Circadian rhythms in Neurospora crassa: oscillations in fatty acids

Characterization of NpgA, a 4'-phosphopantetheinyl transferase of Aspergillus nidulans, and evidence of its involvement in fungal growth and formation of conidia and cleistothecia for development

The null pigmentation mutant (npgA1) in Aspergillus nidulans results in a phenotype with colorless organs, decreased branching growth, delayed of asexual spore development, and aberrant cell wall structure. The npgA gene was isolated from A. nidulans to investigate these pleiomorphic phenomena of npgA1 mutant. Sequencing analysis of the complementing gene indicated that it contained a 4'-phosphopantetheinyl transferase (PPTase) superfamily domain. Enzymatic assay of the PPTase, encoded by the npgA gene, was implemented in vivo and in vitro. Loss-of-function of LYS5, which encoded a PPTase in Saccharomyces cerevisiae, was functionally complemented by NpgA, and Escherichia coli-derived NpgA revealed phosphopantetheinylation activity with the elaboration of 3'5'-ADP. Deletion of the npgA gene caused perfectly a lethal phenotype and the absence of asexual/sexual sporulation and secondary metabolites such as pigments in A. nidulans. However, a cross feeding effect with A. nidulans wild type allowed recovery from deletion defects, and phased-culture filtrate from the wild type were used to verify that the npgA gene was essential for formation of metabolites needed for development as well as growth. In addition, forced expression of npgA promoted the formation of conidia and cleistothecia as well as growth. These results indicate that the npgA gene is involved in the phosphopantetheinylation required for primary biological processes such as growth, asexual/sexual development, and the synthesis of secondary metabolites in A. nidulans.

Light regulation of metabolic pathways in fungi

Light represents a major carrier of information in nature. The molecular machineries translating its electromagnetic energy (photons) into the chemical language of cells transmit vital signals for adjustment of virtually every living organism to its habitat. Fungi react to illumination in various ways, and we found that they initiate considerable adaptations in their metabolic pathways upon growth in light or after perception of a light pulse. Alterations in response to light have predominantly been observed in carotenoid metabolism, polysaccharide and carbohydrate metabolism, fatty acid metabolism, nucleotide and nucleoside metabolism, and in regulation of production of secondary metabolites. Transcription of genes is initiated within minutes, abundance and activity of metabolic enzymes are adjusted, and subsequently, levels of metabolites are altered to cope with the harmful effects of light or to prepare for reproduction, which is dependent on light in many cases. This review aims to give an overview on metabolic pathways impacted by light and to illustrate the physiological significance of light for fungi. We provide a basis for assessment whether a given metabolic pathway might be subject to regulation by light and how these properties can be exploited for improvement of biotechnological processes.

Morphological transitions governed by density dependence and lipoxygenase activity in Aspergillus flavus

Aspergillus flavus differentiates to produce asexual dispersing spores (conidia) or overwintering survival structures called sclerotia. Results described here show that these two processes are oppositely regulated by density-dependent mechanisms and that increasing the cell density (from 10(1) to 10(7) cells/plate) results in the lowest numbers of sclerotial and the highest numbers of conidial. Extract from spent medium of low-cell-density cultures induced a high-sclerotium-number phenotype, whereas high-cell-density extract increased conidiation. Density-dependent development is also modified by changes in lipid availability. Exogenous linoleic acid increased sclerotial production at intermediate cell densities (10(4) and 10(5) cells/plate), whereas oleic and linolenic acids inhibited sclerotium formation. Deletion of Aflox encoding a lipoxygenase (LOX) greatly diminished density-dependent development of both sclerotia and conidia, resulting in an overall increase in the number of sclerotia and a decrease in the number of conidia at high cell densities (>10(5) cells/plate). Aflox mutants showed decreased linoleic acid LOX activity. Taken together, these results suggest that there is a quorum-sensing mechanism in which a factor(s) produced in dense cultures, perhaps a LOX-derived metabolite, activates conidium formation, while a factor(s) produced in low-density cultures stimulates sclerotium formation.

