Inhibitor effects during the cell cycle in Chlamydomonas reinhardtii. Determination of transition points in asynchronous cultures

A role for the carbon source of the cell and protein kinase A in regulating the S. pombe septation initiation network

The septation initiation network (SIN) is a conserved signal transduction network, which is important for cytokinesis in Schizosaccharomyces pombe. The SIN component Etd1p is required for association of some SIN proteins with the spindle pole body (SPB) during anaphase and for contractile ring formation. We show that tethering of Cdc7p or Sid1p to the SIN scaffold Cdc11p at the SPB, rescues etd1-Δ. Analysis of a suppressor of the mutant etd1-M9 revealed that SIN signalling is influenced by the carbon source of the cell. Growth on a non-fermentable carbon source glycerol reduces the requirement for SIN signalling but does not bypass it. The decreased need for SIN signalling is mediated largely by reduction of protein kinase A activity, and it is phenocopied by deletion of pka1 on glucose medium. We conclude that protein kinase A is an important regulator of the SIN, and that SIN signalling is regulated by the carbon source of the cell.

To Divide or Not to Divide? How Deuterium Affects Growth and Division of Chlamydomonas reinhardtii

Extensive in vivo replacement of hydrogen by deuterium, a stable isotope of hydrogen, induces a distinct stress response, reduces cell growth and impairs cell division in various organisms. Microalgae, including Chlamydomonas reinhardtii, a well-established model organism in cell cycle studies, are no exception. Chlamydomonas reinhardtii, a green unicellular alga of the Chlorophyceae class, divides by multiple fission, grows autotrophically and can be synchronized by alternating light/dark regimes; this makes it a model of first choice to discriminate the effect of deuterium on growth and/or division. Here, we investigate the effects of high doses of deuterium on cell cycle progression in C. reinhardtii. Synchronous cultures of C. reinhardtii were cultivated in growth medium containing 70 or 90% D2O. We characterize specific deuterium-induced shifts in attainment of commitment points during growth and/or division of C. reinhardtii, contradicting the role of the "sizer" in regulating the cell cycle. Consequently, impaired cell cycle progression in deuterated cultures causes (over)accumulation of starch and lipids, suggesting a promising potential for microalgae to produce deuterated organic compounds.

Single-cell RNA sequencing of batch Chlamydomonas cultures reveals heterogeneity in their diurnal cycle phase

The photosynthetic unicellular alga Chlamydomonas (Chlamydomonas reinhardtii) is a versatile reference for algal biology because of its ease of culture in the laboratory. Genomic and systems biology approaches have previously described transcriptome responses to environmental changes using bulk data, thus representing the average behavior from pools of cells. Here, we apply single-cell RNA sequencing (scRNA-seq) to probe the heterogeneity of Chlamydomonas cell populations under three environments and in two genotypes differing by the presence of a cell wall. First, we determined that RNA can be extracted from single algal cells with or without a cell wall, offering the possibility to sample natural algal communities. Second, scRNA-seq successfully separated single cells into nonoverlapping cell clusters according to their growth conditions. Cells exposed to iron or nitrogen deficiency were easily distinguished despite a shared tendency to arrest photosynthesis and cell division to economize resources. Notably, these groups of cells not only recapitulated known patterns observed with bulk RNA-seq but also revealed their inherent heterogeneity. A substantial source of variation between cells originated from their endogenous diurnal phase, although cultures were grown in constant light. We exploited this result to show that circadian iron responses may be conserved from algae to land plants. We document experimentally that bulk RNA-seq data represent an average of typically hidden heterogeneity in the population.

Cell-cycle regulation in green algae dividing by multiple fission

Green algae dividing by multiple fission comprise unrelated genera but are connected by one common feature: under optimal growth conditions, they can divide into more than two daughter cells. The number of daughter cells, also known as the division number, is relatively stable for most species and usually ranges from 4 to 16. The number of daughter cells is dictated by growth rate and is modulated by light and temperature. Green algae dividing by multiple fission can thus be used to study coordination of growth and progression of the cell cycle. Algal cultures can be synchronized naturally by alternating light/dark periods so that growth occurs in the light and DNA replication(s) and nuclear and cellular division(s) occur in the dark; synchrony in such cultures is almost 100% and can be maintained indefinitely. Moreover, the pattern of cell-cycle progression can be easily altered by differing growth conditions, allowing for detailed studies of coordination between individual cell-cycle events. Since the 1950s, green algae dividing by multiple fission have been studied as a unique model for cell-cycle regulation. Future sequencing of algal genomes will provide additional, high precision tools for physiological, taxonomic, structural, and molecular studies in these organisms.

