GENETICAL AND CYTOLOGICAL EVIDENCE FOR CHROMOSOMAL ELIMINATION IN A TRUE SLIME MOLD,DIDYMIUM IRIDIS

In crosses involving a polyploid myxamoebal clone. CR 2-25*, F₁ plasmodia and myxamoebae display a variety of unexpected ploidy levels as indicated by nuclear DNA measurements. Genetic analyses of the F₁ generations reveal either complete elimination of certain genetic markers or greatly skewed segregation ratios. On the basis of these two kinds of evidence, it is assumed that chromosome elimination occurs at some stage (or stages) following karyogamy between parental nuclei. The possible significance of polyploidy in relation to myxomycete speciation and evolution is discussed.     

DEVELOPMENT OF APOGAMIC AMOEBAE FROM HETEROTHALLIC LINES OF A MYXOMYCETE,DIDYMIUM IRIDIS

By making appropriate crosses between heterothallic sexual clones of Didymium iridis we can recover apogamic lines in the F₁ generation. In this organism, heterothallic forms typically produce a haploid myxamoebal stage, but recently two diploid myxamoebal clones homozygous for mating types were discovered. When these are crossed, A²A² x A⁵A⁵, tetraploid Plasmodia are produced which later yield diploid F₁ meiospores. Sixty-four percent of the single-spore-derived clones produce both myxamoebae and Plasmodia, while the remainder do not progress past the myxamoebal stage. These results are consistent with the predictions that from tetraploid nuclei, mating types should segregate in the meiospores in a ratio of 1A²A²:4A²A⁵:1A⁵A⁵, and that myxamoebae heterozygous, A²A⁵, for mating type should yield Plasmodia apogamously. As the means for verifying relative ploidy levels of myxamoebae and Plasmodia, nuclear DNA was measured with a scanning microspectrophotometer.     

HETEROTHALLISM AND HOMOTHALLISM IN TWO MYXOMYCETES

Collins , O'Neil Ray . (Queens Coll., New York City.) Heterothallism and homothallism in two Myxomycetes. Amer. Jour. Bot. 48(8): 674–683. Illus. 1961.—Single-spore studies of 2 Myxomycetes, Didymium iridis and Fuligo cinerea, revealed that the former is heterothallic and the latter is homothallic. In D. iridis, 256 single-spore isolations were made from sporangia which developed in mass-spore cultures. Of these, 101 germinated and 22 yielded plasmodia that later fructified in most cases. The remaining 79 single-spore cultures produced clones of myxamoebae and swarm cells only. When 18 of the 79 clones were mated in all possible combinations, plasmodia developed in a pattern which showed that the clones were either (+) or (–) with regard to mating type. Fructifications were readily obtained from these plasmodia. Fifty-three single spores of the F₁ generation were isolated. Of the 44 that germinated, 9 yielded plasmodia in monospore cultures, and 35 produced clones of myxamoebae and swarm cells only. Twenty-five of the F₁ clones were back-crossed with their parents. Results of the back crosses show that each F₁ clone is capable of yielding plasmodia with either the (+) or the (–) parent, never with both. When 14 of the F₁ clones were mated among themselves, a (+) and (–) mating type system was again revealed. Most of the 22 original single-spore cultures which produced plasmodia, later formed sporangia. From these sporangia, 88 spores were isolated. Seventy-two of these germinated and yielded large populations of swarm cells and myxamoebae, but none produced plasmodia. Twenty of the 72 clones were then mated among themselves. Some matings resulted in plasmodial formation, but the pattern was difficult to interpret. However, when these 20 clones were mated with known (+) and (–) clones, the results appear to be in keeping with a (+) and (–) mating type system. In F. cinerea, 219 single spores were isolated from aethalia derived from mass-spore cultures. Of these, 144 germinated and the same number yielded plasmodia. Fructifications were easily obtained from such plasmodia. Thirty-five second-generation single spores were isolated, of which 15 germinated and 15 yielded plasmodia. These results indicate that F. cinerea is homothallic.     