Defects in conidiophore development and conidium-macrophage interactions in a dioxygenase mutant of Aspergillus fumigatus

Oxygenated fatty acids, or oxylipins, play an essential role in physiological signaling and developmental processes in animals, plants, and fungi. Previous characterization of three Aspergillus fumigatus dioxygenases (PpoA, PpoB, and PpoC), similar in sequence to mammalian cyclooxygenases, showed that PpoA is responsible for the production of the oxylipins 8R-hydroperoxyoctadecadienoic acid and 5S,8R-dihydroxy-9Z,12Z-octadecadienoic acid and that PpoC is responsible for 10R-hydroxy-8E,12Z-hydroperoxyoctadecadienoic acid. Here, Delta ppo mutants were characterized to elucidate the role of fungal dioxygenases in A. fumigatus development and host interactions. The Delta ppoC strain displayed distinct phenotypes compared to those of other Delta ppo mutants and the wild type, including altered conidium size, germination, and tolerance to oxidative stress as well as increased uptake and killing by primary alveolar macrophages. These experiments implicate oxylipins in pathogen development and suggest that Delta ppoC represents a useful model for studying the A. fumigatus-host interaction.

Circadian rhythms in Neurospora crassa: clock mutant effects in the absence of a frq-based oscillator

In Neurospora, the circadian rhythm is expressed as rhythmic conidiation driven by a feedback loop involving the protein products of frq (frequency), wc-1 (white collar-1), and wc-2, known as the frq/wc (FWC) oscillator. Although strains carrying null mutations such as frq(10) or wc-2Delta lack a functional FWC oscillator and do not show a rhythm under most conditions, a rhythm can be observed in them by the addition of geraniol or farnesol to the media. Employing this altered media as an assay, the effect of other clock mutations in a frq(10)- or wc-2Delta-null background can be measured. It was found that the existing clock mutations fall into three classes: (1) those, such as prd-3 or prd-4 or frq(1), that showed no effect in a clock null background; (2) those, such as prd-1 or prd-2 or prd-6, that did have a measurable effect in the frq(10) background; and (3) those, such as the new mutation ult, that suppressed the frq(10) or wc-2Delta effect, i.e., geraniol/farnesol was not required for a visible rhythm. This classification suggests that some of the known clock mutations are part of a broader multioscillator system.

Oxylipins act as determinants of natural product biosynthesis and seed colonization in Aspergillus nidulans

Secreted, hormone-like lipogenic molecules, called oxylipins, mediate the balance of asexual to sexual spore ratio in Aspergillus nidulans. Oxylipin production in this fungus is dependent on developmental regulation of three conserved fatty acid oxygenases, PpoA, PpoB and PpoC. Here, we show that in addition to altering spore ratios, loss of ppo genes affect natural product biosynthesis and seed colonization. DeltappoA;DeltappoC and DeltappoA;DeltappoB;DeltappoC mutants were unable to produce the mycotoxin sterigmatocystin (ST) in vitro or in planta but in contrast overproduced the antibiotic penicillin (PN). These findings were correlated with decreased expression of genes involved in ST biosynthesis and increased expression of a PN biosynthetic gene, thus suggesting that oxylipin species regulate secondary metabolites at the transcriptional level. Additionally, the DeltappoA;DeltappoC and the DeltappoA;DeltappoB;DeltappoC mutants were defective in colonization of peanut seeds as reflected by a decrease in conidiation and production of the seed degradative enzyme lipase. These results indicate that oxylipin production is important for host colonization and mycotoxin production and may provide a promising target for future control strategies.

Circadian rhythms in Neurospora crassa: clock gene homologues in fungi

Computer-based analysis of a total of 17 filamentous fungal and yeasts genomes has shown: (1) homologues of frq, wc-1, wc-2, and vvd, key gene components of the Neurospora crassa clock, are present in Magnaporthe grisea, Gibberella zeae, and Podospora anserina, suggesting an frq-based oscillator in these organisms; (2) some fungal species that are more distantly related to Neurospora, such as Rhizopus oryzae do not appear to have frq homologues; (3) many fungal species that do not appear to contain frq, such as Aspergillus nidulans, do contain wc homologues; (4) of 11 well-described genes classified as clock-controlled genes (ccgs), in Neurospora, all of them were found to have homologues in other fungi; (5) the ccg-8 gene of N. crassa has homologies to opi1p, a transcriptional regulatory gene in Saccharomyces cerevisiae involved in inositol regulation. This suggests the possibilities of rhythmic inositol regulation, and/or a cascade of rhythmic activation of other genes in N. crassa.

