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Biomedical subjects

D N Lee

Publications and source records attributed to D N Lee.

At least 19 recordsLinked to original sources
PubMed1992

Structure of RNA and DNA chains in paused transcription complexes containing Escherichia coli RNA polymerase.

RNA polymerases pause conspicuously at certain positions on a DNA template. At the well-studied pause sites in the attenuation control regions that precede the trp and his operons, both formation of secondary structure in the nascent transcript and the DNA sequence immediately downstream contribute to pausing. The mechanisms of these effects are unknown. We report here studies on the structure of the RNA and DNA strands in purified trp and his paused transcription complexes in comparison to ten elongation complexes halted by nucleoside triphosphate deprivation. A 14 to 22 nucleotide region of the DNA strands was accessible to modification by KMnO4 or diethylpyrocarbonate in both the paused and halted transcription complexes. However, the region in front of the nucleotide-addition site was reactive only in some halted complexes. In both types of complexes, approximately eight nucleotides on the template strand immediately preceding the 3' end were protected from modification. We also examined the sensitivity of the nascent transcript to RNase A and found that the 3'-proximal eight nucleotide region could be cleaved without complete loss of the potential for elongation. However, a model RNA:DNA hybrid designed to mimic a hybrid in the transcription complex could also be cleaved under similar conditions. Together, the results suggest that the 3'-proximal eight nucleotides of transcript may pair with the DNA template and that this structure is not disrupted by hairpin formation at a pause site. Rather, pausing may result from distinct interactions between RNA polymerase and both the pause RNA hairpin and the downstream DNA sequence.

Base Sequence↗
PubMed1992

Common principle of guidance by echolocation and vision.

1. Using echolocation, bats move as gracefully as birds through the cluttered environment, suggesting common principles of optic and acoustic guidance. We tested the idea by analysing braking control of bats (Macroderma gigas) flying through a narrow aperture with eyes covered and uncovered. 2. Though braking control would seem to require rapid detection of distance and velocity and computation of deceleration, simpler control is possible using the tau function of any sensory variable S that is a power function of distance to aperture. Tau function of S is tau (S) = S/S (the dot means time derivative). Controlled braking is achievable by keeping tau (S) constant. 3. Previous experiments indicated the tau (S) constant procedure is followed by humans and birds in visually controlling braking. Analysis of the bats' flight trajectories indicated they too followed the braking procedure using echolocation. 4. The tau function of echo-delay or of echo-intensity or of angle subtended by directions of echoes from two points on the approach surface could be used to control braking. Aperture size was modulated during flight on some trials in an attempt to test between these possibilities, but the results were inconclusive.

Animals↗
PubMed1992

How do somersaulters land on their feet?

Body movements of trampolinists landing upright from forward somersaults, with eyes open and closed, were analyzed to test a theory of how braking and timing of actions are conjointly controlled. In regulating landing, rotation has to be slowed by extending the body so that it reaches the upright just as the feet hit the trampoline. Extending a theory of visual control of linear braking, it was hypothesized that upright landing is achieved by perceptually regulating body extension so as to keep the ratio angle of body to upright: rate of change of angle (the tau function of the angle) proportional to time-to-landing. The data support the hypothesis, indicate that vision improves precision of control, and argue further for the value of the tau function in the perceptual regulation of action.

Attention↗
PubMed1992

Errors in young children's decisions about traffic gaps: experiments with roadside simulations.

Young children's vulnerability as pedestrians has often been attributed to deficiencies in their decision making about vehicle approach times. Some studies have found a preponderance of risky decisions below the age of eight years. In contrast, studies using a closer simulation of road crossing, known as the pretend road, have found a preponderance of overcautious decisions in young children: traffic gaps of adequate size were frequently rejected (missed opportunities). However, the pretend road has potentially distorting characteristics which may account for this divergent pattern of findings. The experiments reported below show that new simulations that eradicate distortions nevertheless validate the pattern of results produced with the pretend road. Differences between adults and young children were pronounced for missed opportunities, but not for risky decisions. Subsidiary analyses suggest that the risky decisions of the youngest children may have arisen through lapses in attention, rather than deficits in timing. These findings run contrary to the view that attributes young children's pedestrian vulnerability to perceptuo-motor deficiency.