CYTOPHOTOMETRIC MEASUREMENT OF NUCLEAR DNA IN SEVEN HETEROTHALLIC ISOLATES OF DIDYMIUM IRIDIS,A MYXOMYCETE

Absorption cytophotometry was used to measure nuclear Feulgen-DNA content of myxamoebae and Plasmodia in seven heterothallic isolates of Didymium iridis. Measurements of myxamoebal nuclei from clones of four isolates (Hon 1, Pan 1, Pan 2, and CR 5) gave a mean DNA value of 0.34, whereas the nuclei of Plasmodia which develop from each of the four intraisolate crosses had a mean value of 0.63. These values correspond to the 2C haploid level in myxamoebae and the 4C diploid level in Plasmodia. DNA values in two additional isolates (Pan 3 and CR 2) are much higher than the mean for the other five. Accordingly, it is proposed that these may be polyploid. The question of polyploidy in D. iridis and in other myxomycetes is evaluated. The seventh isolate, Ky 1, is taxonomically very close to D. nigripes and was not included in calculations of mean values for D. iridis.     

NUCLEAR DNA CONTENT OF MYXAMOEBAE AND PLASMODIA IN SIX NON-HETEROTHALLIC ISOLATES OF A MYXOMYCETE,DIDYMIUM IRIDIS

The nuclear DNA content of six non-heterothallic isolates of the myxomycete Didymium iridis was measured by combining the Feulgen reaction with absorption microspectrophotometry. This allowed us to distinguish between homothallic (sexual) and apogamic (non-sexual) isolates. Four of the isolates studied, Panamanian 4 and 5, California 1, and Missouri 1 are homothallic. Moreover, the average DNA content of the myxamoebal and plasmodial nuclei (0.32 and 0.61 respectively) does not differ significantly from the calculated haploid and diploid values for heterothallic isolates of D. iridis (0.34 and 0.63). Hence, it is concluded that in each of these isolates the myxamoebae are haploid and the plasmodia diploid. In two of the isolates investigated, Georgia 1 and Hawaii 1, the DNA content of the myxamoebal and plasmodial nuclei did not differ significantly. Therefore, in both of these isolates the plasmodia appear to develop apogamically. In addition the mean DNA values recorded for the Ha-1 isolate suggest that it is aneuploid.     

Genetic Analysis of Resistance to Benzimidazoles in Physarum: Differential Expression of β-Tubulin Genes

Physarum displays two vegetative cell types, uninucleate myxamoebae and multinucleate plasmodia. Mutant myxamoebae of Physarum resistant to the antitubulin drug methylbenzimidazole-2-yl-carbamate (MBC) were isolated. All mutants tested were cross-resistant to other benzimidazoles but not to cycloheximide or emetine. Genetic analysis showed that mutation to MBC resistance can occur at any one of four unlinked loci, benA, benB, benC or benD. MBC resistance of benB and benD mutants was expressed in plasmodia, but benA and benC mutant plasmodia were MBC sensitive, suggesting that benA and benC encode myxamoeba-specific products. Myxamoebae carrying the recessive benD210 mutation express a β-tubulin with noval electrophoretic mobility, in addition to a β-tubulin with wild-type mobility. This and other evidence indicates that benD is a structural gene for β-tubulin, and that at least two β-tubulin genes are expressed in myxamoebae. Comparisons of the β-tubulins of wildtype and benD210 strains by gel electrophoresis revealed that, of the three (or more) β-tubulin genes expressed in Physarum, one, benD, is expressed in both myxamoebae and plasmodia, one is expressed specifically in myxamoebae and one is expressed specifically in plasmodia. However, mutation in only one gene, benD, is sufficient to confer MBC resistance on both myxamoebae and plasmodia.     