Three putative oxylipin biosynthetic genes integrate sexual and asexual development in Aspergillus nidulans

Oxylipins called psi factors have been shown to alter the ratio of asexual to sexual sporulation in the filamentous fungusAspergillus nidulans. Analysis of theA. nidulansgenome has led to the identification of three fatty acid oxygenases (PpoA, PpoB and PpoC) predicted to produce psi factors. Here, it is reported that deletion ofppoB(ΔppoB) reduced production of the oleic-acid-derived oxylipin psiBβand increased the ratio of asexual to sexual spore development. Generation of the triple mutant ΔppoAΔppoBΔppoCresulted in a strain deficient in producing oleic- and linoleic-acid-derived 8′-hydroxy psi factor and caused increased and mis-scheduled activation of sexual development. Changes in asexual to sexual spore development were positively correlated to alterations in the expression ofbrlAandveA, respectively. PpoB and/or its products antagonistically mediate the expression levels ofppoAandppoC, thus revealing regulatory feedback loops among these three genes. Phylogenetic analyses showed thatppogenes are present in both saprophytic and pathogenic Ascomycetes and Basidiomycetes, suggesting a conserved role for Ppo enzymes in the life cycle of fungi.

Molecular cloning of developmentally specific genes by representational difference analysis during the fruiting body formation in the basidiomycete Lentinula edodes

To understand molecular mechanisms of the fruiting body development in basidiomycetes, we attempted to isolate developmentally regulated genes expressed specifically during the fruiting body formation of Lentinula edodes (Shiitake-mushroom). cDNA representational difference analysis (cDNA-RDA) between vegetatively growing mycelium and two developmental substages, primordium and mature fruiting body, resulted in an isolation of 105 individual genes (51 in primordium and 54 in mature fruiting body, respectively). A search of homology with the protein databases and two basidiomycetous genomes in Phanerochaete chrysosporium and Coprinopsis cinerea revealed that the obtained genes encoded various proteins similar to those involved in general metabolism, cell structure, signal transduction, and responses to stress; in addition, there were apparently several metabolic pathways and signal transduction cascades that could be involved in the fruiting body development. The expression products of several genes revealed no significant homologies to those in the databases, implying that those genes are unique in L. edodes and the encoding products may possess possible functions in the course of fruiting body development. RT-PCR analyses revealed that 20 candidates of the obtained genes were specifically or abundantly transcribed in the course of the fruiting body formation, suggesting that the obtained genes in this work play roles in fruiting body development in L. edodes.

Membrane lipid profile of an edible basidiomycete Lentinula edodes during growth and cell differentiation

The basidiomycetous mushroom Lentinula edodes (Shiitake) exhibits a unique process of cell differentiation termed "fruiting-body formation". To clarify the relationship between membrane lipids and fruiting-body formation in this fungus, we investigated variations in levels of phospholipids, cerebrosides, fatty acyl residues in the major phospholipids, and fatty acyl and sphingoid base residues in cerebrosides during vegetative growth and fruiting-body formation. PC, PE, and PS were the primary phospholipids in the cells of L. edodes. After a shift in growth temperature of L. edodes mycelia has been shifted from 25 to 18 degrees C, the proportion of unsaturated FA (UFA), such as linoleic acid (18:2) and oleic acid (18:1), increased. In contrast, during fruiting-body formation induced by the temperature downshift to 18 degrees C, 18:2 of PC in the primordia and fruiting bodies decreased, and the UFA of PE and 18:1 of PC increased compared with the proportions in mycelia growing at 18 degrees C. These results showed that the proportions of fatty acyl residues in PC and PE differed during fruiting-body formation in L. edodes. Moreover, the amount of cerebrosides in primordia increased compared with those in mycelia and fruiting bodies and, in these differentiating tissues, the proportion of 2-hydroxypentadecanoic acid increased whereas that of 2-hydroxyoctadecanoic acid decreased compared with that in the mycelia. However, the proportion of sphingoid base residues in cerebrosides did not change during fruiting-body formation in L. edodes.

Genetic Connection between Fatty Acid Metabolism and Sporulation in Aspergillus nidulans

In the Ascomycete fungus Aspergillus nidulans, the ratio of conidia (asexual spores) to ascospores (sexual spores) is affected by linoleic acid moieties including endogenous sporogenic factors called psi factors. Deletion of odeA (Delta odeA), encoding a Delta-12 desaturase that converts oleic acid to linoleic acid, resulted in a strain depleted of polyunsaturated fatty acids (18:2 and 18:3) but increased in oleic acid (18:1) and total percent fatty acid content. Linoleic acid-derived psi factors were absent in this strain but oleic acid-derived psi factors were increased relative to wild type. The Delta odeA strain was reduced in conidial production and mycelial growth; these effects were most noticeable when cultures were grown at 26 degrees C in the dark. Under these environmental conditions, the Delta odeA strain was delayed in ascospore production but produced more ascospores than wild type over time. This suggests a role for oleic acid-derived psi factors in affecting the asexual to sexual spore ratio in A. nidulans. Fatty acid composition and spore development were also affected by veA, a gene previously shown to control light driven conidial and ascospore development. Taken together our results indicate an interaction between veA and odeA alleles for fatty acid metabolism and spore development in A. nidulans.

