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PubMed1994

Reducing informational masking by sound segregation.

Informational masking was reduced using three stimulus presentation schemes that were intended to perceptually segregate the signal from the masker. The maskers were sets of sinusoids chosen randomly in frequency and intensity on each stimulus interval or, in some conditions, on every masker burst in a series of bursts within intervals. Masker components were excluded from the frequency region surrounding the 1000-Hz signal to minimize the energetic masking. Masked thresholds as great as 60-70 dB above quiet threshold were observed for some subjects in some conditions. It was shown that this informational masking could be reduced as much as 40 dB by: (1) presenting the masker to both ears and signal to one ear; (2) playing different masker samples sequentially in each interval of every trial; or (3) presenting the signal in alternate bursts of multiple, identical masker samples. For the binaural manipulation, informational masking was reduced because the masker and signal were perceived as originating from different interaural locations. In the latter two manipulations, a difference in the spectral or temporal pattern of the signal and masker provided the detection cue. These effects were interpreted as evidence of the importance of perceptual segregation of sounds in noisy listening environments where signal reception is not limited by energetic masking.

Acoustic Stimulation↗
PubMed1995

The time course of auditory segregation: concurrent vowels that vary in duration.

Human listeners perform well when identifying both members of simultaneous steady-state vowel pairs, even when the vowels start and stop at the same time, are presented monaurally, have approximately equal intensities, and have the same fundamental frequency (f0). The sensation described by listeners is of one dominant, vowel "colored" by the second, less easily identified, or nondominant vowel. Introducing a small separation in f0 between the vowels improves performance and listeners now report that there is a sensation of two voice sources rather than one. It has been suggested that listeners use an f0-guided segregation strategy in identifying two vowels that differ in f0. An experiment is reported in which four listeners attempted to identify both members of a pair of concurrent vowels which varied in duration from a single cycle of the stimulus waveform (one pitch period) up to eight cycles. A dominant vowel was identified with near 100% accuracy even in the single-cycle condition, whereas identification of the nondominant vowel showed a slow improvement up to eight cycles. A difference in f0 between the vowels improved identification of the nondominant vowel, but between three and four cycles of the vowels were necessary for this advantage. It is first concluded that the improvement in performance with stimulus duration is due to an improvement in identification of the nondominant vowel; and, second, a difference in f0 is not required for segregation of the dominant vowel which is available from stimuli which are too brief to provide a useful estimate of f0.

Adult↗
PubMed1996

Computer simulation of auditory stream segregation in alternating-tone sequences.

A computer model is described that takes a novel approach to the problem of accounting for perceptual coherence in alternating pure-tone sequences by using simple physiological principles that operate at a low level. Using the same set of parameter values, the model is able to reproduce a number of phenomena associated with auditory stream segregation. These are (1) the buildup of stream segregation over time, (2) the temporal coherence and fission boundaries obtained from human listeners, and (3) the trill threshold. Whereas these phenomena are generally accounted for in terms of an auditory scene-analysis process that works on the basis of Gestalt perceptual principles, the operation of the model suggests that some Gestalt auditory grouping may be the product of low-level processes.

Acoustic Stimulation↗
PubMed2002

Crystal engineering: from structure to function.

Modern crystal engineering has emerged as a rich discipline whose success requires an iterative process of synthesis, crystallography, crystal structure analysis, and computational methods. By focusing on the molecular recognition events during nucleation and growth, chemists have uncovered new ways of controlling the internal structure and symmetry of crystals and of producing materials with useful chemical and physical properties.

Journal Article↗
PubMed2003

Eye-specific retinogeniculate segregation independent of normal neuronal activity.

The segregation of initially intermingled left and right eye inputs to the dorsal lateral geniculate nucleus (DLGN) during development is thought to be in response to precise spatial and temporal patterns of spontaneous ganglion cell activity. To test this hypothesis, we disrupted the correlated activity of neighboring ganglion cells in the developing ferret retina through immunotoxin depletion of starburst amacrine cells. Despite the absence of this type of correlated activity, left and right eye inputs segregated normally in the DLGN. By contrast, when all spontaneous activity was blocked, the projections from the two eyes remained intermingled. Thus, certain features of normal neural activity patterns are not required for the formation of eye-specific projections to the DLGN.

