Synergetic combination of different types of defect to optimize pinning landscape using BaZrO(3)-doped YBa(2)Cu(3)O(7).

Retaining a dissipation-free state while carrying large electrical currents is a challenge that needs to be solved to enable commercial applications of high-temperature superconductivity. Here, we show that the controlled combination of two effective pinning centres (randomly distributed nanoparticles and self-assembled columnar defects) is possible and effective. By simply changing the temperature or growth rate during pulsed-laser deposition of BaZrO(3)-doped YBa(2)Cu(3)O(7) films, we can vary the ratio of these defects, tuning the field and angular critical-current (Ic) performance to maximize Ic. We show that the defects' microstructure is governed by the growth kinetics and that the best results are obtained with a mixture of splayed columnar defects and random nanoparticles. The very high Ic arises from a complex vortex pinning landscape where columnar defects provide large pinning energy, while splay and nanoparticles inhibit flux creep. This knowledge is used to produce thick films with remarkable Ic(H) and nearly isotropic angle dependence.

Anisotropic spin fluctuations and superconductivity in "115" heavy fermion compounds: 59Co NMR study in PuCoGa5.

We report results of 59Co nuclear magnetic resonance measurements on a single crystal of superconducting PuCoGa5 in its normal state. The nuclear spin-lattice relaxation rates and the Knight shifts as a function of temperature reveal an anisotropy of spin fluctuations with finite wave vector q. By comparison with the isostructural members, we conclude that antiferromagnetic XY-type anisotropy of spin fluctuations plays an important role in mediating superconductivity in these heavy fermion materials.

Hybridization and superconducting gaps in the heavy-fermion superconductor PuCoGa5 probed via the dynamics of photoinduced quasiparticles.

We have examined the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor PuCoGa5. The deduced electron-phonon coupling constant is incompatible with the measured superconducting transition temperature Tc=18.5 K, which speaks against phonon-mediated superconductivity. Upon lowering the temperature, we observe an order-of-magnitude increase of the quasiparticle relaxation time in agreement with the phonon bottleneck scenario--evidence for a hybridization gap in the electronic density of states. The modification of photoinduced reflectance in the superconducting state is consistent with the heavy character of the quasiparticles that participate in Cooper pairing.

dMi-2, a hunchback-interacting protein that functions in polycomb repression.

Early in Drosophila embryogenesis, gap gene products directly repress transcription of homeotic (HOX) genes and thereby delimit HOX expression domains. Subsequently, Polycomb-group proteins maintain this repression. Currently, there is no known molecular link between gap and Polycomb-group proteins. Here, dMi-2 is identified as a protein that binds to a domain in the gap protein Hunchback that is specifically required for the repression of HOX genes. Genetic analyses show that dMi-2 participates in both Hunchback and Polycomb repression in vivo. Hence, recruitment of dMi-2 may serve as a link between repression of HOX genes by Hunchback and Polycomb proteins.

Transcriptional activation of Drosophila homeotic genes from distant regulatory elements.

In Drosophila the genes responsible for specifying segment identity (the homeotic genes) are transcribed in complex patterns during development. Mutations that mimic loss of homeotic gene activity identify cis-acting DNA sequences and trans-acting proteins required for transcriptional activation. Some of the trans-acting proteins may facilitate interactions between cis-regulatory elements and the promoter by bringing together distant chromosomal elements.

The Genetic and Cytological Organization of the Y Chromosome of DROSOPHILA MELANOGASTER.

Cytological and genetic analyses of 121 translocations between the Y chromosome and the centric heterochromatin of the X chromosome have been used to define and localize six regions on the Y chromosome of Drosophila melanogaster necessary for male fertility. These regions are associated with nonfluorescent blocks of the Y chromosome, as revealed using Hoechst 33258 or quinacrine staining. Each region appears to contain but one functional unit, as defined by failure of complementation among translocations with breakpoints within the same block. The distribution of translocation breakpoints examined appears to be nonrandom, in that breaks occur preferentially in the nonfluorescent blocks and not in the large fluorescent blocks.