Genetic and molecular analysis of circadian rhythms in Neurospora

Over the course of the past 40 years Neurospora has become a well-known and uniquely tractable model system for the analysis of the molecular basis of eukaryotic circadian oscillatory systems. Molecular bases for the period length and sustainability of the rhythm, light, and temperature resetting of the circadian system and for gating of light input and light effects are becoming understood, and Neurospora promises to be a suitable system for examining the role of coupled feedback loops in the clock. Many of these insights have shown or foreshadow direct parallels in mammalian systems, including the mechanism of light entrainment, the involvement of PAS:PAS heterodimers as transcriptional activators in essential clock-associated feedback loops, and dual role of FRQ in the loop as an activator and a repressor; similarities extend to the primary sequence level in at least one case, that of WC-1 and BMAL1. Work on circadian output in Neurospora has identified more than a dozen regulated genes and has been at the forefront of studies aimed at understanding clock control of gene expression.

sn-1,2-diacylglycerol levels in the fungus Neurospora crassa display circadian rhythmicity

The fungus Neurospora crassa is a model organism for investigating the biochemical mechanism of circadian (daily) rhythmicity. When a choline-requiring strain (chol-1) is depleted of choline, the period of the conidiation rhythm lengthens. We have found that the levels of sn-1,2-diacylglycerol (DAG) increase in proportion to the increase in period. Other clock mutations that change the period do not affect the levels of DAG. Membrane-permeant DAGs and inhibitors of DAG kinase were found to further lengthen the period of choline-depleted cultures. The level of DAG oscillates with a period comparable to the rhythm of conidiation in wild-type strains, choline-depleted cultures, and frq mutants, including a null frq strain. The DAG rhythm is present at the growing margin and also persists in older areas that have completed development. The phase of the DAG rhythm can be set by the light-to-dark transition, but the level of DAG is not immediately affected by light. Our results indicate that rhythms in DAG levels in Neurospora are driven by a light-sensitive circadian oscillator that does not require the frq gene product. High levels of DAG may feed back on that oscillator to lengthen its period.

Sporogenic effect of polyunsaturated fatty acids on development of Aspergillus spp

Aspergillus spp. are frequently occurring seed-colonizing fungi that complete their disease cycles through the development of asexual spores, which function as inocula, and through the formation of cleistothecia and sclerotia. We found that development of all three of these structures in Aspergillus nidulans, Aspergillus flavus, and Aspergillus parasiticus is affected by linoleic acid and light. The specific morphological effects of linoleic acid include induction of precocious and increased asexual spore development in A. flavus and A. parasiticus strains and altered sclerotium production in some A. flavus strains in which sclerotium production decreases in the light but increases in the dark. In A. nidulans, both asexual spore production and sexual spore production were altered by linoleic acid. Spore development was induced in all three species by hydroperoxylinoleic acids, which are linoleic acid derivatives that are produced during fungal colonization of seeds. The sporogenic effects of these linoleic compounds on A. nidulans are similar to the sporogenic effects of A. nidulans psi factor, an endogenous mixture of hydroxylinoleic acid moieties. Light treatments also significantly increased asexual spore production in all three species. The sporogenic effects of light, linoleic acid, and linoleic acid derivatives on A. nidulans required an intact veA gene. The sporogenic effects of light and linoleic acid on Aspergillus spp., as well as members of other fungal genera, suggest that these factors may be significant environmental signals for fungal development.

Changes in fatty acid composition of Neurospora crassa accompany sexual development and ascospore germination

Fatty acid composition was determined during several stages of sexual development in Neurospora crassa. Triacylglycerol was the predominant acyl lipid in cultures undergoing sexual development. The absolute amounts of triacylglycerol in fertilized cultures varied over time, in contrast to control (unfertilized or mock-fertilized) cultures, in which the amount of triacylglycerol decreased linearly with age. In cultures competent to undergo sexual development, alpha-linoleate was the predominant fatty acid, ranging from 53 to 65% of the total fatty acid mass. alpha-Linolenate was 3% or less of the total fatty acid, in marked contrast to the much higher levels (10-35%) typically reported for vegetative cultures. In fertilized cultures, a slightly higher mass ratio of oleate was also observed. This difference was due to the developing asci: in developing asci and mature ascospores, oleate replaced alpha-linoleate as the predominant fatty acid (45 to 50% of the total). In germinating ascospores, the fatty acid composition approached that of vegetative cultures 6 h after inducing germination by heat activation. These results show that the fatty acid composition of sexual tissues of Neurospora differs substantially from the composition of asexual tissues, and that extensive changes in fatty acid composition correlate with several events in the sexual stage of development.