Acceleration↗
PubMed1991

Effect of task on movement control in cerebral palsy: implications for assessment and therapy.

In order to examine the possibility that children with cerebral palsy (CP) find abstract tasks, such as extending the arm as much as possible, more difficult than concrete tasks, such as reaching to grasp an object, nine hemiparetic children with CP and 12 nursery-school children were tested with both a concrete and an abstract task. The children with CP achieved a significantly larger range of movement in the concrete task, whereas the nursery-school children showed no difference between tasks. Thus the CP children's poorer performance on the abstract task did not fully reflect their movement capability. This means that conventional neurological measurements of limb function in cerebral palsy, which mainly use abstract tasks or passive movements, will give an incomplete picture of the child's action capability.

Cerebral Palsy↗
PubMed1990

Transcription pausing by Escherichia coli RNA polymerase is modulated by downstream DNA sequences.

Escherichia coli RNA polymerase pauses immediately after transcription of certain sequences that can form stable secondary structures in the nascent RNA transcript; pausing appears to be essential for several types of bacterial transcription attenuation mechanisms. Because base changes that weaken the RNA secondary structures reduce the half-life of pausing by RNA polymerase, nascent transcript RNA hairpins are thought to cause pausing at these sites. We show here that, for the well characterized trpL pause site, the determinants of transcription pausing are not limited to the RNA hairpin, but include the not-yet-transcribed sequence of DNA immediately downstream from the pause site. We show that this effect extends to bases up to fourteen nucleotides downstream from the pause site, that placement of a oligo(dT) tract in the nontranscribed strand in this region does not convert the pause site to a termination site, and that shifting the position of pausing by one nucleotide downstream almost eliminates pausing. From an analysis of many variants of this downstream sequence, we argue that the effect of downstream sequence is not related simply to its GC content. We suggest that these effects are mediated by altered interactions between RNA polymerase and the DNA template downstream from the enzyme's active site.

Base Sequence↗
PubMed1990

Development of looking with head and eyes.

Research into stabilization of gaze has concentrated on how the eyes counterrotate to compensate for head rotation. There is little information on how head movements function as an integral part of gaze stabilization. The head and eye coordination of six adults and six infants was tested under two conditions: tracking a moving target when the body was stationary; fixating a stationary target when the body was turning. Under each condition, both infants and adults turned their heads more than their eyes in stabilizing gaze. The infants were tested at 3-week intervals between the ages of 11 and 28 weeks. During this period, the precision with which head turning was coupled to their own or the target movement developed to near adult level, showing rapid growth between 11 and 16 weeks. However, the infants' ability to couple the eyes onto the target did not change over the tested period, remaining much less precise than the adults'. These findings have important implications for the assessment of abnormal gaze stabilization, which could facilitate the early diagnosis of perceptuomotor dysfunction.

Arousal↗
PubMed1990

Amino acid changes in conserved regions of the beta-subunit of Escherichia coli RNA polymerase alter transcription pausing and termination.

