Controllable physicochemical properties of WOx thin films grown under glancing angle

In this work, various physicochemical properties are investigated in nanostructured WOx thin films prepared by radio-frequency magnetron sputtering for optoelectronic applications. A glancing angle of 87° is employed to grow films of different thicknesses, which are then exposed to post-growth annealing. Detailed local probe analyses in terms of morphology and work function of WOx films are carried out to investigate thickness-dependent property modulations of the as-deposited and annealed films. The analyses show a reasonably good correlation with photoelectron spectroscopic measurements on the films and the bulk I-V characteristics acquired on a series of WOx/p-Si heterojunction diodes. The presence of a critical WOx thickness is identified to regulate the rectification ratio values at the WOx/p-Si heterostructures and increase in series resistance within the bulk of the films. The present study provides valuable insights to correlate optical, electrical, and structural properties of WOx thin films, which will be beneficial for fabricating WOx-based optoelectronic devices, including photovoltaic cells.

Site-Specific Emulation of Neuronal Synaptic Behavior in Au Nanoparticle-Decorated Self-Organized TiOx Surface

Neuromorphic computing is a potential approach for imitating massive parallel processing capabilities of a bio-synapse. To date, memristors have emerged as the most appropriate device for designing artificial synapses for this purpose due to their excellent analog switching capacities with high endurance and retention. However, to build an operational neuromorphic platform capable of processing high-density information, memristive synapses with nanoscale footprint are important, albeit with device size scaled down, retaining analog plasticity and low power requirement often become a challenge. This paper demonstrates site-selective self-assembly of Au nanoparticles on a patterned TiOx layer formed as a result of ion-induced self-organization, resulting in site-specific resistive switching and emulation of bio-synaptic behavior (e.g., potentiation, depression, spike rate-dependent and spike timing-dependent plasticity, paired pulse facilitation, and post tetanic potentiation) at nanoscale. The use of local probe-based methods enables nanoscale probing on the anisotropic films. With the help of various microscopic and spectroscopic analytical tools, the observed results are attributed to defect migration and self-assembly of implanted Au atoms on self-organized TiOx surfaces. By leveraging the site-selective evolution of gold-nanostructures, the functionalized TiOx surface holds significant potential in a multitude of fields for developing cutting-edge neuromorphic computing platforms and Au-based biosensors with high-density integration.

Perspectives on metal induced crystallization of a-Si and a-Ge thin films

In recent times, the metal induced crystallization (MIC) process in amorphous semiconductors (a-Si and a-Ge) has been extensively investigated by many researchers due to potential applications of crystalline semiconductors in high-density data storage devices, flat panel displays, and high performance solar cells. In this context, we have presented a review on different schemes of MIC in metal/a-Si and metal/a-Ge bilayer films (with stacking change) on various substrates under different annealing conditions. The parameters, which limit crystallization of a-Si and a-Ge have been analyzed and discussed extensively keeping in mind their applications in solar cells and flat panel displays. The MIC of a-Si and a-Ge films under ion beam irradiation has also been discussed in detail. At the end, some suggestions to overcome the limitations of the MIC process in producing better crystalline semiconductors have been proposed. We believe that this review article will inspire readers to perform a thorough investigation on various aspects of MIC for further development of high efficiency solar cells and high quality flat panel displays.

An assessment on crystallization phenomena of Si in Al/a-Si thin films via thermal annealing and ion irradiation