Analysis of Y-Linked Mutations to Male Sterility in DROSOPHILA MELANOGASTER.

The frequencies of newly induced male-sterilizing lesions on both the X and Y chromosomes of Drosophila melanogaster were determined after either 4000 r of gamma-irradiation or adult feeding of ethyl methanesulfonate. The Y chromosome is approximately twice as sensitive as the X chromosome to newly induced male-sterilizing lesions after gamma-irradiation, but slightly less sensitive after ethyl methanesulfonate treatment. A large proportion of the radiation-induced lesions are associated with Y-autosome or X-autosome translocations, with the Y chromosome recovered in translocations far in excess of the frequency expected from metaphase lengths. Although translocations between the X and Y chromosomes or between autosomes do not appear to sterilize heterozygous males, interchanges between sex chromosomes and autosomes often sterilize males carrying them in a dominant manner, suggesting that the organization of the genome is critical for normal spermatogenesis. Complementation tests between recessive Y-linked male-sterilizing mutants do not reveal the existence of any additional fertility loci beyond the six previously defined.

Spontaneous mitotic recombination and evidence for an X-ray-inducible system for the repair of DNA damage in Drosophila melanogaster.

Spontaneous mitotic recombination in the left and of chromosome 3 was examined in both unirradiated control flies and sibs irradiated early in development by determining the sizes and frequencies of multiple-wing-hair (mwh) clones in the wing blade of heterozygous mwh/+ flies. Approximately 16% of the spontaneous mwh clones arise from events generating cells with normal division rates. The remaining 84% result from events generating cells with an average cell division rate one-third that of the surrounding cells; these are thought to result from events that generate aneuploid cells. Such clones probably arise from a failure correctly to repair spontaneous DNA damage. The frequency of spontaneous events late in development decreases significantly after irradiation as much as 150 hours earlier in development. The suppression of spontaneous events decreases with a longer period of time between irradiation and the final cell divisions in the wing blade. These results suggest the existence of a repair system for DNA damage in Drosophila that is induced by irradiation. The decrease in effect with time following irradiation could result from slow degradation or dilution by subsequent cell growth and division.

Dosage-Dependent Modifiers of Homoeotic Mutations in Drosophila melanogaster.

The determination of segment identity in Drosophila melanogaster appears to be controlled by a small number of genes. In order to identity new components in the process, we have systematically screened the autosomal complement for loci that show a dosage-dependent interaction with mutations in previously characterized genes thought to be important in the determination of segment identity. The dominant homoeotic phenotype of mutations at four loci involved in thoracic leg determination (Pc, Pcl, Antp and Scr) were quantitated in flies bearing a series of synthetic duplications covering more than 99% of the autosomal complement. Twelve regions were identified that when present in three wild-type copies strongly enhanced or suppressed the phenotype of mutations at one or more of the four homoeotic loci examined. The effects of five of these regions appear to correspond to previously described homoeotic loci; the effects of the remaining seven appear to identify new loci involved in the determination of segment identity.

Reciprocal recombination and the evolution of the ribosomal gene family of Drosophila melanogaster.

The role of reciprocal recombination in the coevolution of the ribosomal RNA gene family on the X and Y chromosomes of Drosophila melanogaster was assessed by determining the frequency and nature of such exchange. In order to detect exchange events within the ribosomal RNA gene family, both flanking markers and restriction fragment length polymorphisms within the tandemly repeated gene family were used. The vast majority of crossovers between flanking markers were within the ribosomal RNA gene region, indicating that this region is a hotspot for heterochromatic recombination. The frequency of crossovers within the ribosomal RNA gene region was approximately 10(-4) in both X/X and X/Y individuals. In conjunction with published X chromosome-specific and Y chromosome-specific sequences and restriction patterns, the data indicate that reciprocal recombination alone cannot be responsible for the observed variation in natural populations.