Action Potentials↗
PubMed1966

Segregation of sister chromatids in mammalian cells.

Segregation of sister chromatids in embryonic mouse cells in primary tissue culture is not random. In mitosis those chromatids replicated on a DNA template synthesized during the preceding division cycle are separated from those constructed on a template synthesized two division cycles previously. Segregation in cells of the Chinese hamster follows a similar, but less pronounced, pattern.

Animals↗
PubMed1996

DPY-26, a link between dosage compensation and meiotic chromosome segregation in the nematode.

The DPY-26 protein is required in the nematode Caenorhabditis elegans for X-chromosome dosage compensation as well as for proper meiotic chromosome segregation. DPY-26 was shown to mediate both processes through its association with chromosomes. In somatic cells, DPY-26 associates specifically with hermaphrodite X chromosomes to reduce their transcript levels. In germ cells, DPY-26 associates with all meiotic chromosomes to mediate its role in chromosome segregation. The X-specific localization of DPY-26 requires two dosage compensation proteins (DPY-27 and DPY-30) and two proteins that coordinately control both sex determination and dosage compensation (SDC-2 and SDC-3).

Animals↗
PubMed1998

Synaptic segregation at the developing neuromuscular junction.

Throughout the developing nervous system, competition between axons causes the permanent removal of some synaptic connections. In mouse neuromuscular junctions at birth, terminal branches of different axons are intermingled. However, during the several weeks after birth, these branches progressively segregated into nonoverlapping compartments before the complete withdrawal of all but one axon. Segregation was caused by selective branch atrophy, detachment, and withdrawal; the axon branches that were nearest to the competitor's branches were removed before the more distant branches were removed. This progression suggests that the signals that mediate the competitive removal of synapses must decrease in potency over short distances.

Animals↗
PubMed1983

Segregation and mapping analysis of polymorphic HLA class I restriction fragments: detection of a novel fragment.

An HLA-B7 complementary DNA clone was used as a hybridization probe to analyze the segregation pattern of polymorphic class I restriction fragments in several families whose HLA types had been determined by serological techniques. In one family in which a crossover in the HLA region had occurred, a specific genomic fragment was mapped with respect to the crossover. In another family, a novel genomic fragment present in one child and absent in all other family members was observed. With the exception of this novel fragment, all polymorphic class I fragments observed in this study segregated with a serologically defined parental haplotype, a result consistent with HLA linkage.

Chromosome Mapping↗
PubMed1995

Segregation in DNA solutions induced by electric fields.

DNA solutions subjected to an electric field exhibit an instability that leads to DNA segregation in aggregates tilted with regard to the field. With the use of epifluorescence videomicroscopy, the evolution of DNA patterns in capillaries as a function of DNA concentration, DNA size, field strength, and field frequency was studied. The field threshold for segregation was decreased when the frequency was lowered or when the DNA molecular weight or concentration was increased. Aggregation is attributed to an electrohydrodynamic instability triggered by the dipole-dipole interaction. This phenomenon explains the failure of earlier attempts to separate large DNA in capillaries.

DNA↗
PubMed1991

Effects of quinolones on nucleoid segregation in Escherichia coli.

The effects of quinolone antibiotics on nucleoid segregation in growing Escherichia coli were examined by using fleroxacin (Ro 23-6240, AM 833) as a prototype compound. At levels that were close to its MIC and induced growth arrest and filamentation, fleroxacin caused large nucleoids to appear in midcell, suggesting inhibition of nucleoid segregation. With increasing fleroxacin concentrations, nucleoids became progressively smaller, suggesting inhibition of DNA replication. Removal of fleroxacin restored normal cell and nucleoid morphology in filaments with large nucleoids but not in filaments with small nucleoids. The results are consistent with inhibition of chromosome decatenation at low quinolone concentrations (bacteriostatic effect) and DNA supercoiling at high concentrations (bactericidal effect).