Control of transcription at pause and termination sites is common in bacteria. Many transcriptional pause and termination events are thought to occur in response to formation of an RNA hairpin in the nascent transcript. Some mutations in the beta-subunit of Escherichia coli RNA polymerase that confer resistance to the transcription inhibitor rifampicin also alter the response to transcriptional pause and termination signals. Here, we report isolation of termination-altering mutations that do not confer rifampicin resistance and show that such mutations occur predominantly in limited regions of the beta-subunit polypeptide. One region is between amino acid residues 500 and 575, which encompasses the locations of almost all known rifampicin-resistance mutations. Many termination-altering mutations also occur in two other regions: between amino acid residues 740 and 840 and near the carboxyl terminus of the beta-subunit (amino acid residues 1225-1342). Amino acid sequences in these three regions of the beta-subunit are conserved between prokaryotic and eukaryotic beta-subunit homologs. Several mutations that alter transcription termination in vitro affect amino acid residues that are identical in prokaryotic and eukaryotic RNA polymerase beta-subunit homologs, suggesting that they alter an important function common to multisubunit RNA polymerases. We propose that these three regions of the beta-subunit may contact the nascent RNA transcript, the RNA-DNA heteroduplex, or the DNA template in the transcription complex and that mutations in these regions alter transcription pausing and termination by affecting these contacts.

Amino Acid Sequence↗
PubMed1987

Training children in road crossing skills using a roadside simulation.

Five-year-old children were trained in road-crossing skill using a new method which allows them to act safely in relation to vehicles on a normal road. The children learned to time their crossings of a "pretend road" as if the vehicles were on this, rather than on the adjacent road. A previous study, using a single lane of traffic, showed that many children performed well in this simulation with minimal instruction, but that five-year-olds were generally less proficient than older children. In the present study, the method was extended to the more realistic case of two-way traffic, and training programmes for five-year-olds were assessed. (In addition, the performances of adults in the two-way pretend task and in actually crossing the road were compared; the results confirmed the validity of the simulation. After a few sessions of guided practice, the children's efficiency in making use of gaps by setting off promptly after a vehicle had passed improved markedly; in single-lane crossing they reached almost adult standard. In both single-lane and two-way crossing they became as successful as adults in completing their crossings before a second vehicle passed and they took account of the duration of the gap by crossing more quickly when time was short. They remained very cautious, rejecting many adequate gaps. The standard of performance after a few sessions of two-way crossing was maintained over a three week break in training. Practice with one-way traffic did not help them with two-way crossing. In sum, after practice in the simulation, five year olds develop a degree of competence normally shown by older children, whose experience on the roads puts them at lower risk. This suggests that the simulation, which allows children to safely explore and develop their capabilities more fully than when actually crossing the road, would be a valuable addition to road safety programmes.

Accident Prevention↗
PubMed1986

Visual control of step length during running over irregular terrain.

Running over uneven ground requires visually regulating step length to secure proper footing. To examine how this is achieved, we studied subjects running on a treadmill on a series of irregularly spaced targets. The movements of their lower limbs and coccyx relative to the targets were monitored opto-electronically by a Selspot system. The results indicated that step length was adjusted to strike the targets primarily by varying the vertical component of impulse applied to the ground during the stance phase. In contrast, horizontal impulse was not varied significantly, and changing the reach forward of the foot on landing contributed little to variation in step length. Changing the vertical impulse simply altered the step time proportionately. Thus the data are consistent with a time-based model in which vertical impulse is modulated by the optic variable delta tau (Lee, 1976) specifying the time gap that has to be bridged by the runner between two targets.

Coccyx↗
PubMed1981

Why two eyes are better than one: the two views of binocular vision.

Despite centuries of research on the topic, the answer to the question "'Are two eyes significantly better than one, independent of stereopsis?" is still uncertain. In this investigation, steps are taken toward answering the question in a behavioral context. Three sets of experiments are reported in which human binocular and monocular performance are compared in a variety of exteroceptive and visuomotor tasks. In all of the experiments, two eyes facilitated performance. The findings suggest that the binocular system is able to detect the matching information, that is, the concordance, in the monocular optic arrays and to use that information to increase visual efficiency. Furthermore, stereopsis was not found to be important in the performance of visuomotor skills in three dimensions when the subjects were free to move their heads. Thus, the results indicate that an important ecological benefit of binocular frontal vision is having binocular concordance, rather than having binocular disparity.