In the present study, crystallization of amorphous-Si (a-Si) in Al/a-Si bilayer thin films under thermal annealing and ion irradiation has been investigated for future solar energy materials applications. In particular, the effect of thickness ratio (e.g. in Al : a-Si, the ratio of the Al and a-Si layer thickness) and temperature during irradiation on crystallization of the Si films has been explored for the first time. Two sets of samples with thickness ratio 1 : 1 (set-A: 50 nm Al/50 nm a-Si) and thickness ratio 1 : 3 (set-B: 50 nm Al/150 nm a-Si) have been prepared on thermally oxidized Si-substrates. In one experiment, thermal annealing of the as-prepared sample (of both the sets) has been done at different temperatures of 100 °C, 200 °C, 300 °C, 400 °C, and 500 °C. Significant crystallization was found to initiate at 200 °C with the help of thermal annealing, which increased further by increasing the temperature. In another experiment, ion irradiation on both sets of samples has been carried out at 100 °C and 200 °C using 100 MeV Ni7+ ions with fluences of 1 × 1012 ions per cm2, 5 × 1012 ions per cm2, 1 × 1013 ions per cm2, and 5 × 1013 ions per cm2. Significant crystallization of Si was observed at a remarkably low temperature of 100 °C under ion irradiation. The samples irradiated at 100 °C show better crystallization than the samples irradiated at 200 °C. The maximum crystallization of a-Si has been observed at a fluence of 1 × 1012 ions per cm2, which was found to decrease with increasing ion fluence at both temperatures (i.e. 100 °C & 200 °C). The crystallization of a-Si is found to be better for set-B samples as compared to set-A samples at all the fluences and irradiation temperatures. The present work is aimed at developing the understanding of the crystallization process, which may have significant advantages for designing crystalline layers at lower temperature using appropriate masks for irradiation at the desired location. The detailed mechanisms behind all the above observations are discussed in this paper.

Crystallization of Ge in ion-irradiated amorphous-Ge/Au thin films

Herein, the structural, optical, and electrical properties of Au-induced crystallization in amorphous germanium (a-Ge) thin films are presented for future solar energy material applications.

Growth of Wafer-Scale ReS2 with "Tunable" Geometry toward Electron Field-Emission Application

Despite high potential, the promise of 2D materials has not been realized practically because of limits of tiny grown size and difficult manipulation of the active spot. The utilization of 2D layers is the ultimate approach, which should be supported by large-scale production. In this very first report, we demonstrate the wafer-scale production of ReS₂ using the conventional sputtering method. The controllability of ReS₂ geometry has been investigated to form typical thin films or vertically aligned layers that are further applied for field emission. The vertically aligned ReS₂ layers exhibit ultralow turn-on electric field (0.6 V μm^-1) with the current density (0.6 mA cm^-2) and significantly low threshold electric field (0.8 V μm^-1), respectively, along with outstanding emission stability. The results are attributed to weakly coupled ReS₂ layers and the high geometrical field enhancement factor (∼1.08 × 10⁵). Further, Kelvin probe force microscopy measurements confirm that lowering the work function is not solely responsible to achieve the ultralow operative field. Moreover, finite element simulation suggests that not only the length, width, and separation of the nanostructures but also the local slope plays an important role in suppressing screening effects.

A Transparent Photonic Artificial Visual Cortex

Mimicking brain-like functionality with an electronic device is an essential step toward the design of future technologies including artificial visual and memory applications. Here, a proof-of-concept all-oxide-based (NiO/TiO₂ ) highly transparent (54%) heterostructure is proposed and demonstrated, which mimics the primitive functions of the visual cortex. Specifically, orientation selectivity and spatiotemporal processing similar to that of the visual cortex are demonstrated using direct optical stimuli under the self-biased condition due to photovoltaic effect, illustrating an energy-efficient approach for neuromorphic computing. The photocurrent of the device can be modulated from zero to 80 µA by simply rotating the slit by 90°. The device shows fast rise and fall times of 3 and 6 ms, respectively. Based on Kelvin probe force measurements, the observed results are attributed to a lateral photovoltaic effect. This highly transparent, self-biased, photonic triggered device paves the way for the advancement of energy-efficient neuromorphic computation.

