Effect of Exoskeleton-Assisted Rehabilitation Over Prefrontal Cortex in Multiple Sclerosis Patients: A Neuroimaging Pilot Study

Application of a passive and fully articulated exoskeleton, called Human Body Posturizer (HBP), has been demonstrated to improve mobility, response accuracy and ambulation in multiple sclerosis (MS) patients. By using functional magnetic imaging (fMRI) during a visuomotor discrimination task, we performed a pilot study to evaluate the effect of HBP over the neural correlates of motor and cognitive functions which are typically impaired in MS patients. Specifically, we tested the effect of a 6-week multidisciplinary rehabilitation intervention on two groups of MS patients: a control group who followed a standard physiotherapeutic rehabilitation protocol, and an experimental group who used the HBP during physical exercises in addition to the standard protocol. We found that, after treatment, the experimental group exhibited a significant lower activity (as compared to the control group) in the inferior frontal gyrus. This post-treatment activity reduction can be explained as a retour to a normal range, being the amount of iFg activity observed in the experimental patients very similar to that observed in healthy subjects. These findings indicate that the use of HBP during rehabilitation intervention normalizes the prefrontal activity, mitigating the cortical hyperactivity associated to MS.

Effect of task complexity on motor and cognitive preparatory brain activities

In the present study, we investigated scalp-recorded activities of motor and cognitive preparation preceding stimulus presentation in relatively simple and complex visual motor discriminative response tasks (DRTs). Targets and non-targets were presented (with equal probability) in both tasks, and the complexity of the task depended on the discrimination and categorization processing load, which was based on the number of stimuli used (two stimuli in the simple- and four in the complex-DRT, respectively). We recorded event-related potentials (ERPs) in 16 participants in simple-DRT and 16 participants in complex-DRT. At the behavioral level, the performance was faster and more accurate in simple-DRT. Two pre-stimulus ERPs were considered: the central Bereitschaftspotential (BP) and the prefrontal negativity (pN). Both components showed earlier onset and larger amplitude in the complex-DRT. Overall, the simple-DRT required less motor and cognitive preparation in premotor and prefrontal areas compared to the complex-DRT. Present findings also suggest that the pN component was not reported in previous studies, likely because most ERP literature focusing on pre-stimulus ERP used simple-DRTs, and with such a task the pN amplitude is small and can easily go undetected.

Exercise-related cognitive effects on sensory-motor control in athletes and drummers compared to non-athletes and other musicians

Both playing a musical instrument and playing sport produce brain adaptations that might affect sensory-motor functions. While the benefits of sport practice have traditionally been attributed to aerobic fitness, it is still unknown whether playing an instrument might induce similar brain adaptations, or if a specific musical instrument like drums might be associated to specific benefits because of its high energy expenditure. Since the aerobic costs of playing drums was estimated to be comparable to those of average sport activities, we hypothesized that these two groups might show both behavioral and neurocognitive similarities. To test this hypothesis, we recruited 48 young adults and divided them into four age-matched groups: 12 drummers, 12 athletes, 12 no-drummer musicians and 12 non-athletes. Participants performed a visuo-motor discriminative response task, namely the Go/No-go, and their cortical activity was recorded by means of a 64-channel electroencephalography (EEG). Behavioral performance showed that athletes and drummers were faster than the other groups. Electrophysiological results showed that the pre-stimulus motor preparation (i.e. the Bereitschaftspotential or BP) and attentional control (i.e., the prefrontal negativity or pN), and specific post-stimulus components like the P3 and the pP2 (reflecting the stimulus categorization process) were enhanced in the athletes and drummers' groups. Overall, these results suggest that playing sport and drums led to similar benefits at behavioral and cognitive level as detectable in a cognitive task. Explanations of these findings, such as on the difference between drummers and other musicians, are provided in terms of long-term neural adaptation mechanisms and increased visuo-spatial abilities.

Objects are highlighted by spatial attention

Selective attention may be focused upon a region of interest within the visual surroundings, thereby improving the perceptual quality of stimuli at that location. It has been debated whether this spatially selective mechanism plays a role in the attentive selection of whole objects in a visual scene. The relationship between spatial and object-selective attention was investigated here through recordings of event-related brain potentials (ERPs) supplemented with functional magnetic brain imaging (fMRI). Subjects viewed a display consisting of two bar-shaped objects and directed attention to sequences of stimuli (brief corner offsets) at one end of one of the bars. Unattended stimuli belonging to the same object as the attended stimuli elicited spatiotemporal patterns of neural activity in the visual cortex closely resembling those elicited by the attended stimuli themselves, albeit smaller in amplitude. This enhanced neural activity associated with object-selective attention was localized by use of ERP dipole modeling and fMRI to the lateral occipital extrastriate cortex. We conclude that object-selective attention shares a common neural mechanism with spatial attention that entails the facilitation of sensory processing of stimuli within the boundaries of an attended object.

Effects of spatial congruity on audio-visual multimodal integration

Spatial constraints on multisensory integration of auditory (A) and visual (V) stimuli were investigated in humans using behavioral and electrophysiological measures. The aim was to find out whether cross-modal interactions between A and V stimuli depend on their spatial congruity, as has been found for multisensory neurons in animal studies (Stein & Meredith, 1993). Randomized sequences of unimodal (A or V) and simultaneous bimodal (AV) stimuli were presented to right- or left-field locations while subjects made speeded responses to infrequent targets of greater intensity that occurred in either or both modalities. Behavioral responses to the bimodal stimuli were faster and more accurate than to the unimodal stimuli for both same-location and different-location AV pairings. The neural basis of this cross-modal facilitation was studied by comparing event-related potentials (ERPs) to the bimodal AV stimuli with the summed ERPs to the unimodal A and V stimuli. These comparisons revealed neural interactions localized to the ventral occipito-temporal cortex (at 190 msec) and to the superior temporal cortical areas (at 260 msec) for both same- and different-location AV pairings. In contrast, ERP interactions that differed according to spatial congruity included a phase and amplitude modulation of visual-evoked activity localized to the ventral occipito-temporal cortex at 100-400 msec and an amplitude modulation of activity localized to the superior temporal region at 260-280 msec. These results demonstrate overlapping but distinctive patterns of multisensory integration for spatially congruent and incongruent AV stimuli.