Genetic analysis of the brahma gene of Drosophila melanogaster and polytene chromosome subdivisions 72AB.

The brahma gene is required for activation of the homeotic genes of the Antennapedia and bithorax complexes in Drosophila. We have isolated and characterized 21 mutations in brahma. We show that both maternal and zygotic functions of brahma are required during embryogenesis. In addition, the severe abnormalities caused by loss of maternal brahma expression show that the homeotic genes are not the only targets for brahma activation. The complex pattern of interallelic complementation for the 21 brahma alleles suggests that brahama may act as a multimer. In addition to mutations in brahma, we have isolated mutations in four other essential genes within polytene chromosome subdivisions 72AB. Based on a compilation of similar studies that include about 24% of the genome, we estimate that about 3600 genes in Drosophila can mutate to cause recessive lethality, with fewer than 900 additional genes essential only for gametogenesis. We have identified three times more transcripts than lethal complementation groups in 72AB. One transcript in 72AB is the product of the essential arf-like gene and encodes a member of the ARF subfamily of small GTP-binding proteins. Two other transcripts are probably the products of a single gene whose protein products are similar to the catalytic subunits of cAMP-dependent protein kinases.

Genetic analysis of brahma: the Drosophila homolog of the yeast chromatin remodeling factor SWI2/SNF2.

The Drosophila brahma (brm) gene encodes an activator of homeotic genes related to the yeast chromatin remodeling factor SWI2/SNF2. Here, we report the phenotype of null and dominant-negative brm mutations. Using mosaic analysis, we found that the complete loss of brm function decreases cell viability and causes defects in the peripheral nervous system of the adult. A dominant-negative brm mutation was generated by replacing a conserved lysine in the ATP-binding site of the BRM protein with an arginine. This mutation eliminates brm function in vivo but does not affect assembly of the 2-MD BRM complex. Expression of the dominant-negative BRM protein caused peripheral nervous system defects, homeotic transformations, and decreased viability. Consistent with these findings, the BRM protein is expressed at relatively high levels in nuclei throughout the developing organism. Site-directed mutagenesis was used to investigate the functions of conserved regions of the BRM protein. Domain II is essential for brm function and is required for the assembly or stability of the BRM complex. In spite of its conservation in numerous eukaryotic regulatory proteins, the deletion of the bromodomain of the BRM protein has no discernible phenotype.

The Drosophila homeotic gene moira regulates expression of engrailed and HOM genes in imaginal tissues.

moira is a member of the trithorax group of homeotic gene regulators in Drosophila melanogaster. We show that moira is required for the function of multiple homeotic genes of the Antennapedia and bithorax complexes (HOM genes) in most imaginal tissues and that the requirement for moira function is at the level of transcription. moira is also required for transcription of the engrailed segmentation gene in the imaginal wing disc. The abnormalities caused by the loss of moira function in germ cells suggests that at least one other target gene requires moira for normal oogenesis.

Cloning and characterization of the Drosophila melanogaster CDK5 homolog.

The D. melanogaster homolog of mammalian CDK5 has been cloned and its chromosomal location determined. The gene for Cdk5 consists of 4 exons separated by 3 short introns ranging in size from 61-160 bp. Northern blot analysis revealed a single mRNA of approximately 1.6 kb that is expressed at highest levels in the adult fly. The putative amino acid sequence for Drosophila Cdk5 predicts a protein with a mass of approximately 32 kDa that is 77% identical to its mammalian counter-parts. Drosophila Cdk5 gene is located in polytene chromosomal region 52BC of the right arm of chromosome 2. This study provides the framework for a molecular genetic analysis of CDK5 function.

Regulation of the Sex combs reduced gene in Drosophila.