Effect of Fe ion implantation on the thermoelectric properties and electronic structures of CoSb3 thin films

In the present study, thin films of single-phase CoSb₃ were deposited onto Si(100) substrates via pulsed laser deposition (PLD) method using a polycrystalline target of CoSb₃. These films were implanted by 120 keV Fe-ions with three different fluences: 1 × 10¹⁵, 2.5 × 10¹⁵ and 5 × 10¹⁵ ions per cm². All films were characterised by X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), Rutherford backscattering (RBS) spectrometry and X-ray absorption spectroscopy (XAS). XRD data revealed that the ion implantation decreased the crystalline nature of these films, which are recovered after the rapid thermal annealing process. The Seebeck coefficient S vary with the fluences in the temperature range of 300 K to 420 K, and is found to be highest (i.e., 254 μV K⁻¹) at 420 K for the film implanted with 1 × 10¹⁵ ions per cm². The high S and low resistivity lead to the highest power factor for the film implanted with 1 × 10¹⁵ ions per cm² (i.e., 700 μW m⁻¹ K⁻²) at 420 K. The changing of the sign of S from negative for the pristine film to positive for the Fe-implanted samples confirm that the Fe ions are electrically active and act as electron acceptors by replacing the Co atoms. XAS measurements confirm that the Fe ions occupied the Co site in the cubic frame of the skutterudite and exist in the 3+ oxidation state in this structure.

The power factor for the Fe ion-implanted samples is greater than that of the pristine sample with a value of 700 mW m⁻¹ K⁻² at 420 K for the I_1E15A sample.

Temporal evolution on SiO2 surface under low energy Ar+-ion bombardment: roles of sputtering, mass redistribution, and shadowing

Self-organized pattern evolution on SiO₂ surface under low energy Ar-ion irradiation has been investigated extensively at varied ion energies, angles of ion incidence, and ion flux. Our investigations reveal an instability on SiO₂ surface in an angular window of 40° ̶ 70° and for a comprehensive range of Ar-ion energies (200-1000 eV). Different topographical features, viz. ripples, mounds, and elongated nanostructures evolve on the surface, depending upon the angle of incidence and ion fluence. The results are compiled in the form of a parametric phase diagram (ion energy versus angle of incidence) which summarizes the pattern formation on SiO₂ surface. To understand the evolution of observed patterns, we have carried out theoretical estimation, taking into account the synergetic roles of ion induced curvature-dependent sputter erosion and prompt atomic redistribution. It is shown that irradiation-induced mass redistribution of target atoms plays a crucial role in determining the critical angle of ion incidence for pattern formation on SiO₂ under the present experimental conditions, whereas the contribution of curvature-dependent sputtering needs to be considered to understand the existence of the angular window of pattern formation. In addition, ion-beam shadowing by surface features are shown to play a dominant role in the formation of mounds and elongated structures at higher ion fluences.

Strong uniaxial magnetic anisotropy in Co films on highly ordered grating-like nanopatterned Ge surfaces

We present a systematic investigation on uniaxial magnetic anisotropy (UMA) in Co thin films induced by high aspect ratio nanopatterned anisotropic substrates. Self-organized long grating-like nanostructures, with extreme regularities, are fabricated on Ge surfaces using Au-ion implantation at room temperature. Subsequently deposition of Co films are carried out on the same at two different angles. Magneto-optical Kerr effect measurements show strong UMA in Co films grown on ion-patterned Ge substrates, fabricated under different ion fluences, along and perpendicular to the direction of the patterns (long grating-like nanostructures). Magnetic force microscopy measurements under different externally applied magnetic fields reveal an easy domain wall motion when the field is applied along the grating-like nanostructures. On the other hand, high amplitude grating-like nanostructures hinder the spin rotation when the field is applied along the hard axis. The present study will be useful for magnetic recording media and ultra-small magnetic field sensors.