COMPARATIVE MEASUREMENTS OF NUCLEAR DNA IN A HETEROTHALLIC AND A SELF-FERTILE ISOLATE OF THE MYXOMYCETE,DIDYMIUM IRIDIS

Comparative measurements were made of the nuclear Feulgen-DNA content of a heterothallic and a self-fertile isolate of the myxomycete Didymium iridis. Plasmodial nuclei of both isolates contain the diploid amount of DNA. The replicated diploid (4C) values for the heterothallic and the self-fertile isolates are 5.66 and 5.95, respectively. Myxamoebae, however, are quite dissimilar in their nuclear DNA content. Those of the heterothallic isolates, Honduran 1–2 (A¹) and Panamanian 2–4 (A⁷), have mean values of 3.81 and 3.69, whereas myxamoebae of the self-fertile Philippine-1 isolate were found to have a mean value of 6.07. Myxamoebae of the Ph-1 isolate are, therefore, at the same ploidy level as the Ph-1 Plasmodium. Mean DNA values for Ph-1 sporangial nuclei were in category 4C. Measurement of the DNA content of mitotic metaphases in sporangia at T = 6 hr confirmed that the mean DNA content of both Ph-1 myxamoebae and plasmodial nuclei is equivalent to 4C. It is concluded that nuclear phase alternance is lacking in the Ph-1 isolate and that the Plasmodium of this isolate develops by apogamy.     

QUANTITATIVE MICROSPECTROPHOTOMETRY OF NUCLEAR DNA IN SELFING STRAINS OF THE MYXOMYCETE DIDYMIUM IRIDIS

Genetic and cytochemical investigations of the origin, development, nuclear activity, and ploidy level of Plasmodia obtained from selfed clones S-2 and B₁P-33 of the heterothallic myxomycete, Didymium iridis, are presented. To demonstrate that selfing did not result from contamination of the clones, or mutations at the mating-type locus, crosses were made between F₁ clones and clones of known mating types. The data were inconsistent with these two possibilities. DNA was quantified by Feulgen-DNA microspectrophotometry. All cellular phases studied (logarithmic amoebae, swarmers, and encysted amoebae) appear to be haploid, with the nuclear DNA being in the replicated (2C) state. The plasmodia are in all cases diploid; however, the data indicate that the selfed Plasmodia are in an extended G₁ condition. The nuclear DNA content of these is therefore 2C, whereas that of the cross Plasmodium is 4C. Sporangial nuclei exhibit DNA in diploid replicated (4C) category.     

Fine genetic and physical mapping of the CRb gene conferring resistance to clubroot disease in Brassica rapa

The use of clubroot resistance (CR) genes is an effective and economical approach for controlling Plasmodiophora brassicae, the causal agent of clubroot disease in Chinese cabbage (Brassica rapa) and other Brassica crops. In a previous study, we identified and mapped the CRb locus on chromosome A03 of B. rapa in the doubled-haploid (DH) line ‘CR Shinki DH line’ of Chinese cabbage. In this study, CRb, a dominant gene conferring resistance to pathotype 4 of P. brassicae, was finely mapped in combination with bulked segregant analysis and bioinformatics analysis (BIA). Using 1,486 highly susceptible individuals and 2,896 individuals from two separate F₂ populations of ‘702-5’ (B. rapa ssp. chinensis) ×  ‘CR Shinki DH line,’ the CRb locus was narrowed to a region of approximately 0.14 cM between two flanking markers, TCR79 and TCR108. The sequences of seven newly developed markers linked to CRb were landed on bacterial artificial chromosome (BAC) of the reference B. rapa ‘Chiifu-401-42’ by BIA, and a physical map consisting of three BAC clones was constructed. The CRb locus was defined as an interval of approximately 83.5 kb on a BAC clone (KBrB085J21). The target interval contained one Toll-interleukin-1 receptor/nucleotide-binding site/leucine-rich repeat (TIR–NBS–LRR) gene, one NBS–LRR gene, and several putative regulatory genes in the B. rapa genome. The CRb gene was tightly linked to two other CR genes, CRa and CRb ^Kato . These results provide useful information for isolation of the CRb gene and tightly linked molecular markers for breeding CR in B. rapa.     