Molecular circadian oscillators: an alternative hypothesis

Results from experiments in different organisms have shown that elements of input pathways can themselves be under circadian control and that outputs might feed back into the oscillator. In addition, it has become clear that there might be redundancies in the generation of circadian rhythmicity, even within single cells. In view of these results, it is worth reevaluating our current working hypotheses about the pacemaker's molecular mechanisms and the involvement of single autoregulatory genes. On one hand, redundancies in the generation of circadian rhythmicity might make the approach of defining a discrete circadian oscillator with the help of single gene mutations extremely difficult. On the other hand, many examples show that components of signal transduction pathways can indeed be encoded by single genes. The authors have constructed a model placing an autoregulatory gene and its products on an input pathway feeding into a separate oscillator. The behavior of this model can explain the majority of results of molecular circadian biology published to date. In addition, it shows that different qualities of the circadian system might be associated with different cellular functions that can exist independently and, only if put together, will lead to the known circadian phenotype.

Identification of an annexin-like protein and its possible role in the Aplysia eye circadian system

Light and serotonin regulate the phase of the circadian rhythm of the isolated eye of Aplysia. To screen for possible protein components of the eye circadian oscillator, we identified a number of proteins whose synthesis was altered in opposite ways by light and serotonin. The cellular function of one of these proteins was investigated by obtaining a partial amino acid sequence of it and by examining its immunoreactivity. A 38-amino acid sequence was obtained from a 40-kDa (isoelectric point 5.6) protein. A greater than 60% amino acid identity existed between this sequence and sequences of a family of calcium/phospholipid-binding proteins called annexins. Furthermore, the 40-kDa protein reacted with antibodies generated against a conserved amino acid sequence of annexins and with antibodies raised against human annexin I. The identification of the 40-kDa, light- and serotonin-regulated protein as an annexin led us to hypothesize that arachidonic acid metabolism plays a role in the Aplysia eye circadian system. To test this hypothesis, we examined the ability of an inhibitor of the arachidonic acid metabolic pathway to perturb the eye rhythm. Pulse treatments of isolated eyes with a lipoxygenase inhibitor, nordihydroguaiaretic acid, phase shifted the rhythm. The phase-shifting ability of nordihydroguaiaretic acid suggests that arachidonic acid and some of its metabolites may play a role in the eye circadian system. The results of our studies raise the possibility that links may exist between the 40-kDa annexin-like protein, arachidonic acid metabolism, and the circadian oscillator.

The hunt for mechanisms of circadian timing in the eye of Aplysia

The goals of our research are to understand how circadian oscillations in the eye of Aplysia california are generated and how entraining agents regulate these oscillations. These goals require identification of the molecular components of the oscillator and entrainment pathways as well as elucidation of the biochemical processes by which these components interact with one another. Our experimental strategy entails tracing environmental information along an entrainment pathway until the last component of the pathway is reached. The isolated eye of Aplysia exhibits a circadian rhythm of optic nerve impulses. This rhythm is regulated by at least two entrainment pathways. A photic pathway entrains the rhythm to light-dark cycles and an efferent serotonergic pathway relays neural information from the CNS to the oscillator. Phase shifting by light appears to involve an increase in the levels of cGMP, depolarization, and protein synthesis. Phase shifting by serotonin appears to involve an increase in the levels of cAMP, hyperpolarization, and protein synthesis. The involvement of protein synthesis in the entrainment pathways, together with the findings that brief treatments of inhibitors of protein synthesis phase shift the rhythm and that continuous treatments of these inhibitors alter the period of the rhythm, indicates that translation is part of the oscillator mechanism. Recent evidence indicates that transcription may also be part of the oscillator mechanism. Brief treatments with DRB, a reversible transcription inhibitor, phase shift the rhythm while continuous treatments with DRB lengthen the period of the rhythm. A comparison of the effects of transcription and translation inhibitors on the rhythm indicates that transcription and translation are closely coupled in the eye circadian system. To know the precise role of transcription and translation in the circadian system, it is necessary to identify and then study specific proteins and mRNAs important for circadian timing. To identify putative oscillator proteins (POPs), we have hunted for proteins whose synthesis or phosphorylation was altered by the entraining agents light and 5-HT and by other agents that perturb the circadian rhythm. By exposing eyes to labeled amino acids in the presence of phase-shifting treatments and then using two-dimensional gel electrophoresis to separate proteins, we found eight proteins that may be considered POPs. To elucidate the cellular function of POPs, we have begun to obtain their amino acid sequences. A 40,000, pI 5.6 protein (POP-1) was identified as a member of the lipocortin family of proteins. Lipocortins are Ca(2+)-phospholipid binding proteins whose functions include inhibition of PLA2.(ABSTRACT TRUNCATED AT 400 WORDS)