Surfing Silicon Nanofacets for Cold Cathode Electron Emission Sites

Point sources exhibit low threshold electron emission due to local field enhancement at the tip. In the case of silicon, however, the realization of tip emitters has been hampered by unwanted oxidation, limiting the number of emission sites and the overall current. In contrast to this, here, we report the fascinating low threshold (∼0.67 V μm^-1) cold cathode electron emission from silicon nanofacets (Si-NFs). The ensembles of nanofacets fabricated at different time scales, under low energy ion impacts, yield tunable field emission with a Fowler-Nordheim tunneling field in the range of 0.67-4.75 V μm^-1. The local probe surface microscopy-based tunneling current mapping in conjunction with Kelvin probe force microscopy measurements revealed that the valleys and a part of the sidewalls of the nanofacets contribute more to the field emission process. The observed lowest turn-on field is attributed to the absence of native oxide on the sidewalls of the smallest facets as well as their lowest work function. In addition, first-principle density functional theory-based simulation revealed a crystal orientation-dependent work function of Si, which corroborates well with our experimental observations. The present study demonstrates a novel way to address the origin of the cold cathode electron emission sites from Si-NFs fabricated at room temperature. In principle, the present methodology can be extended to probe the cold cathode electron emission sites from any nanostructured material.

Wavelet-Based Recognition of Handwritten Characters Using Artificial Neural Network

In the present chapter, the widely common problem of handwritten character recognition has been tackled with multiresolution technique using discrete wavelet transform and artificial neural networks. The technique has been tested and found to be more accurate and economic in respect of the recognition process time of the system. Features of the handwritten character images are extracted by discrete wavelet transform used with appropriate level of multiresolution technique, then the artificial neural networks is trained by extracted features. The unknown input handwritten character images are recognized by trained artificial neural networks system. The proposed method provides good recognition accuracy for handwritten characters with less training time, less no. of samples and less no. of iterations.

Gold-decorated highly ordered self-organized grating-like nanostructures on Ge surface: Kelvin probe force microscopy and conductive atomic force microscopy studies

Nanoarchitecture by atomic manipulation is considered to be one of the emerging trends in advanced functional materials. It has a gamut of applications to offer in nanoelectronics, chemical sensing, and nanobiological science. In particular, highly ordered one-dimensional semiconductor nanostructures fabricated by self-organization methods are in high demand for their high aspect ratios and large number of applications. An efficient way of fabricating semiconductor nanostructures is by molecular beam epitaxy, where atoms are added to a crystalline surface at an elevated temperature during growth, yielding the desired structures in a self-assembled manner. In this article, we offer a room temperature process, in which atoms are sputtered away by ion impacts. Using gold ion implantation, the present study reports on the formation of highly ordered self-organized long grating-like nanostructures, with grooves between them, on a germanium surface. The ridges of the patterns are shown to have flower-like protruding nanostructures, which are mostly decorated by gold atoms. By employing local probe microscopic techniques like Kelvin probe force microscopy and conductive atomic force microscopy, we observe a spatial variation in the work function and different nanoscale electrical conductivity on the ridges of the patterns and the grooves between them, which can be attributed to gold atom decorated ridges. Thus, the architecture  presented offers the advantage of using the patterned germanium substrates as periodic arrays of conducting ridges and poorly conducting grooves between them.

Spread and exchange of bla NDM-1 in hospitalized neonates: role of mobilizable genetic elements

To investigate the mobilizable elements associated with bla _NDM-1 in Enterobacteriaceae isolated from septicaemic neonates at a NICU in India, during December, 2008-2011. An attempt was also made to understand whether there was a pattern in the temporal acquisition of bla _NDM-1 within the unit. Transferability of carbapenem resistance was tested by conjugation and transformation. Plasmid types and addiction systems were analysed. The genetic background of bla _NDM-1 and association with class 1 integron were evaluated by PCR mapping. RFLP was carried out to discriminate plasmids of same incompatibility group. Transfer of carbapenem resistance was successful in 13/15 cases. bla _NDM-1 was associated with different plasmid scaffolds (IncFII, IncL/M, IncN, IncR, IncHIB-M/FIB-M), IncF type being the prevalent one. Addiction systems ccdAB and hok/sok were associated with transferable plasmids. Genetic structures surrounding bla _NDM-1 showed its association with at least a remnant of ISAba125 at its 5'-end. The spread of NDM-1 was not related to class 1 integron which possessed resistance determinants against trimethoprim (dfrA12, dfrA1, dfrA5), streptomycin (aadA2, aacA4), and rifampicin (arr-3). RFLP showed that three isolates possessed the same FII/FIIs plasmid; two of these three isolates were from a single neonate, implying interspecies transfer of bla _NDM-1. The predominance of FII plasmids and ISAba125 along with bla _NDM-1 was noted, but no specific pattern in the temporal acquisition of mobile genetic elements could be identified. To the best of our knowledge, this report is the first to inform the in-vivo interspecies plasmid transfer event of bla _NDM-1 in a neonate.