Evidence for a new early acceptor in Photosystem I of plants. An ESR investigation of reaction center triplet yield and of the reduced intermediary acceptors

The yield of the triplet state of the primary electron donor of Photosystem I of photosynthesis (P^T-700) and the characteristic parameters (g value, line shape, saturation behavior) of the ESR signal of the photoaccumulated intermediary acceptor A have been measured for two types of Photosystem I subchloroplast particles: Triton particles (TSF 1, about 100 chlorophyll molecules per P-700) that contain the iron-sulfur acceptors F_X, F_B and F_A, and lithium dodecyl sulfate (LDS) particles (about 40 chlorophyll molecules per P-700) that lack these iron-sulfur acceptors. The results are: (i) In Triton particles the yield of P^T-700 upon illumination is independent of the redox state of A and of F_X,B,A and is maximally about 5% of the active reaction centers at 5 K. The molecular sublevel decay rates are k_x = 1100 s^?1 ± 10%, k_y = 1300 s^?1 ± 10% and k_z = 83 s^?1 ± 20%. In LDS particles the triplet yield decreases linearly with concentration of reduced intermediary acceptors, the maximal yield being about 4% at 5 K assuming full P-700 activity. (ii) In Triton particles the acceptor complex A consists of two acceptors A₀ and A₁, with A₀ preceding A₁. In LDS particles at temperatures below ?30°C only A₀ is photoactive. (iii) The spin-polarized ESR signal found in the time-resolved ESR experiments with Triton particles is attributed to a polarized P-700-A^?₁ spectrum. The decay kinetics are complex and are influenced by transient nutation effects, even at low microwave power. It is concluded that the lifetime at 5 K of P-700A₀A^?₁ must exceed 5 ms. We conclude that P^T-700 originates from charge recombination of P-700A^?₀, and that in Triton particles A₀ and A₁ are both photoaccumulated upon cooling at low redox potential in the light. Since the state P-700AF^?_X does not give rise to triplet formation the 5% triplet yield in Triton particles is probably due to centers with damaged electron transport.     

Theileria parva: cell culture analysis of clones of macroschizont-infected bovine lymphoblastoid cells

Three clones (H7, D7, and C5) were established from single cells of a bovine lymphoblastoid cell line (IR.TPM.1) infected with macroschizonts of the protozoan parasite Theileria parva. The cloning efficiency using feeder layers was 0.3–0.4. The mean parasite size (the number of parasite nuclei per cell) was different in each clone and was correlated to the growth rate. The fast growing clone, C5 (population doubling time 24 hr), contained smaller (mean parasite nuclear number, 12) parasites than a slow growing clone, D7 (population doubling time, 73 hr; mean number of parasite nuclei per cell, 35.3). The third clone, H7, had an intermediate growth rate (population doubling time, 49 hr) and parasite size (mean nuclei number, 18.1). There was variation in the incidence of microschizonts among the clones but microschizont-free clones were not isolated. When the clones were subjected to 4.3 × 10^?7M aminopterin, 20–25% of the cell population of clones H7 and C5 and the uncloned parent line lost their parasites in 4 days, while it took 7 days to reach a similar result (31% parasite-free cells) in clone D7. We were unable to isolate parasite-free clones from cells treated with aminopterin. Hydroxyurea (4 × 10^?4M) inhibited the growth of clone C5, but the macroschizonts continued to proliferate, and the incidence of cells with microschizonts increased. The size profile analysis showed that most of the aminopterin-treated cells were 9.0 μm, the hydroxyurea-treated cells 14.7 μm, and the untreated cells 10.8 μm in diameter.     