Role of timer and sizer in regulation of Chlamydomonas cell cycle

To estimate the role that time and size had in controlling the Chlamydomonas cell cycle, we used a new on-chip single-cell microcultivation system, which involved the direct observation of single cells captured in microchambers made on a thin glass slide. The dependence of the pattern of energy supply for cells on its cell cycle was examined through a series of different intensities of continuous illumination in a minimal medium, and we found that cell division occurred when cells reached the critical size, which was 2.2 times larger than that of the newly created cells. When illumination stopped before cells reached the critical size, even though growth had stopped, they continued dividing during the delay time, which was shorter when cells were larger. With re-illumination after darkness, cells began to grow again and the timing of cell division was again controlled by the critical size. This indicates that the co-existence of two cell cycle regulation mechanisms and the sizer mechanism had a stronger influence than the timer.

Kinetics and regulation of de novo centriole assembly. Implications for the mechanism of centriole duplication

BACKGROUND

Centriole duplication is a key step in the cell cycle whose mechanism is completely unknown. Why new centrioles always form next to preexisting ones is a fundamental question. The simplest model is that preexisting centrioles nucleate the assembly of new centrioles, and that although centrioles can in some cases form de novo without this nucleation, the de novo assembly mechanism should be too slow to compete with normal duplication in order to maintain fidelity of centriole duplication.

RESULTS

We have measured the rate of de novo centriole assembly in vegetatively dividing cells that normally always contain centrioles. By using mutants of Chlamydomonas that are defective in centriole segregation, we obtained viable centrioleless cells that continue to divide, and find that within a single generation, 50% of these cells reacquire new centrioles by de novo assembly. This suggests that the rate of de novo assembly is approximately half the rate of templated duplication. A mutation in the VFL3 gene causes a complete loss of the templated assembly pathway without eliminating de novo assembly. A mutation in the centrin gene also reduced the rate of templated assembly.

CONCLUSIONS

These results suggest that there are two pathways for centriole assembly, namely a templated pathway that requires preexisting centrioles to nucleate new centriole assembly, and a de novo assembly pathway that is normally turned off when centrioles are present.

Ribonucleotide reductases and their occurrence in microorganisms: a link to the RNA/DNA transition

The evolution of a deoxyribonucleotide synthesizing ribonucleotide reductase might have initiated the transition from the ancient RNA world into the prevailing DNA world. At least five classes of ribonucleotide reductases have evolved. The ancient enzyme has not been identified. A reconstruction of the first ribonucleotide reductase requires knowledge of contemporary enzymes and of microbial evolution. Experimental work on the former focuses on few organisms, whereas the latter is now well understood on the basis of ribosomal RNA sequences. Deoxyribonucleotide formation has not been investigated in many evolutionary important microorganisms. This review covers our knowledge on deoxyribonucleotide synthesis in microorganisms and the distribution of ribonucleotide reductases in nature. Ecological constraints on enzyme evolution and knowledge deficiencies emerge from complete coverage of the phylogenetic groups.

Effects of light and acetate on the liberation of zoospores by a mutant strain ofChlamydomonas reinhardtii