Field-induced doping-mediated tunability in work function of Al-doped ZnO: Kelvin probe force microscopy and first-principle theory

We demonstrate that the work function of Al-doped ZnO (AZO) can be tuned externally by applying an electric field. Our experimental investigations using Kelvin probe force microscopy show that by applying a positive or negative tip bias, the work function of AZO film can be enhanced or reduced, which corroborates well with the observed charge transport using conductive atomic force microscopy. These findings are further confirmed by calculations based on first-principles theory. Tuning the work function of AZO by applying an external electric field is not only important to control the charge transport across it, but also to design an Ohmic contact for advanced functional devices.

Tunable wettability of Si through surface energy engineering by nanopatterning

Schematic diagram of a water droplet on an isotropic (flat) and anisotropic (rippled) surfaces.

Facile synthesis of a superhydrophobic and colossal broadband antireflective nanoporous GaSb surface

Ion implantation creates a superhydrophobic and colossal antireflective nanoporous GaSb.

Structural defect-dependent resistive switching in Cu-O/Si studied by Kelvin probe force microscopy and conductive atomic force microscopy

In this study, we show structural defect-dependent presence or absence of resistive switching in Cu-O films. We use Kelvin probe force microscopy and conductive atomic force microscopy to show the presence of resistive switching. In addition, local current mapping provides direct evidence on the formation of nanoscale filament. These findings match well with the existing theoretical model on resistive switching. In particular, understanding the role of structural defects in resistive switching can be considered as critically important to take a step forward for designing advanced nanoscale memory devices.

Semi-insulating behaviour of self-assembled tin(IV)corrole nanospheres

Three novel tin(iv)corrole complexes have been prepared and characterized by various spectroscopic techniques including single crystal X-ray structural analysis. Packing diagrams of the tin(iv)corroles revealed that corrolato-tin(iv)-chloride molecules are interconnected by intermolecular C-HCl hydrogen bonding interactions. HCl distances are 2.848 Å, 3.051 Å, and 2.915 Å, respectively, for the complexes. In addition, the C-HCl angles are 119.72°, 144.70°, and 147.08°, respectively, for the complexes. It was also observed that in one of the three synthesized complexes dimers were formed, while in the other two cases 1D infinite polymer chains were formed. Well-defined and nicely organized three-dimensional hollow nanospheres (SEM images on silicon wafers) with diameters of ca. 676 nm and 661 nm are obtained in the complexes, forming 1D polymer chains. By applying a thin layer of tin(iv)corrole nanospheres to an ITO surface (AFM height images of ITO films; ∼200 nm in height), a device was fabricated with the following composition: Ag/ITO-coated glass/tin(iv)corrole nanospheres/ITO-coated glass/Ag. The resistivity (ρ) of the nanostructured film was calculated to be ∼2.4 × 10(8) Ω cm, which falls in the range of semi-insulating semiconductors. CAFM current maps at 10 V bias show bright spots with a 10-20 pA intensity and indicate that the nanospheres (∼250 nm in diameter) are the electron-conducting pathway in the device. The semi-insulating behavior arises from the non-facile electron transfer in the HOMOs of the tin(iv)corrole nanospheres.

Tunable antireflection from conformal Al-doped ZnO films on nanofaceted Si templates

Photon harvesting by reducing reflection loss is the basis of photovoltaic devices. Here, we show the efficacy of Al-doped ZnO (AZO) overlayer on ion beam-synthesized nanofaceted silicon for suppressing reflection loss. In particular, we demonstrate thickness-dependent tunable antireflection (AR) from conformally grown AZO layer, showing a systematic shift in the reflection minima from ultraviolet to visible to near-infrared ranges with increasing thickness. Tunable AR property is understood in light of depth-dependent refractive index of nanofaceted silicon and AZO overlayer. This improved AR property significantly increases the fill factor of such textured heterostructures, which reaches its maximum for 60-nm AZO compared to the ones based on planar silicon. This thickness matches with the one that shows the maximum reduction in surface reflectance.