A role for unsaturated fatty acids in mitochondrial movement and inheritance

Yeast cells with the mdm2 mutation display temperature-sensitive growth and defective intracellular mitochondrial movement at the non-permissive temperature. The latter phenotype includes both an absence of mitochondrial transfer into daughter buds of mitotically growing cells and an aberrant mitochondrial distribution in cells exposed to mating pheromone. The wild-type MDM2 gene was cloned by complementation, and DNA sequence analysis revealed a large open reading frame encoding a putative protein of 58.4 kD. The predicted protein sequence is identical to that reported for the yeast OLE1 gene encoding fatty acid desaturase. Unsaturated fatty acid levels are substantially decreased in mdm2 cells after a prolonged incubation at the non-permissive temperature. The addition of oleic acid complements the temperature-sensitive growth and mitochondrial distribution defects of the mutant cells. These results indicate that mdm2 is a temperature-sensitive allele of OLE1 and demonstrate an essential role for unsaturated fatty acids in mitochondrial movement and inheritance.

The biological clock of Neurospora in a microgravity environment

The circadian rhythm of conidiation in Neurospora crassa is thought to be an endogenously derived circadian oscillation; however, several investigators have suggested that circadian rhythms may, instead, be driven by some geophysical time cue(s). An experiment was conducted on space shuttle flight STS-9 in order to test this hypothesis; during the first 7-8 cycles in space, there were several minor alterations observed in the conidiation rhythm, including an increase in the period of the oscillation, an increase in the variability of the growth rate and a diminished rhythm amplitude, which eventually damped out in 25% of the flight tubes. On day seven of flight, the tubes were exposed to light while their growth fronts were marked. Some aspect of the marking process reinstated a robust rhythm in all the tubes which continued throughout the remainder of the flight. These results from the last 86 hours of flight demonstrated that the rhythm can persist in space. Since the aberrant rhythmicity occurred prior to the marking procedure, but not after, it was hypothesized that the damping on STS-9 may have resulted from the hypergravity pulse of launch. To test this hypothesis, we conducted investigations into the effects of altered gravitational forces on conidiation. Exposure to hypergravity (via centrifugation), simulated microgravity (via the use of a clinostat) and altered orientations (via alterations in the vector of a 1 g force) were used to examine the effects of gravity upon the circadian rhythm of conidiation.

Neurospora mitochondria contain an acyl-carrier protein

Mitochondria of Neurospora crassa were found to contain a protein which was labelled with [14C]pantothenic acid and which carried an acyl group. This protein, when purified 6000-fold, closely resembled the bacterial and chloroplast acyl-carrier protein(s) [ACP(s)] in its physical and chemical properties. The predominant acyl group esterified to the purified protein was 3-hydroxytetradecanoate, as determined by gas chromatographic mass spectrometry. The amino acid sequence of the tryptic peptide carrying the 4'-phosphophantetheine moiety showed a high degree of sequence similarity to the analogous bacterial and chloroplast ACP peptide sequences. The possible functions of this ACP in lipid metabolism are discussed in view of the fact that Neurospora has a separate cytoplasmic enzyme complex which carries out the de novo biosynthesis of fatty acids.

Circadian rhythms in Neurospora crassa: a clock mutant, prd-1, is altered in membrane fatty acid composition

The fatty acid compositions of the phospholipids of Neurospora crassa mutants with altered periods were determined to test the possibility that some of these mutants might have altered membrane composition. In liquid shaker culture in constant light the bd (band) strain, which has a normal period (21.6 h), exhibited a growth-dependent increase in linoleic acid content and a decrease in linolenic acid content during early log phase growth. By late log phase, fatty acid composition was essentially constant. The phospholipid fatty acid compositions of bd strains containing mutations at the frq (frequency) and chr (chrono) loci were indistinguishable from that of the bd strain under the conditions used. However, a bd strain containing a mutation at the prd-1 (period) locus, as well as prd-1 segregants from a cross of this strain to a bd strain, had altered patterns of growth-dependent fatty acid composition; linoleic and linolenic acid contents changed more slow than in the bd strain and continued to change throughout growth. In addition, the fatty acid composition of a bd prd-1 strain on solid medium differed from that of the bd strain. It is proposed that the prd-1 mutation leads to altered membrane homeostasis, which in turn affects circadian rhythmicity because some or all components of the rhythm-generating system are membrane-localized.

