phototaxis. Relative to the anterior flagella, the eyespot is asymmetrically positioned adjacent to the daughter four-membered rootlet (D4), a unique bundle of acetylated microtubules extending from the daughter basal body toward the posterior of the cell. Here, we detail the relationship between the rhodopsin eyespot photoreceptor Channelrhodopsin 1 (ChR1) and acetylated microtubules. In wild-type cells, ChR1 was observed in an equatorial patch adjacent to D4 near the end of the acetylated microtubules and along the D4 rootlet. In cells with cytoskeletal protein mutations, supernumerary ChR1 patches remained adjacent to acetylated microtubules. In mlt1 (multieyed) mutant cells, supernumerary photoreceptor patches were not restricted to the D4 rootlet, and more anterior eyespots correlated with shorter acetylated microtubule rootlets. The data suggest a model in which photoreceptor localization is dependent on microtubule-based trafficking selective for the D4 rootlet, which is perturbed in mlt1 mutant cells.
PMCID: PMC3166873  PMID: 21555459
6.  Thioredoxin-family protein EYE2 and Ser/Thr kinase EYE3 play interdependent roles in eyespot assembly 
Molecular Biology of the Cell  2011;22(9):1421-1429.
EYE2 is a key protein in connecting the positioning information of the microtubule rootlet cytoskeleton and channelrhodopsin 1 (ChR1) photoreceptor to the formation and positioning of the eyespot pigment granules in the chloroplast of Chlamydomonas. EYE3, a ser/thr kinase of the ABC1 family, is found in pigment granules and is required for their biogenesis.
The eyespot of the biflagellate unicellular green alga Chlamydomonas reinhardtii is a complex organelle that facilitates directional responses of the cell to environmental light stimuli. The eyespot, which assembles de novo after every cell division and is associated with the daughter four-membered (D4) microtubule rootlet, comprises an elliptical patch of rhodopsin photoreceptors on the plasma membrane and stacks of carotenoid-rich pigment granule arrays in the chloroplast. Two loci, EYE2 and EYE3, define factors involved in the formation and organization of the eyespot pigment granule arrays. Whereas EYE3, a serine/threonine kinase of the ABC1 family, localizes to pigment granules, EYE2 localization corresponds to an area of the chloroplast envelope in the eyespot. EYE2 is positioned along, and adjacent to, the D4 rootlet in the absence of pigment granules. The eyespot pigment granule array is required for maintenance of the elliptical shape of both the overlying EYE2 and channelrhodopsin-1 photoreceptor patches. We propose a model of eyespot assembly wherein rootlet and photoreceptor direct EYE2 to an area of the chloroplast envelope, where it acts to facilitate assembly of pigment granule arrays, and EYE3 plays a role in the biogenesis of the pigment granules.
PMCID: PMC3084665  PMID: 21372178
7.  Miniature- and Multiple-Eyespot Loci in Chlamydomonas reinhardtii Define New Modulators of Eyespot Photoreception and Assembly 
G3: Genes|Genomes|Genetics  2011;1(6):489-498.
The photosensory eyespot of the green alga Chlamydomonas reinhardtii is a model system for the study of organelle biogenesis and placement. Eyespot assembly and positioning are governed by several genetic loci that have been identified in forward genetic screens for phototaxis-defective mutants. These include the previously described miniature-eyespot mutant min1, the multiple-eyespot mutant mlt1, the eyeless mutants eye2 and eye3, and two previously uncharacterized eyespot mutants, min2 and mlt2. In this study, effects of miniature- and multiple-eyespot mutations and their combinations on the localization and expression levels of the rhodopsin photoreceptor channelrhodopsin-1 (ChR1) and the localization of the eyespot-assembly proteins EYE2 and EYE3 were examined. min2 mutants assemble a properly organized, albeit nonfunctional, eyespot that is slightly smaller than wild-type; however, combination of the min2 and mlt1 mutations resulted in drastic reduction of photoreceptor levels. Both stationary-phase mlt1 and mlt2 cells have supernumerary, mislocalized eyespots that exhibit partial or total dissociation of the eyespot layers. In these mutant strains, photoreceptor patches in the plasma membrane were never associated with pigment granule arrays in the chloroplast stroma unless EYE2 was present in the intervening envelope. The data suggest that MIN2 is required for the photoreceptive ability of the eyespot and that MLT2 plays a major role in regulating eyespot number, placement, and integrity.