Nitrofuran induced mutagenesis and error prone repair in Escherichia coli

The binding of cis(c)- and trans(t)-Pt(NH₃)₂Cl₂ to DNA at platinum/DNA-nucleotide ratios (R_i) of 0.1 or less has been studied by means of radioactive ^195mPt-labeled compounds. Kinetic data are consistent with the following scheme:

At 25°C and pH 5–6 in 5 mM NaClO₄, the values for the rate constants in the above scheme for the c-isomer are k₂ = 2.2 × 10^?5 sec^?1, k₇ = 0.32 (sec M)^?1, and k₈ = 143 (sec M)^?1; for the t-isomer the values are k₂ < 0.5 × 10^?5 sec^?1 and k₇ = 0.95 (sec M)^?1. Platinum-DNA adducts do not undergo detectable exchange after 3 days at 37°C, indicating the absence of a dynamic equillibrium. For both isomers the rate of binding is the same for single- and double-stranded DNA. The conclusions derived from Ag⁺ and H⁺ titration studies are consistent with binding at guanine N(7) for R_i < 0.1. The reaction rate is competitively inhibited by various salts and buffers and is suppressed by raising the pH (50% inhibition of initial rates at pH 7.3). At 37°C and pH 7 in 0.15 M NaCl, 6–8% of both the c- and t-isomers bind to DNA in 24 h, suggesting that both compounds should bind to DNA under biological conditions.     

Cell type-dependent expression of tubulins in Physarum

Three alpha-tubulins and two beta-tubulins have been resolved by two-dimensional gel electrophoresis of whole cell lysates of Physarum myxamoebae or plasmodia. Criteria used to identify the tubulins included migration on two-dimensional gels with myxamoebal tubulins purified by self-assembly into microtubules in vitro, peptide mapping with Staphylococcus V8 protease and with chymotrypsin, immunoprecipitation with a monoclonal antibody specific for beta-tubulin, and, finally, hybrid selection of specific mRNA by cloned tubulin DNA sequences, followed by translation in vitro. Differential expression of the Physarum tubulins was observed. The alpha 1- and beta 1-tubulins were detected in both myxamoebae and plasmodia; alpha 2 and beta 2 were detected only in plasmodia, alpha 3 was detected only in the myxamoebal phase, and may be specific to the flagellate. Observation of more tubulin species in plasmodia than in myxamoebae was remarkable; the only microtubules detected in plasmodia are those of the mitotoic spindle, whereas myxamoebae display cytoplasmic, centriolar, flagellar, and mitotic-spindle microtubules. In vitro translation of myxamoebal and plasmodial RNAs indicated that there are distinct mRNAs, and therefore probably separate genes, for the alpha 1-, alpha 2-, beta 1-, and beta 2-tubulins. Thus, the different patterns of tubulin expression in myxamoebae and plasmodia reflect differential expression of tubulin genes.     

STABLE DIPLOIDS FROM INTRAGROUP ZYGOSPORES OF CLOSTERIUM EHRENBERGII MENEGH. (CONJUGATOPHYCEAE)1

Two pairs of stable diploid clones were obtained as aberrant forms among F₁ progeny of an intragroup (intraspecific) cross between R-11-4 (mating type +) and M-16-4b (mating type -) of Group A of Closterium ehrenbergii Menegh. Each pair was derived from the two germination products of a single zygospore, and both clones were mating type minus. The cell size range of these four diploid minus clones was considerably above that of normal (haploid) Group A clones. Chromosome counts at the second meiotic metaphase indicated that these clones were diploid with approximately 200 chromosomes, which was double the number for normal Group A clones. Diploid minus clones conjugated normally with any haploid Group A plus clones, and yielded many triploid zygospores. Triploid zygospores germinated normally as did intragroup diploid zygospores. In metaphase I preparations, only bivalents were observed except on a few occasions where some uni- and multivalents were also detected. Viability of F₁ progeny from triploid zygospores (55–74%) was somewhat lower than from diploid zygospores of Japanese Group A populations (65–90%), but higher than intergroup (interspecific) hybrid zygospores from Groups A, B and H (0–12%). In addition to lower viability, some F₁ progeny from triploid zygospores exhibited slow vegetative growth. Almost all pairs of F₁ clones from single triploid zygospores were of opposite mating type, similar to normal diploid zygospores of the intragroup cross. Morphological variability of F₁ progeny of triploid zygospores was great. The apparently normal meiosis of triploid zygospores and the high viability of F₁ progeny suggested that the genome of Group A contains several sets of chromosome complements with mechanisms by which bivalents are regularly formed in the first meiotic division.     