Circadian rhythms in Neurospora crassa: membrane composition of a mutant defective in temperature compensation

The cel mutant of Neurospora, partially blocked in fatty acid synthesis and lacking temperature compensation of its circadian rhythm below 22 degrees C, had a phospholipid fatty acid composition in liquid shaker culture distinctly different from that of a cel+ control strain. During growth, cel+ exhibited a reproducible increase in its linoleic acid level from about 32 to a plateau at 63 mol%, and a corresponding decrease in its linolenic acid level from about 40 to a plateau at 10 mol%. The level of palmitic acid was constant at 19 mol%. In the cel strain, the linoleic acid level was constant at 54 mol% while the palmitic acid level increased from about 12 to about 23 mol%. Supplementation with palmitic or linoleic acids altered the patterns of fatty acid composition of cel, but did not affect the pattern of cel+. Altered fatty acid composition cosegregated with the cel marker. The mitochondrial phospholipids of cel in liquid culture also had abnormal fatty acid composition, as did the whole mycelial phospholipids on solid medium. These results are consistent with the involvement of membrane homeostasis in the temperature compensation of circadian rhythms.

The kinetics of changes in the fatty acid composition of Neurospora crassa lipids after a temperature increase

The fatty acid composition of the total lipids, phospholipids and neutral lipids of log-phase shaker cultures of the bd (band) strain of Neurospora crassa, were measured every 2 h for an 8-h period following a temperature increase from 22 to 40 degrees C. For purposes of comparison, the fatty acid composition was also measured when cultures were grown from inoculation at temperatures between 22 and 40 degrees C. In the phospholipids, the temperature jump produced, over a 4-6 hour span, a linear decrease in the linolenic acid (18:3) content from 31 to 10 mol% and an increase in the linoleic acid (18:2) content from 41 to 45 mol% for a few hours, followed by a decrease to 38 mol%. The oleic acid (18:1) content increased, after a 2 h lag, from 5 to 26 mol%. The temperature increase caused a decrease in the double bond index from 180 to 135 but produced no changes in the mol% of the saturated fatty acids, the ratio of saturated to unsaturated fatty acids, the total amount of fatty acids per gram dry weight, or the growth rate of the cultures. After the switch to 40 degrees C the total amount of 18:3 per flask increased only slightly over the 8 h period, indicating that there was little loss of 18:3 from the mycelia by beta-oxidation, or by conversion to other fatty acids. Since the mass of the culture increased some 4-fold in this time, it suggests that the decrease in the mole percent of 18:3 is probably due to a decrease in the rate of synthesis of 18:3.

Preparation of functional group analogs of unsaturated fatty acids and their effects on the circadian rhythm of a fatty-acid-deficient mutant of Neurospora crassa

Functional group analogs of oleic, linoleic and linolenic acids were prepared by coverting their double bonds to dibromo, cyclopropyl, epoxy, methoxy, and, in the case of oleic acid, hydroxy groups. These compounds were supplemented to the bd csp cel strain of the mold Neurospora crassa. The cel mutation confers a partial requirement for saturated fatty acids and, also, perturbs the circadian rhythm of spore formation. For example, the period of bd csp cel's rhythm is dramatically lengthened upon supplementation by natural cis-unsaturated fatty acids. Of the analogs tested, only the monoepoxy, monomethoxy, dibromo, and hexabromo stearic acids gave significant period lengthening. Other analogs, which should have comparable abilities to disrupt lipid bilayer packing, gave no rhythm effect. Further, the inactive di- and tri-methoxystearic acid analogs were incorporated to a greater extent than the active mono-methoxystearic acid. The results do not, therefore, support a direct alteration in membrane "fluidity' as the mode of action of the period-lengthening fatty acids.

Circadian rhythms in Neurospora crassa: the effects of point mutations on the proteolipid portion of the mitochondrial ATP synthetase

Five oligomycin-resistant (olir) mutant strains of Neurospora crassa were analyzed for their growth rate and for the periodicity of their circadian rhythm. The most resistant strains had periods of 18-19 h while the least resistant strain had a normal period of 21.0 h. There was a rough correlation between the in vivo degree of oligomycin-resistance and the amount of change in the period. Several of the olir mutations have been previously described by Sebald et al. (1977) in terms of known amino acid changes in the primary structure of the proteolipid, or DCCD-binding protein, found in the F0 membrane portion of the mitochondrial ATP synthetase. Amino acid changes in the structure of this protein are reported here for two other olir mutations. The proteolipid isolation procedures were slightly modified to include a delipidation step, and an HPLC procedure was developed to separate the hydrophobic peptides of this protein. Analysis of heterocaryons carrying both the olir and olis markers indicated that the olir and olis mutations were codominant to each other in terms of period and growth rate. The changes in the primary structure of this DCCD-binding protein reported here are the first known examples of changes in the primary structure of a protein which alter the period of a circadian rhythm.