PMCID: PMC3276157  PMID: 22384359
eyespot; photoreception; organelle biogenesis; MIN2; MLT2
8.  The Chlamydomonas Genome Reveals the Evolution of Key Animal and Plant Functions 
Merchant, Sabeeha S. | Prochnik, Simon E. | Vallon, Olivier | Harris, Elizabeth H. | Karpowicz, Steven J. | Witman, George B. | Terry, Astrid | Salamov, Asaf | Fritz-Laylin, Lillian K. | Maréchal-Drouard, Laurence | Marshall, Wallace F. | Qu, Liang-Hu | Nelson, David R. | Sanderfoot, Anton A. | Spalding, Martin H. | Kapitonov, Vladimir V. | Ren, Qinghu | Ferris, Patrick | Lindquist, Erika | Shapiro, Harris | Lucas, Susan M. | Grimwood, Jane | Schmutz, Jeremy | Cardol, Pierre | Cerutti, Heriberto | Chanfreau, Guillaume | Chen, Chun-Long | Cognat, Valérie | Croft, Martin T. | Dent, Rachel | Dutcher, Susan | Fernández, Emilio | Ferris, Patrick | Fukuzawa, Hideya | González-Ballester, David | González-Halphen, Diego | Hallmann, Armin | Hanikenne, Marc | Hippler, Michael | Inwood, William | Jabbari, Kamel | Kalanon, Ming | Kuras, Richard | Lefebvre, Paul A. | Lemaire, Stéphane D. | Lobanov, Alexey V. | Lohr, Martin | Manuell, Andrea | Meier, Iris | Mets, Laurens | Mittag, Maria | Mittelmeier, Telsa | Moroney, James V. | Moseley, Jeffrey | Napoli, Carolyn | Nedelcu, Aurora M. | Niyogi, Krishna | Novoselov, Sergey V. | Paulsen, Ian T. | Pazour, Greg | Purton, Saul | Ral, Jean-Philippe | Riaño-Pachón, Diego Mauricio | Riekhof, Wayne | Rymarquis, Linda | Schroda, Michael | Stern, David | Umen, James | Willows, Robert | Wilson, Nedra | Zimmer, Sara Lana | Allmer, Jens | Balk, Janneke | Bisova, Katerina | Chen, Chong-Jian | Elias, Marek | Gendler, Karla | Hauser, Charles | Lamb, Mary Rose | Ledford, Heidi | Long, Joanne C. | Minagawa, Jun | Page, M. Dudley | Pan, Junmin | Pootakham, Wirulda | Roje, Sanja | Rose, Annkatrin | Stahlberg, Eric | Terauchi, Aimee M. | Yang, Pinfen | Ball, Steven | Bowler, Chris | Dieckmann, Carol L. | Gladyshev, Vadim N. | Green, Pamela | Jorgensen, Richard | Mayfield, Stephen | Mueller-Roeber, Bernd | Rajamani, Sathish | Sayre, Richard T. | Brokstein, Peter | Dubchak, Inna | Goodstein, David | Hornick, Leila | Huang, Y. Wayne | Jhaveri, Jinal | Luo, Yigong | Martínez, Diego | Ngau, Wing Chi Abby | Otillar, Bobby | Poliakov, Alexander | Porter, Aaron | Szajkowski, Lukasz | Werner, Gregory | Zhou, Kemin | Grigoriev, Igor V. | Rokhsar, Daniel S. | Grossman, Arthur R.
Science (New York, N.Y.)  2007;318(5848):245-250.
Chlamydomonas reinhardtii is a unicellular green alga whose lineage diverged from land plants over 1 billion years ago. It is a model system for studying chloroplast-based photosynthesis, as well as the structure, assembly, and function of eukaryotic flagella (cilia), which were inherited from the common ancestor of plants and animals, but lost in land plants. We sequenced the ∼120-megabase nuclear genome of Chlamydomonas and performed comparative phylogenomic analyses, identifying genes encoding uncharacterized proteins that are likely associated with the function and biogenesis of chloroplasts or eukaryotic flagella. Analyses of the Chlamydomonas genome advance our understanding of the ancestral eukaryotic cell, reveal previously unknown genes associated with photosynthetic and flagellar functions, and establish links between ciliopathy and the composition and function of flagella.
PMCID: PMC2875087  PMID: 17932292
9.  C2 Domain Protein MIN1 Promotes Eyespot Organization in Chlamydomonas reinhardtii▿ †  
Eukaryotic Cell  2008;7(12):2100-2112.
Assembly and asymmetric localization of the photosensory eyespot in the biflagellate, unicellular green alga Chlamydomonas reinhardtii requires coordinated organization of photoreceptors in the plasma membrane and pigment granule/thylakoid membrane layers in the chloroplast. min1 (mini-eyed) mutant cells contain abnormally small, disorganized eyespots in which the chloroplast envelope and plasma membrane are no longer apposed. The MIN1 gene, identified here by phenotypic rescue, encodes a protein with an N-terminal C2 domain and a C-terminal LysM domain separated by a transmembrane sequence. This novel domain architecture led to the hypothesis that MIN1 is in the plasma membrane or the chloroplast envelope, where membrane association of the C2 domain promotes proper eyespot organization. Mutation of conserved C2 domain loop residues disrupted association of the MIN1 C2 domain with the chloroplast envelope in moss cells but did not abolish eyespot assembly in Chlamydomonas. In min1 null cells, channelrhodopsin-1 (ChR1) photoreceptor levels were reduced, indicating a role for MIN1 in ChR1 expression and/or stability. However, ChR1 localization was only minimally disturbed during photoautotrophic growth of min1 cells, conditions under which the pigment granule layers are disorganized. The data are consistent with the hypothesis that neither MIN1 nor proper organization of the plastidic components of the eyespot is essential for localization of ChR1.