PACS

81.07.-b; 42.79.Wc; 81.16.Rf; 81.15.Cd.

Synthesis, electron transport, and charge storage properties of fullerene–zinc porphyrin hybrid nanodiscs

A MIS-type diode (Ag/C₆₀–ZnTANP circular discs/p-Si/Ag) has been fabricated and its charge storage properties have been explored.

Transition from ripples to faceted structures under low-energy argon ion bombardment of silicon: understanding the role of shadowing and sputtering

In this study, we have investigated temporal evolution of silicon surface topography under 500-eV argon ion bombardment for two angles of incidence, namely 70° and 72.5°. For both angles, parallel-mode ripples are observed at low fluences (up to 2 × 1017 ions cm-2) which undergo a transition to faceted structures at a higher fluence of 5 × 1017 ions cm-2. Facet coarsening takes place at further higher fluences. This transition from ripples to faceted structures is attributed to the shadowing effect due to a height difference between peaks and valleys of the ripples. The observed facet coarsening is attributed to a mechanism based on reflection of primary ions from the facets. In addition, the role of sputtering is investigated (for both the angles) by computing the fractional change in sputtering yield and the evolution of surface roughness. PACS: 81.05.Cy, 81.16.Rf, 61.80.Jh, 87.64.Dz.

Paramagnetism and antiferromagnetic interactions in Cr-doped GaN

We report on the magnetic and structural properties of Cr-doped GaN prepared by ion implantation of epitaxial thin films. Based on a detailed analysis of the magnetometry data, we demonstrate that the magnetic interactions between Cr moments in GaN are antiferromagnetic (AFM). Increasing the Cr fractional concentration up to 0.35, we observe that strong nearest cation neighbor AFM coupling results in the reduction of the effective moment per Cr atom. The uncompensated Cr moments exhibit paramagnetic behavior and we discuss to what extent the effects of an anisotropic crystal field and AFM interactions can be inferred from the magnetization data. We discuss the observed changes in magnetic and structural properties induced by thermal annealing in terms of defect annealing and Cr aggregation.

Effect of substrate temperature on implantation doping of Co in CdS nanocrystalline thin films

We demonstrate doping of nanocrystalline CdS thin films with Co ions by ion implantation at an elevated temperature of 573 K. The modifications caused in structural and optical properties of these films are investigated. Co-doping does not lead to amorphization or formation of any secondary phase precipitate for dopant concentrations in the range of 0.34-10.8 at.% used in the present study. However, we observe a systematic reduction in the d-spacing with increasing cobalt concentration. Optical band gap of CdS does not show any obvious change upon Co-doping. In addition, implantation gives rise to grain growth and increase in the surface roughness. The results are discussed in the light of ion-matter interaction in the keV regime.

Modification of WO(3) thin films by MeV N(+)-ion beam irradiation

In this paper, we report on modification of the structural, optical, vibrational, and surface morphological properties of 2 MeV N(+)-ion irradiated WO(3) thin films at different fluences (up to 1 × 10(15) ions cm(-2)). Although we observe irradiation induced grain growth, no structural phase transition takes place in the WO(3) films. These are accompanied by a systematic reduction in the optical band gap of the films. These observations are corroborated by our independent micro-Raman and optical absorption spectroscopy studies. We have made an attempt to correlate these results with MeV ion-matter interaction.