Anti-Infective Agents↗
PubMed2000

Geographical segregation of the neurotoxin-producing cyanobacterium Anabaena circinalis.

Blooms of the cyanobacterium Anabaena circinalis are a major worldwide problem due to their production of a range of toxins, in particular the neurotoxins anatoxin-a and paralytic shellfish poisons (PSPs). Although there is a worldwide distribution of A. circinalis, there is a geographical segregation of neurotoxin production. American and European isolates of A. circinalis produce only anatoxin-a, while Australian isolates exclusively produce PSPs. The reason for this geographical segregation of neurotoxin production by A. circinalis is unknown. The phylogenetic structure of A. circinalis was determined by analyzing 16S rRNA gene sequences. A. circinalis was found to form a monophyletic group of international distribution. However, the PSP- and non-PSP-producing A. circinalis formed two distinct 16S rRNA gene clusters. A molecular probe was designed, allowing the identification of A. circinalis from cultured and uncultured environmental samples. In addition, probes targeting the predominantly PSP-producing or non-PSP-producing clusters were designed for the characterization of A. circinalis isolates as potential PSP producers.

Anabaena↗
PubMed1990

Construction of stable cloning vectors that do not segregate from a human fecal Escherichia coli strain in the streptomycin-treated mouse large intestine.

Escherichia coli F-18 Col- was previously shown to be a poor colonizer of the streptomycin-treated mouse large intestine, relative to its parent, E. coli F-18. Prior to attempting to clone genes responsible for the colonization phenotype of E. coli F-18 into E. coli F-18 Col-, a suitable cloning vector had to be found. In this investigation, we report that the commonly used cloning vectors pBR322, pHC79, and pBR329 all segregate from E. coli F-18 Col- both when grown in L broth under conditions of nonselection (i.e., in vitro) and when fed to streptomycin-treated mice (i.e., in vivo). Insertion of the cer region (which promotes resolution of replicating plasmids into monomeric forms) into pHC79 stabilized this plasmid in E. coli F-18 Col- in vitro and in vivo. In contrast, two independent cer insertions into pBR329 did not stabilize the plasmid completely in E. coli F-18 Col- in vitro, and feeding the strain to streptomycin-treated mice resulted in rapid segregation of the plasmids in vivo. Also, stability of all three plasmids in E. coli F-18 Col- in vitro was achieved by insertion of the parB region of plasmid R1, which encodes a cell-killing protein, Hok, that is active only postsegregationally. However, as with cer, complete in vitro and in vivo stabilization was achieved only in parB constructs of pBR322 and pHC79.

Animals↗
PubMed1981

Regular segregation of composite plasmid Rms201.

Copy number mutants Rms201ts15 and Rms201ts16 were isolated at 30 degrees C from a temperature-sensitive replication mutant (Rms201ts14) of the conjugative plasmid Rms201. The numbers of plasmids per chromosome of ML1410(Rms201ts14), ML1410(Rms201ts15), and ML1410(Rms201ts16) grown at 30 degrees C were 2.2, 7.4, and 20, respectively. The synthesis of covalently closed circular plasmid deoxyribonucleic acid stopped in Rms201ts14, Rms201ts15, and Rms201ts16 immediately after a "shift-up" in temperature (42 degrees C). At 42 degrees C, antibiotic-sensitive derivatives appeared after a certain lag time: the lag times of ML1410(Rms201ts14), ML1410(Rms201ts15), and ML1410(Rms201ts16) were 2.5, 5, and 6.8 generations, respectively. After these times, plasmid-positive cells in the populations decreased at a rate of about 50% per generation in all of the mutants. From these results we conclude that plasmid segregation (partition) of Rms201 occurs by regular segregation (partition).

Ampicillin↗
PubMed1982

Identification of cell wall subunits in bacillus subtilis and analysis of their segregation during growth.