PMCID: PMC2593190  PMID: 18849467
10.  Intersection of RNA Processing and the Type II Fatty Acid Synthesis Pathway in Yeast Mitochondria▿  
Molecular and Cellular Biology  2008;28(21):6646-6657.
Distinct metabolic pathways can intersect in ways that allow hierarchical or reciprocal regulation. In a screen of respiration-deficient Saccharomyces cerevisiae gene deletion strains for defects in mitochondrial RNA processing, we found that lack of any enzyme in the mitochondrial fatty acid type II biosynthetic pathway (FAS II) led to inefficient 5′ processing of mitochondrial precursor tRNAs by RNase P. In particular, the precursor containing both RNase P RNA (RPM1) and tRNAPro accumulated dramatically. Subsequent Pet127-driven 5′ processing of RPM1 was blocked. The FAS II pathway defects resulted in the loss of lipoic acid attachment to subunits of three key mitochondrial enzymes, which suggests that the octanoic acid produced by the pathway is the sole precursor for lipoic acid synthesis and attachment. The protein component of yeast mitochondrial RNase P, Rpm2, is not modified by lipoic acid in the wild-type strain, and it is imported in FAS II mutant strains. Thus, a product of the FAS II pathway is required for RNase P RNA maturation, which positively affects RNase P activity. In addition, a product is required for lipoic acid production, which is needed for the activity of pyruvate dehydrogenase, which feeds acetyl-coenzyme A into the FAS II pathway. These two positive feedback cycles may provide switch-like control of mitochondrial gene expression in response to the metabolic state of the cell.
PMCID: PMC2573234  PMID: 18779316
11.  Analysis of transcription asymmetries along the tRNAE-COB operon: evidence for transcription attenuation and rapid RNA degradation between coding sequences 
Nucleic Acids Research  2004;32(21):6276-6283.
Mitochondrial gene expression in yeast is believed to be regulated predominantly at the post-transcriptional level. However, the contribution of mitochondrial transcription and RNA-turnover rates to differential gene regulation is still largely unknown. Mitochondrial run-on transcription and hybrid selection assays showed that some of the multigenic transcription units of the mitochondrial genome are transcribed evenly, whereas others are transcribed asymmetrically, with higher transcription rates for promoter-proximal genes, than for promoter-distal genes. The tRNAE-cytochrome b (COB) operon was analyzed in detail to investigate the mechanisms underlying transcription rate asymmetries in yeast mitochondria. We showed that a drop in transcription rates occurs in a particular region between the coding sequences and is independent of the coding sequence of the downstream COB gene. Deletion of the region between tRNAE and COB coding sequences decreases the drop in transcription rates. Deletion of the nuclear gene encoding the Pet 127 protein, which is involved in mitochondrial RNA 5′ processing and degradation, also partially relieves transcriptional asymmetry. Therefore, asymmetry is probably due to a combination of attenuated transcription at specific sites between the coding sequences and very rapid RNA degradation.
PMCID: PMC535675  PMID: 15576354
12.  The Mitochondrial Message-specific mRNA Protectors Cbp1 and Pet309 Are Associated in a High-Molecular Weight Complex 
Molecular Biology of the Cell  2004;15(6):2674-2683.
In Saccharomyces cerevisiae, the nuclear-encoded protein Cbp1 promotes stability and translation of mitochondrial cytochrome b transcripts through interaction with the 5′ untranslated region. Fusion of a biotin binding peptide tag to the C terminus of Cbp1 has now allowed detection in mitochondrial extracts by using peroxidase-coupled avidin. Cbp1 is associated with the mitochondrial membranes when high ionic strength extraction conditions are used. However, the protein is easily solubilized by omitting salt from the extraction buffer, which suggests Cbp1 is loosely associated with the membrane through weak hydrophobic interactions. Gel filtration analysis and blue native PAGE showed that Cbp1 is part of a single 900,000-Da complex. The complex was purified using the biotin tag and a sequence-specific protease cleavage site. In addition to Cbp1, the complex contains several polypeptides of molecular weights between 113 and 40 kDa. Among these, we identified another message-specific factor, Pet309, which promotes the stability and translation of mitochondrial cytochrome oxidase subunit I mRNA. A hypothesis is presented in which the Cbp1–Pet309 complex contains several message-specific RNA binding proteins and links transcription to translation of the mRNAs at the membrane.
PMCID: PMC420092  PMID: 15047869

Results 1-12 (12)