Binding and phosphorylation of tubulin by G protein-coupled receptor kinases

Although the beta-adrenergic receptor kinase (betaARK) mediates agonist-dependent phosphorylation and desensitization of G protein-coupled receptors, recent studies suggest additional cellular functions. During our attempts to identify novel betaARK interacting proteins, we found that the cytoskeletal protein tubulin could specifically bind to a betaARK-coupled affinity column. In vitro analysis demonstrated that betaARK and G protein-coupled receptor kinase-5 (GRK5) were able to stoichiometrically phosphorylate purified tubulin dimers with a preference for beta-tubulin and, under certain conditions, the betaIII-isotype. Examination of the GRK/tubulin binding characteristics revealed that tubulin dimers and assembled microtubules bind GRKs, whereas the catalytic domain of betaARK contains the primary tubulin binding determinants. In vivo interaction of GRK and tubulin was suggested by the following: (i) co-purification of betaARK with tubulin from brain tissue; (ii) co-immunoprecipitation of betaARK and tubulin from COS-1 cells; and (iii) co-localization of betaARK and GRK5 with microtubule structures in COS-1 cells. In addition, GRK-phosphorylated tubulin was found preferentially associated with the microtubule fraction during in vitro assembly assays suggesting potential functional significance. These results suggest a novel link between the cytoskeleton and GRKs that may be important for regulating GRK and/or tubulin function.

Developmental decisions in Aspergillus nidulans are modulated by Ras activity

To better understand how Ras controls development of multicellular organisms, we have chosen Aspergillus nidulans as a model system. When grown on solid medium, this fungus follows a well-defined program of development, sequentially giving rise to several cell types which produce three distinct structures: vegetative hyphae, aerial hyphae, and the conidiophore structure. Here we describe a ras homolog found in this fungus (Aras) and demonstrate that it is an essential gene that regulates the ordered program of development. We created dominant alleles of this gene and expressed them to different levels in order to vary the ratio of GTP-bound (active) to GDP-bound (inactive) A-Ras protein. When the amount of active Ras is large, nuclear division proceeds, but further development is inhibited at the early step of germ tube formation. At an intermediate level of active Ras, aerial hypha formation is inhibited, while at a low level, conidiophore formation is inhibited. Maintenance of an even lower level of the active Ras is essential for initiation and progression of conidiophore formation, the final stage of development. When the level of active Ras is artificially lowered, each stage of development is initiated prematurely except germination, the initial stage of development. Therefore, the progression of the ordered developmental pathway of A. nidulans is dependent upon an initial high level of active Ras followed by its gradual decrease. We propose that several concentration threshold exist, each of which allows development to proceed to a certain point, producing the proper cell type while inhibiting further development.

Autoregulation of 2 micron circle gene expression provides a model for maintenance of stable plasmid copy levels

The yeast plasmid 2 micron circle actively maintains high but stable copy levels in the cell, even though the plasmid confers no selective advantage to its host. To address the mechanism by which stable copy control is achieved, we have examined the level of expression of the genes resident on the yeast plasmid 2 micron circle as a function of the presence of proteins encoded by the plasmid. We find that transcription of the site-specific recombinase gene, FLP, is repressed at least 100-fold by the concerted action of the products of two other plasmid genes, REP1 and REP2. In addition, these products repress transcription of the REP1 gene itself. These results can be formulated into a consistent model for plasmid copy control.

Control of ColE1 plasmid replication: enhancement of binding of RNA I to the primer transcript by the Rom protein

RNA I prevents a transcript (RNA II) from the ColE1 primer promoter to form a hybrid with the template DNA and thereby inhibits formation of primer for DNA replication. Binding of RNA I to RNA II is responsible for the inhibition. The Rom protein, a plasmid-specified 63 amino acid protein, enhances the inhibitory effect of RNA I on primer formation by enhancing the binding of RNA I to RNA II. In vivo, RNA I controls plasmid copy number and incompatibility and inhibits expression of a galK gene fused to the primer promoter. The rom gene modulates these activities of RNA I. These functions of the rom gene can be explained by alteration of the binding of RNA I to RNA II by the Rom protein.