Adult↗
PubMed1980

The optic flow field: the foundation of vision.

As a basis for understanding the visual system, we need to consider the functions that vision has to perform, which are pre-eminently in the service of activity, and the circumstances in which it normally operates, namely when the head is moving. The fundamental ecological stimulus for vision is not a camera-like time-frozen image but a constantly changing optic array or flow field, the description of which must be in spatio-temporal terms. A mathematical analysis of the optic flow field is presented, revealing the informamtion that it affords for controlling activity - information both about the topography of the environment and about the movement of the organism relative to the environment. Results of human behavioral experiments are also reported. It is suggested that the optic flow field should be the starting point in attempting to discover the physiological workings of the visual system.

Information Theory↗
PubMed1979

Normophosphatemic phosphate depletion in growing rat.

To study the influence of phosphate depletion (PD) on the serum, urinary, bone, and soft tissue phosphorus, we studied growing rats given a high-phosphorus (HP), normal-phosphorus (NP), or low-phosphorus (LP) diet. We obtained the following results. a) With an LP diet, animals did not grow but developed the characteristic biochemical changes of PD. b) The NP rats had an unexplained accelerated rate of growth, and developed all the biochemical changes of PD, although they were normophosphatemic. c) Bone P and Ca were significantly lower in the LP and NP rats compared to the HP rats, suggesting that minerals were mobilized from bone to support soft tissue P needs. d) Soft tissue P was not different in HP, NP, and LP rats, confirming previous observations that soft tissue P is maintained even in PD. We conclude that 1) the physiologic adaptation to PD may occur in growing rats on an apparently normal phosphorus diet when the metabolic demand is unusually high, e.g. accelerated growth; 2) the evolution of the biochemical parameters of PD in the face of normophosphatemia suggest a sensor mechanism sensitive to change in dietary P concentration.

Animals↗
PubMed1976

A theory of visual control of braking based on information about time-to-collision.

A theory is presented of how a driver might visually control his braking. A mathematical analysis of the changing optic array at the driver's eye indicates that the simplest type of visual information, which would be sufficient for controlling braking and would also be likely to be easily picked up by the driver, is information about time-to-collision, rather than information about distance, speed, or acceleration/deceleration. It is shown how the driver could, in principle, use visual information about time-to-collision in registering when he is on a collision course, in judging when to start braking, and in controlling his ongoing braking. Implications of the theory for safe speeds and safe following distances are discussed, taking into account visual angular velocity detection thresholds, and some suggestions are made as to how safety on the roads might be improved.

Automobile Driving↗
PubMed1975

Electrophoresis of cottage cheese whey proteins and their polymers.

Cottage cheese whey solutions containing sodium dodecyl sulfate were resolved electrophoretically on 5% sodium dodecyl sulfate polyacrylamide gels. Major bands were identified by comparing their electrophoretic mobilities to those of known whey components. Other components were identified principally from molecular weights determined from a calibration correlating mobility and molecular weight. Conditions which affect polymerization of proteins were studied in whey and solutions of purified beta-lactoglobulin. Formation of a number of polymers was induced by concentrating the whey samples, lowering the temperature, adjusting pH, or adding salts. The dimer, trimer, tetramer, and octamer of beta-lactoglobulin, the dimer and trimer of bovine serum albumin, and several unidentified components in the 100,000 to 300,000 molecular weight range were observed. The octamer state of beta-lactoglobulin was observed in whey at pH values between 5.1 and 8.0, at temperatures below 10 C, and with .2M addition of potassium thiocyanate, potassium iodide, calcium chloride, or sodium acetate. Similar polymer formation, and temperature and pH effects, were observed with solutions of purified beta-lactoglobulin, which contained dimers, trimers, and tetramers. The beta-lactoglobulin octamer in whey samples could be dissociated by the addition of acid. The bovine serum albumin dimer appeared in whey at pH above 6.2 and at -minus 1 C.

Animals↗