Continuous as well as pulse-labeling and chase experiments with Bacillus subtilis demonstrated that the cell wall (both peptidoglycan and teichoic acid) is composed of a limited number of blocks which, once completed, segregate during subsequent growth without undergoing any mixing with newly synthesized blocks. This observation suggests that new wall material is inserted in a limited number of zones. Previously reported observations which suggested diffuse intercalation of new wall material are reinterpreted on the basis of our results. Experiments performed on different media showed that the number of segregation units per unit of cell length and thus the density of insertion zones increases with medium richness. This finding suggests analogies between the regulation of cell wall and DNA synthesis.

Bacillus subtilis↗
PubMed1987

Effects of segregation and selection on instability of plasmid pACYC184 in Escherichia coli B.

We use a mathematical model to analyze the dynamics of loss of nonconjugative pACYC184 from populations of Escherichia coli B in glucose-limited continuous culture. This model incorporates both plasmid segregation and selection against plasmid carriage. It is concluded that there is intense selection against plasmid carriage (s = 0.3 per culture generation), which amplifies the frequency of segregants arising de novo.

Conjugation, Genetic↗
PubMed1989

Delta dnaK52 mutants of Escherichia coli have defects in chromosome segregation and plasmid maintenance at normal growth temperatures.

Major heat shock proteins, such as the Escherichia coli DnaK protein, not only are required for cell growth after heat shock but seem to possess important functions in cellular metabolism at normal growth temperatures as well. E. coli delta dnaK52 mutants have severe cellular defects at 30 degrees C, one of which is in cell division (B. Bukau and G. C. Walker, J. Bacteriol, 171:2337-2346, 1989). Here we show that at 30 degrees C, delta dnaK52 mutants have defects in chromosome segregation and in maintenance of low-copy-number plasmids. Fluorescence microscopic analysis revealed that chromosomes were frequently lacking at peripheries of cell filaments of delta dnaK52 mutants and clustered at other locations. In other parts of the cell filaments, chromosomes were apparently normally distributed and they were also present in most of the small cells found in populations of delta dnaK52 cells. These defects might be at the level of DNA replication, since delta dnaK52 mutants have a threshold lower rate of DNA synthesis than wild-type cells. Chromosome segregation defects of delta dnaK52 mutants were also observed in an rnh dnaA mutant background, in which initiation of DNA replication is DnaA-oriC independent. We also found that low-copy-number P1 miniplasmids could not be stably maintained in delta dnaK52 mutants at 30 degrees C. delta par P1 miniplasmids that carry the P1-encoded rep functions required for their replication but lack the P1-encoded par functions required for faithful partitioning of the plasmids during cell division were also unstable in delta dnaK52 mutants. Taken together, our results indicate important, although not absolutely essential, functions for DnaK at 30 degrees C in one or more processes necessary for correct replication and/or partitioning of chromosomes and P1 miniplasmids. Furthermore, we found that P1 miniplasmids were also highly unstable in dnaJ259 mutants, indicating a role for the DnaJ heat shock protein in maintenance of these plasmids.

Cell Division↗
PubMed1989

Replication and segregational stability of Bacillus plasmid pBAA1.

A cryptic plasmid, pBAA1, was identified in an industrial Bacillus strain. The plasmid is 6.8 kilobases in size and is present in cells at a copy number of approximately 5 per chromosome equivalent. The plasmid has been maintained under industrial fermentation conditions without apparent selective pressure and so is assumed to be partition proficient. The minimal replicon was localized to a 1.4-kilobase fragment which also contains the functions required for copy number control. The very low level of segregational instability of the minimal replicon suggests that it also contains functions involved in plasmid maintenance. Comparison with other plasmids indicates that pBAA1 belongs to the group of small gram-positive plasmids which replicate by a rolling cycle-type mechanism. A sequence was identified which is required for the efficient conversion of the single plus strand to the double-stranded form during plasmid replication. Deletion of this sequence resulted in a low level of segregational plasmid instability.

Amino Acid Sequence↗