Regulatory regions of ColE1 that are involved in determination of plasmid copy number

The copy number of plasmid ColE1 has been known to increase when the Hae II-C segment downstream from the replication origin is deleted. The presence of the 306-base-pair (bp) Hpa II region in the segment is sufficient for reduction of the copy number. Plasmids harboring the region express a trans-acting function that is responsible for the copy number reduction and synthesize a unique protein. A protein specified by the region is purified to near homogeneity and identified as the 63-amino-acid protein encoded by the Hpa II segment. The region, which includes segments 19-25 bp and 53-311 bp downstream from the start site of the primer RNA, is involved in determination of sensitivity to the inhibitory function. In vivo transcription of the galK gene, which is directed by the primer promoter in a segment inserted in front of the structural gene, is inhibited by a plasmid carrying the Hpa II region. The inhibition is strong when the promoter segment contains up to 135 bp downstream from the primer RNA start site, whereas it is weak when only the region up to 52 bp downstream is present.

The origin of replication of plasmid p15A and comparative studies on the nucleotide sequences around the origin of related plasmids

Replication of Escherichia coli plasmid p15A was examined by use of a cell extract or a mixture of three purified E. coli enzymes: RNA polymerase; RNAase H; and DNA polymerase I. In each system, replication initiates at any of three consecutive nucleotides located at a unique site. Primer transcription starts 508 bp upstream of the replication origin. The region between 294 and 524 bp upstream of the origin determines the incompatibility property. This region specifies an RNA (RNA I) of about 105 nucleotides that is involved in regulation of primer formation. We compare the nucleotide sequences around the origins of related plasmids p15A, ColE1, pBR322, RSF1030 and CloDF13, and discuss the significance of possible RNA secondary structures in primer formation.

Origin of replication of Escherichia coli plasmid RSF 1030

The nucleotide sequence of a region of plasmid RSF 1030 that includes the origin of DNA replication was determined using the DNA of a small derivative, pST19. The nucleotide sequence of the pST 19 origin region is very similar to that of the ColE1 origin except for a 25 base pair (bp) deletion about 350 bp upstream of the origin and a considerable difference in the region between 400 and 600 bp upstream of the origin. Replication of pST19 starts at one of three consecutive nucleotides (dA, dA or dC) located at a unique position in the region where the nucleotide sequence is identical to that of the ColE1 origin. There are two major sites of initiation of transcription in the region. Transcription from one of the sites yields the primer precursor that can be cleaved by RNase H to form the primer of about 530 nucleotides long. Transcription from the other site proceeds on the opposite strand and terminates close to the primer initiation site to yield species I RNA (or RNA I) about 105 nucleotides long. The presumed RNA polymerase binding sites in the promoters of these transcripts differ from those of the corresponding ColE1 transcripts. Incompatibility specified by pST19 is different from that specified by ColE1. Hypothetical peptides encoded by the origin region of these plasmids are unlikely to be involved in the determination of incompatibility. It has been shown that RNA I is an incompatibility-group specific inhibitor of primer formation. Despite a significant difference in nucleotide sequence, the primer RNA and RNA I of pST19 can be folded into structures analogous to those of the ColE1 transcripts.

Inhibition of ColE1 RNA primer formation by a plasmid-specified small RNA

Transcription of ColE1 DNA by RNA polymerase in vitro starts at two sites in a region required for maintenance of the plasmid. Certain transcripts that start at one of the sites can be cleaved by RNase H and then act as primers for DNA replication. Transcription from the other site produces a RNA approximately 108 nucleotides long (species I or RNA I). Transcripts analogous to the primer and RNA I of ColE1 are produced when p15A or small derivatives of two other ColE1-compatible plasmids, CloDF13 and RSF1030, are used as template. If purified RNA I is added to the transcription reaction containing RNase H, formation of primer is inhibited. Each RNA I can inhibit primer formation by the plasmid that specifies it but has no effect on primer formation by heterologous templates. Thus, the inhibition of primer formation by RNA I is incompatibility specific. Because RNA I does not inhibit initiation or propagation of transcription or the processing of preformed precursors, the step that is sensitive to inhibition is probably formation of the hybrid between the primer precursor and the template. This hybrid is the required substrate for RNase H. Experiments with recombinant plasmids show the region that determines the specificity of response to RNA I to be greater than 300 base pairs upstream of the origin of DNA replication.