Induction of fruiting in two aggregateless mutants of Dictyostelium discoideum

Five general groups of morphogenetically aberrant mutants of Dictyostelium discoideum were isolated. Each group of mutants was characterized either by the absence of any fruiting structures or by the formation of abnormal fructifications. Among these developmental mutants were two aggregateless isolates, Agg⁻-1 and Agg⁻-2, that could be induced to form normal sorocarps under certain conditions. Sorocarps of the normal D. discoideum type were formed when growing myxamoebae from either of these mutants were allowed to come in contact with myxamoebae of the other mutants, wild-type D. discoideum, D. purpureum, or D. mucoroides. No sorocarps were formed when myxamoebae of Agg⁻-1 and Agg⁻-2 were paired. These two aggregateless mutants, while incapable of aggregating or fruiting when cultivated singly with Escherichia coli B/r on a glucose-salts medium, formed normal fruiting structures after being freed of what appeared to be a product of bacterial growth. The spores produced by Agg⁻-1 and Agg⁻-2 myxamoebae again gave rise to aggregateless clones of the original parental types.     

Emergence of acetylcholine resistance and loss of rhythmic activity associated with the development of hypertension,obesity, and type 2 diabetes

The aim of this work was to study the influence of aging, obesity, metabolic syndrome (MS), hypertension (HT), and type 2 diabetes (T2D) on the endogenous rhythmic activity and the development acetylcholine resistance in aorta rings of male rats. T2D was produced by a free access to fat (lard). It was shown that phenylephrine (PE) or 5-hydroxytryptamine (5-HT) induces two types of rhythmic contractions: with periods T ₁ = 3–10 s and T ₂ = 50–70 s and amplitudes A ₁ = 1–5% and A ₂ = 20–40% of the maximal contraction force (F _max), respectively. Such periodic modes can be caused by the operation of two known positive feedback loops (PFL) based on the Ca²⁺-induced activation of IP3 receptor (IP3R) or phospholipase C PFL1 and PFL2, respectively, and are not eliminated by L-NAME. Slow rhythmic activity induced by acetylcholine (Ach) with period T ₃ = 7–20 min and amplitude A ₃ = 20–30% of F _max was observed only in young animals (under 6 months) and can be determined by the operation of PFL3, involving Ca²⁺, NO, kinase G, cADP-ribose, and the ryanodine receptor (RyR). Fast mode of contractions (T ₁, A ₁) is maintained regardless of age and the presence of MS and HT (140 mm Hg and higher) and disappears only at later stages of the T2D development. Probability of intermediate mode of contractions (T ₂, A ₂) decreases to 0.20–0.25 at the age of 14–16 months or during the development of HT and MS. In these circumstances, Ach could cause relaxation of preconstricted rings only to 40 and 60% of F _max, respectively. At the stages of the T2D development characterized by high values of arterial pressure (above 150 mm Hg) and of the glucose (10–12 mM), ammonium (120–180 μM), and blood lipid levels, as well as by liver dysfunction (fibrosis/cirrhosis), the rhythmic activity of any type is lost and dysfunction of the initial part of the signaling cascade with the participation of PFL3 is manifested by the absence of responses to Ach or L-NAME. Coenzyme NAD (agonist of the P2Y receptors, К⁺ channel activator and a precursor of cADP-ribose) can exert a partial relaxation of aorta rings from healthy animals and animals with MS. Nicotinamide (product and an inhibitor of ADP-ribosyl cyclase) and SNP (donor of NO) produce an effective relaxation of aorta rings from healthy animals and animals with T2D. Relaxing effect of nicotinamide may suggest a tandem operation of IP3R and RyR in the control of intracellular Ca²⁺ stores in vascular cells.     