In light-dark-synchronized cultures of the unicellular green algaChlamydomonas reinhardtii, release of zoospores from the wall of the mother cell normally takes place during the second half of the dark period. The recently isolated mutant 'ls', however, needs light for the liberation of zoospores when grown photoautotrophically under a 12 h light-12 h dark regime. The light-induced release of zoospores was found to be prevented by addition of the photosystem-II inhibitor 3-(3',4'-dichlorophenyl)-1,1-dimethylurea. Furthermore, light dependence of this process was shown to be abolished when the mutant 'ls' was grown either photoautotrophically under a 14 h light-10 h dark regime or in the presence of acetate. Our findings indicate that the light-dependency of zoospore liberation observed in cultures of this particular mutant during photoautotrophic growth under a 12 h light-12 h dark regime might be attributed to an altered energy metabolism. The light-induced release of zoospores was found to be prevented by addition of cycloheximide or chloramphenicol, antibiotics which inhibit protein biosynthesis by cytoplasmic and organellar ribosomes, respectively. Actinomycin D, an inhibitor of RNA synthesis, however, did not affect the light-induced liberation of zoospores.Sporangia accumulate in stationary cultures of the mutant 'ls'. Release of zoospores was observed when these sporangia were collected by centrifugation and incubated in the light after resuspension in fresh culture medium. Since liberation of zoospores was not observed after dilution of the stationary cultures with fresh culture medium, we suppose that components which interfere with the action of the sporangial autolysin are accumulated in the culture medium of the mutant 'ls'.

Control of cell division in Paramecium tetraurelia. Effects of abrupt changes in nutrient level on accumulation of macronuclear DNA and cell mass

In the cell cycle of Paramecium there are three points of interaction between cell growth-related processes and the processes of macronuclear DNA replication and cell division: initiation of DNA synthesis, regulation of the rates of growth and DNA accumulation, and initiation of cell division. This study examines the regulation of the latter two processes by analysis of the response of each to abrupt changes in nutrient level brought about either by transferring dividing cells from a steady-state chemostat culture to medium with unlimited food, or by transferring well-fed dividing cells to exhausted medium. The rates of DNA accumulation and cell growth respond quickly to changes in nutrient level. The amounts of these cell components accumulated during the cell cycle following a shift in nutrient level are typical of those occurring during equilibrium growth under post-shift conditions. Commitment to division occurs at a fixed interval prior to fission that is similar in well-fed and nutrient-limited cells. Initiation of cell division in Paramecium is associated with accumulation of a threshold DNA increment, whose level is largely independent of nutritive conditions. The amount of DNA accumulated during the cell cycle varies with nutritional conditions because the rates of growth and DNA accumulation are affected by nutrient level; slowly growing cells accumulated relatively little DNA during the fixed interval between commitment to cell division and fission.

Regulation of genes encoding the large subunit of ribulose-1,5-bisphosphate carboxylase and the photosystem II polypeptides D-1 and D-2 during the cell cycle of Chlamydomonas reinhardtii

Synthesis of the major chloroplast proteins is temporally regulated in light-dark-synchronized Chlamydomonas cells. We have used cloned chloroplast DNA probes, and in vitro and in vivo protein synthesis to examine the cell cycle regulation of photosystem II polypeptides D-1 and D-2, and the large subunit of ribulose-1,5-bisphosphate carboxylase (RuBPCase LS). Synthesis and accumulation of D-1 and D-2 mRNAs occurs during the first half of the light period (G1), correlating with increasing synthesis of the polypeptides. Rifampicin, added immediately before the light period, inhibited the normal increase in D-1, D-2 polypeptide synthesis. During the dark period D-1, D-2 mRNAs persist at high levels despite reduced rates of mRNA synthesis and translation during this period. Cell-free translation analyses indicate that the D-1 mRNA present during the dark period is efficient at directing synthesis of the D-1 precursor in vitro. We conclude that expression of the psbA (D-1) and psbD (D-2) genes are regulated primarily at the transcriptional level during the light-induction period but at the translational level for the remainder of the cell cycle. Transcripts of the RuBPCase LS gene (rbcL) are also found at high levels during the light and dark periods but, unlike D-1 and D-2, LS mRNA levels do not increase until the last half of the light period and measurable synthesis and accumulation of this mRNA occurs during the dark. Furthermore, induction of LS polypeptide synthesis during the light period is insensitive to rifampicin. We conclude that LS production is regulated primarily at the translational level during the cell cycle.