Unsaturated fatty acid isomers: effects on the circadian rhythm of a fatty-acid-deficient Neurospora crassa mutant

The fatty acids oleic, linoleic, and linolenic, each of which has a cis double bond at the delta 9 position, are known to lengthen the circadian period of conidiation (spore formation) of strains of Neurospora crassa carrying the cel mutation. cel confers a partial fatty acid requirement on the organism and has been used to promote incorporation of exogenous fatty acids. To test whether a physical effect imparted by the cis double bonds, such as increased membrane fluidity, is critical for the perturbation of the rhythm, various isomers of these fatty acids were supplemented to the bd csp cel strain. Positional isomers of oleic acid, such as petroselinic (delta 6) and vaccenic (delta 11) acids, and longer-chain isomers, such as eicosenoic (delta 11) and erucic (delta 13) acids, did not lengthen the rhythm. The shorter-chain palmitoleic (delta 9) acid did not give a consistent lengthening of the rhythm; it may be elongated to vaccenic acid. In contrast, gamma-linolenic acid (delta 6,9,12) dramatically lengthened the period. Linoelaidic acid (the trans,trans isomer of linoleic acid) lengthened the period at 22 degrees C, but elaidic acid (the trans isomer of oleic acid) did not. Elaidic acid was shown to exert a lengthening effect, but only at lower temperatures. The data do not support a direct physical action as the source of the fatty acids' "chronobiotic" ability.

A pantothenate derivative is covalently bound to mitochondrial proteins in Neurospora crassa

The presence of protein-bound pantothenate in Neurospora crassa was investigated by labelling a pantothenate auxotroph (pan-2) with [14C]pantothenate and examining mycelial homogenates on dodecyl sulfate/polyacrylamide gels. Five peaks of radioactivity were found, with apparent molecular masses of 200, 140, 22, 19, and 9 kDa. The 200-kDa peak was identified as fatty acid synthetase, based on its absence in a fatty acid synthetase mutant. The 22-kDa and 19-kDa peaks co-purified with mitochondrial markers on sucrose gradients. When purified mitochondria were fractionated, the 19-kDa protein was associated with the inner membrane and the 22-kDa protein was enriched in the soluble mitochondrial fraction. The label was quantitatively recovered from the mitochondrial proteins as 4'-phosphopantetheine after mild alkaline hydrolysis. Although the function of this post-translational modification of mitochondrial proteins is not known, several possibilities are discussed: the 4'-phosphopantetheine may act as a carrier group in an enzymatic reaction, or it may perform a regulatory function as part of an enzyme complex.

Chronobiology at the cellular and molecular levels: models and mechanisms for circadian timekeeping

This review considers cellular chronobiology and examines, at least in a superficial way, several classes of models and mechanisms that have been proposed for circadian rhythmicity and some of the experimental approaches that have appeared to be most productive. After a brief discussion of temporal organization and the metabolic, epigenetic, and circadian time domains, the general properties of circadian rhythms are enumerated. A survey of independent oscillations in isolated organs, tissues, and cells is followed by a review of selected circadian rhythms in eukaryotic microorganisms, with particular emphasis placed on the rhythm of cell division in the algal flagellate Euglena as a model system illustrating temporal differentiation. In the ensuing section, experimental approaches to circadian clock mechanisms are considered. The dissection of the clock by the use of chemical inhibitors is illustrated for the rhythm of bioluminescence in the marine dinoflagellate Gonyaulax and for the rhythm of photosynthetic capacity in the unicellular green alga Acetabularia. Alternatively, genetic analysis of circadian oscillators is considered in the green alga Chlamydomonas and in the bread mold Neurospora, both of which have yielded clock mutants and mutants having biochemical lesions that exhibit altered clock properties. On the basis of the evidence generated by these experimental approaches, several classes of biochemical and molecular models for circadian clocks have been proposed. These include strictly molecular models, feedback loop (network) models, transcriptional (tape-reading) models, and membrane models; some of their key elements and predictions are discussed. Finally, a number of general unsolved problems at the cellular level are briefly mentioned: cell cycle interfaces, the evolution of circadian rhythmicity, the possibility of multiple cellular oscillators, chronopharmacology and chronotherapy, and cell-cycle clocks in development and aging.