Identification and characterization of the ATP·Mg-dependent protein phosphatase activator (F A) as a microtubule protein kinase in the brain

The activating factor of ATP·Mg-dependent protein phosphatase (F _A) has been identified in brain microtubules. When using purified MAP-2 (microtubule associated protein 2) and tau proteins as substrates,F _A could phosphorylate MAP-2 to 16 moles of phosphates per mole of protein with aK _m value of 0.4 µM, and tau proteins to 4 moles of phosphates per mole of proteins with aK _m value of about 3 µM. When using microtubules as substrates,F _A could enhance many-fold the endogenous phosphorylation of many microtubule-associated proteins including MAP-2, tau proteins, and several low-molecular-weight MAPs. In contrast to other reported MAP kinases, such as cAMP-dependent protein kinase and Ca⁺²/phospholipid-dependent protein kinase, theF _A-catalyzed phosphorylation of tau proteins could cause an electrophoretic mobility shift on sodium dodecyl sulfate polyacrylamide gel electrophoresis, suggesting that a dramatic conformational change of tau proteins was produced byF _A. Peptide mapping analysis of the phosphopeptides derived from SV8 protease digestion revealed thatF _A could phosphorylate MAP-2 and tau proteins on at least four specific sites distinctly different from those phosphorylated by cAMP-dependent and Ca⁺²/phospholipid-dependent MAP kinases. Quantitative analysis further indicated that approximately 19% of the total endogenous kinase activity in brain microtubules was due toF _A. Taken together, the results provide initial evidence that the ATP·Mg-dependent protein phosphatase activating factor (F _A) is a potent and unique MAP kinase, and may represent one of the major factors involved in phosphorylation of brain microtubules.     

The ultrastructure of mitosis in myxamoebae and plasmodia of Physarum flavicomum

Electron microscopy of glutaraldehyde-osmium-fixed samples of haploid myxamoebae and diploid plasmodia of the myxomycete Physarum flavicomum Berk. reveal dissimilar spindle apparatus during mitosis in the two cell types. Myxamoebae exhibit an astral type of mitosis with centrioles at the poles and nuclear envelope breakdown during prophase. Plasmodial nuclei lack centrioles at mitosis and have an intranuclear spindle, with nuclear envelope persisting during the entire division. Coated vesicles are noted during prophase and telophase in myxamoebae and their role in spindle formation and dispersion is suggested.     

Temporal and spectral characterization of the photosynthetic reaction center from Heliobacterium modesticaldum

A time-resolved spectroscopic study of the isolated photosynthetic reaction center (RC) from Heliobacterium modesticaldum reveals that thermal equilibration of light excitation among the antenna pigments followed by trapping of excitation and the formation of the charge-separated state P₈₀₀ ⁺A₀ ^– occurs within ~25 ps. This time scale is similar to that reported for plant and cyanobacterial photosystem I (PS I) complexes. Subsequent electron transfer from the primary electron acceptor A₀ occurs with a lifetime of ~600 ps, suggesting that the RC of H. modesticaldum is functionally similar to that of Heliobacillus mobilis and Heliobacterium chlorum. The (A₀ ^– ? A₀) and (P₈₀₀ ⁺ ? P₈₀₀) absorption difference spectra imply that an 8¹-OH-Chl a _F molecule serves as the primary electron acceptor and occupies the position analogous to ec3 (A₀) in PS I, while a monomeric BChl g pigment occupies the position analogous to ec2 (accessory Chl). The presence of an intense photobleaching band at 790 nm in the (A₀ ^– ? A₀) spectrum suggests that the excitonic coupling between the monomeric accessory BChl g and the 8¹-OH-Chl a _F in the heliobacterial RC is significantly stronger than the excitonic coupling between the equivalent pigments in PS I.