The Sex combs reduced gene of the Antennapedia complex specifics the identities of the anterior thoracic and posterior head segments, including the primordium of the larval salivary gland. The Sex combs reduced transcription unit spans over 30 kb of genomic DNA, with another 40 kb of upstream cis-regulatory sequences. The pattern of Sex combs reduced transcription is set in the early embryo by the segmentation genes and is then maintained by two competing sets of proteins, the Polycomb group and the trithorax group. One of the trithorax group genes required for activation, the brahma gene, encodes an evolutionarily conserved DNA-stimulated ATPase that is part of a large protein complex. This complex facilitates the action of sequence-specific, DNA-binding proteins in regulating target genes, possibly by altering chromatin structure.

Magnetic order in Pu2M3Si5 (M = Co, Ni).

The physical properties including magnetic susceptibility, specific heat, and electrical resistivity of two new plutonium compounds Pu2M3Si5 (M = Co, Ni) are reported. Pu2Ni3Si5 crystallizes in the orthorhombic U2Co3Si5 structure type, which can be considered a variant of the BaAl4 tetragonal structure, while Pu2Co3Si5 adopts the closely related monoclinic Lu2Co3Si5 type. Magnetic order is observed in both compounds, with Pu2Ni3Si5 ordering ferromagnetically at T(C) = 65 K then undergoing a transition into an antiferromagnetic state below T(N) = 35 K. Two successive magnetic transitions are also observed at T(mag1) = 38 K and T(mag2) = 5 K in Pu2Co3Si5. Specific heat measurements reveal that these two materials have a moderately enhanced Sommerfeld coefficient γ ∼ 100 mJ/mol Pu K(2) in the magnetic state with comparable RKKY and Kondo energy scales.

The Polycomb and trithorax group proteins of Drosophila: trans-regulators of homeotic gene function.

The Polycomb and trithorax group genes encode trans-regulators of homeotic gene function in Drosophila. The Polycomb group genes encode transcriptional repressors, while the trithorax group proteins are positive factors required for homeotic gene function. Among the Polycomb group proteins, the POLYCOMB protein has been most extensively characterized. The POLYCOMB protein contains a chromodomain, a conserved domain found in a Drosophila protein with effects on position-effect variegation. Among the trithorax group proteins characterized, the BRAHMA protein appears to be a subunit of a protein complex conserved from yeast to man (the SNF/SWI complex) that modifies chromatin to facilitate the transcriptional activation by gene-specific DNA-binding proteins. The ZESTE protein may help to activate transcription by bringing distant cis-regulatory elements closer to promoter-bound proteins.

Positional signaling by hedgehog in Drosophila imaginal disc development.

We describe a dominant gain-of-function allele of the segment polarity gene hedgehog. This mutation causes ectopic expression of hedgehog mRNA in the anterior compartment of wing discs, leading to overgrowth of tissue in the anterior of the wing and partial duplication of distal wing structures. The posterior compartment of the wing is unaffected. Other imaginal derivatives are affected, resulting in duplications of legs and antennae and malformations of eyes. In mutant imaginal wing discs, expression of the decapentaplegic gene, which is implicated in the hedgehog signaling pathway, is also perturbed. The results suggest that hedgehog protein acts in the wing as a signal to instruct neighboring cells to adopt fates appropriate to the region of the wing just anterior to the compartmental boundary.

Trans-regulation of homeotic genes in Drosophila.

Homeotic genes of the Antennapedia and bithorax complexes control Drosophila development by encoding DNA-binding proteins that regulate the transcription of target genes. Because either the presence or absence of these DNA-binding proteins alters development, regulation of the spatial patterns of expression is crucial to normal development. Numerous gene products are required for properly regulated expression of Antennapedia and bithorax complex genes, but few (if any) are dedicated solely to the regulation of these genes. One of the pleiotropic activators of homeotic genes in Drosophila, the brahma gene, encodes a protein similar to a yeast protein that is required for transcriptional activation of multiple tightly regulated genes. Other components of this system may be conserved as well, suggesting that the biochemical basis for induced gene expression in single-celled organisms may have more in common with programmed developmental pathways in multicellular organisms than previously thought.