Cell cycle control by timer and sizer in Chlamydomonas

Conservation of cell cycle control mechanisms is indicated by the presence of functionally homologous division control genes in unrelated yeasts and by the nonspecific action of oncogenes, but it remains uncertain what property of a growing cell results in the initiation of events leading to division. Response to a critical size is indicated by the longer growth period of smaller cells prior to division, which is consistent with deferment of division events until a minimum size is attained; however, in the same cell types faster growing cells are larger and this is more easily explained if division follows a timed period during which faster growing cells grow more, as is postulated for mammalian cells. Therefore, either time- or size-dependent controls might be the sole significant mechanism; we report here, however, that both controls do function in Chlamydomonas since cycle duration is under timer control and cell size determines the number of division rounds committed at the end of each cycle, and hence whether 2, 4, 8 or 16 daughter cells are formed.

Effects of 3,4-benzopyrene on the course of cell cycle events in the chlorococcal alga Scenedesmus quadricauda

Synchronous cultures of the chlorococcal alga Scenedesmus quadricauda were grown under optimal growth conditions. The mean length of their cell cycle was approximately 20 h. The cultures were treated at the start, at the 4th, and 8th hour of the cell cycle with 3,4-benzo(a)pyrene (BP) in the range of 0.1-0.5 μg ml(-1) of final concentration. A period about 4 h was found within which no inhibitory effects could be detected even at the highest BP concentrations used. In presence of BP the rates of RNA and protein syntheses gradually decreased until complete inhibition of net syntheses occurred. In a similar way chlorophyll synthesis was inhibited, and this was followed by gradual degradation of the chlorophyll. The higher the concentration of BP the more rapid the decrease of the rates of syntheses and the earlier their complete inhibition. At low BP concentrations while DNA replications were initiated, the number of replications was lowered. At higher concentrations the initiations of DNA replications were delayed or completely suppressed. Syntheses of saccharides were the least inhibited processes in presence of BP. Starch synthesis was slowed down at the end of the cell cycle and fructose synthesis (free and sucrose bound) was even stimulated later in the cell cycle. The release of daughter coenobia, and protoplast fissions were most susceptible to BP treatment, being affected at concentrations which produced no measureble disturbances of macromolecular syntheses. At BP concentrations at which the inhibition of macromolecular syntheses occurred, the delay or suppression of mitoses was observed.

Regulation of light-harvesting chlorophyll-binding protein (LHCP) mRNA accumulation during the cell cycle in Chlamydomonas reinhardi

Light-harvesting chlorophyll a/b protein (LHCP) synthesis is highly regulated during the cell cycle in light-dark synchronized C. reinhardi cells. LHCPs are a family of cytoplasmically synthesized proteins which are imported into the chloroplast. LHCPs are derived from at least two precursor proteins (32 kd and 30 kd) that are synthesized in vitro and immunoprecipitated by antiserum against chlorophyll-protein complex II proteins. A DNA copy of the mRNA encoding a 32 kd LHCP precursor was cloned from cDNA synthesized from poly(A) RNA obtained from mid-light-phase synchronous cells. Using cloned cDNA (pHS16) as a hybridization probe, we found that a single 1.2 kb RNA complementary to pHS16 accumulates in a wave-like manner during the mid-light phase of the 12 hr light-12 hr dark cycle and correlates with the pattern of chlorophyll synthesis. Light, during the light phase in the light-dark cycle, is required for accumulation of this RNA.

Regulation of the Chlamydomonas cell cycle by light and dark

By growing cells in alternating periods of light and darkness, we have found that the synchronization of phototrophically grown Chlamydomonas populations is regulated at two specific points in the cell cycle: the primary arrest (A) point, located in early G1, and the transition (T) point, located in mid-G1. At the A point, cell cycle progression becomes light dependent. At the T point, completion of the cycle becomes independent of light. Cells transferred from light to dark at cell cycle position between the two regulatory points enter a reversible resting state in which they remain viable and metabolically active, but do not progress through their cycles. The photosystem II inhibitor dichlorophenyldimethylurea (DCMU) mimics the A point block induced by darkness. This finding indicates that the A point block is mediated by a signal that operates through photosynthetic electron transport. Cells short of the T point will arrest in darkness although they contain considerable carbohydrate reserves. After the T point, a sharp increase occurs in starch degradation and in the endogenous respiration rate, indicating that some internal block to the availability of stored energy reserves has now been released, permitting cell cycle progression.

[Periodic, metabolic and structural phenomena in a protist, Euglena gracilis]

Sychronous divisions of Euglena gracilis strain Z can be obtained by various methods. When the cells are cultivated in a medium containing lactate as the sole carbon source, synchronous divisions are observed, independent of the conditions of illumination. Nevertheless, there exists a relationship between the phase of cell division and ther periods of light and darkness applied to the culture. During the cell cycle, the synthesis of macromolecules is discontinuous--this is true of nuclear and mitochondrial DNA, ribosomal and nonribosomal RNA, and certain proteins (cytochrome c 558). Cyclic variations in the structure of mitochondria and chloroplasts are observed. In the course of the cell cycle, sequential metabolic processes accompany structural modifications of the organelles. Also, at the beginning of the cycle, at the start of phase G1, the cytoplasmic ribosomes are synthesized, and then, in green euglenids, nonribosomal RNAs are formed. These syntheses of RNA precede enlargement of the chondriome and plastids. In mid-G1 phase, a new synthesis of RNA begins, which precedes synthesis of nuclear and mitochondrial DNA. At the end of G1 phase, division of organelles starts, beginning with the chondriome and plastids, arranged in a network.

Gametocytogenesis of Plasmodium falciparum in vitro: the cell-cycle

Reproducible growth of gametocytes of Plasmodium falciparum in vitro was obtained from ring-stages taken directly from naturally infected patients and from the same material following storage in liquid nitrogen. Progressive sexual differentiation in vitro was examined for a finite period of 9 days in microcultures and was, for convenience, divided into 5 stages using established morphological criteria (Hawking, Wilson & Gammage, 1971). This microculture system was adapted as a bioassay for various anti-metabolites. Drug activity was measured by observing the inhibition of the established pattern of sequential development in experimental as compared to control cultures. Inhibitors used were directed against DNA, RNA and protein metabolism and microtubule assembly. As a result of these studies it is proposed that the sexual cell-cycle of P. falciparum is characterized by 4 phases. (1) A G1 period which lasts only a few hours. (2) The S phase, where DNA synthesis occurs, occupies the remainder of the first 2 days of development - both G1 and S are confined to stage I and II gametocytes. (3) G2, which is subdivided into 2 sections: G2A, characterized by stage II and III gametocytes, in which significant RNA and protein synthesis continue to occur; and G2B, where there is a progressive increase in transcription control resulting in the depression of both RNA and protein synthesis. Nonetheless, continued morphological differentiation occurs in the latter section transforming the parasites to stage IV and the morphologically and functionally mature stage V. The final M phase is marked by the brief and exposive events of gametogenesis, during which further protein synthesis occurs de novo. The proposed cell-cycle is examined as a model for studies on the activity of gametocytocidal compounds.

Temporal programming of chloroplast and cytoplasmic ribosomal RNA transcription in the synchronous cell cycle of Chlamydomonas reinhardtii

Approximately 90% of the Chlamydomonas reinhardtii chloroplast and cytoplasmic rRNAs was transcribed in the nuclear G1 phase, which occurred during the light period under an alternating light-dark synchronization regime of 12 h each. The remaining 10% of chloroplast and cytoplasmic rRNAs was transcribed from its respective DNAs in the dark period, in the midst of an apparent turnover of a transcription appeared to be prokaryotic in sophistication. The transcription was not interrupted during chloroplast DNA synthesis which occurred during the light period. However, transcription of the nuclear DNA was repressed severely during the nuclear S phase in the dark period. The patterns of incorporation of 32P into chloroplast and cytoplasmic rRNA species in the cell cycle were similar to those of the actual rRNA synthesis as measured optically. However, the quantity of 32P incorporation per unit amount of rRNA synthesized varied considerably during the cell cycle, increasing in all rRNA's during the dark period. 32P incorporation data obtained from continuous and pulse 32P-labeling experiments also revealed a turnover of a small amount of both cytoplasmic and chloroplast rRNAs at the end of the S phase. The 32P incorporation into cytoplasmic and chloroplast rRNAs was well matched temporally with the 32P incorporation into their corresponding ribosomes, indicating that the newly synthesized rRNA molecules are utilized without delay throughout the cell cycle in the assembly of ribosomes.