Objects Are Highlighted by Spatial Attention

Selective attention may be focused upon a region of interest within the visual surroundings, thereby improving the perceptual quality of stimuli at that location. It has been debated whether this spatially selective mechanism plays a role in the attentive selection of whole objects in a visual scene. The relationship between spatial and object-selective attention was investigated here through recordings of event-related brain potentials (ERPs) supplemented with functional magnetic brain imaging (fMRI). Subjects viewed a display consisting of two bar-shaped objects and directed attention to sequences of stimuli (brief corner offsets) at one end of one of the bars. Unattended stimuli belonging to the same object as the attended stimuli elicited spatiotemporal patterns of neural activity in the visual cortex closely resembling those elicited by the attended stimuli themselves, albeit smaller in amplitude. This enhanced neural activity associated with object-selective attention was localized by use of ERP dipole modeling and fMRI to the lateral occipital extrastriate cortex. We conclude that object-selective attention shares a common neural mechanism with spatial attention that entails the facilitation of sensory processing of stimuli within the boundaries of an attended object.

Cortical plasticity following surgical extension of lower limbs

Human cortical plasticity has been studied after peripheral sensory alterations due to amputations or grafts, while sudden 'quasi-physiological' changes in the dimension of body parts have not been investigated yet. We examined the cortical reorganization in achondroplastic dwarfs submitted to progressive elongation (PE) of lower limbs through the Ilizarov technique. This paradigm is ideal for studying cortical plasticity because it avoids the perturbation connected with deafferentation and re-afferentation. Somatosensory evoked-potentials (SEP) and fMRI studies were performed before and after PE during foot and knee stimulation, above and below the surgical fracture. A body schema test was also performed. Following PE, cortical modifications were observed in the primary somatosensory cortex for foot stimulation and in higher order somatosensory cortices for foot and knee. The former modifications tended to decrease 6 months after the elongation ending, whereas the latter tended to persist. Results are interpreted in terms of cortical adaptation mediated by temporary disorganization.

Separate neural systems for processing action- or non-action-related sounds

The finding of a multisensory representation of actions in a premotor area of the monkey brain suggests that similar multimodal action-matching mechanisms may also be present in humans. Based on the existence of an audiovisual mirror system, we investigated whether sounds referring to actions that can be performed by the perceiver underlie different processing in the human brain. We recorded multichannel ERPs in a visuoauditory version of the repetition suppression paradigm to study the time course and the locus of the semantic processing of action-related sounds. Results show that the left posterior superior temporal and premotor areas are selectively modulated by action-related sounds; in contrast, the temporal pole is bilaterally modulated by non-action-related sounds. The present data, which support the hypothesis of distinctive action sound processing, may contribute to recent theories about the evolution of human language from a mirror system precursor.

Abnormal Motor Preparation in Severe Traumatic Brain Injury with Good Recovery

Movement-related cortical potentials (MRCPs) were examined in seven patients with severe traumatic brain injury (TBI) and 12 matched control subjects. All patients had clinically established good recovery by the time of testing. Flexion movements of the index finger of the left or right hand were recorded in two (alternating and repetitive) self-paced conditions and in one externally triggered condition. In control subjects, the bereitschaftspotential (BP) component of MRCP was detected approximately 2000 msec prior to movement onset in the self-paced conditions and was larger and earlier in the alternating compared to the repetitive condition. The BP component was absent in the externally triggered condition. In TBI patients, the BP was greatly reduced and no difference between the alternating-repetitive conditions was detected; in contrast, only small differences were present in the controls for the negative slope (NS) and MP components and no difference for the reafferent positivity (RAP) component. A dipole analysis indicated the supplementary motor area and the premotor area as the likely generators of BP and NS' components, respectively. Gradientrecalled echo magnetic resonance imaging allowed the detection of a number of small hypointense lesions primarily located in the frontal lobes, as in diffuse axonal injury. This pattern of results indicates a selective deficit in motor preparation and a relatively spared pattern of activation during and following movement in these patients. Imaging data appear generally consistent with the pattern of MRCPs observed in the patient group. Implications of these results for the problem of slowness in TBI patients are discussed.

Effects of sustained, voluntary attention on amplitude and latency of steady-state visual evoked potential: a costs and benefits analysis

OBJECTIVE

Steady-state visual evoked potentials (VEPs) were recorded to study the mechanisms that underlie visual attention.

METHODS

VEPs were recorded from 1 cycle/degree sinusoidal grating contrast reversed at various temporal frequencies (6-10 Hz). This was displayed in one hemifield. A letter search display was flashed at a random rate in the other hemifield. The subject performed a demanding task on the recording stimulus (attended condition) or on the opposite side stimulus (unattended condition). Alternatively, he/she passively fixated on the fixation point (passive condition).

RESULTS

Relative to the passive condition, attended stimuli elicited enhanced-amplitude and shortened-latency VEP (benefits). Costs (i.e. responses to passive vs. unattended stimuli) were more marked for latency.

CONCLUSIONS

VEP latency may be the key of a priority-based attention mechanism acting at an early level.

An analysis of audio-visual crossmodal integration by means of event-related potential (ERP) recordings

Crossmodal integration was studied in humans by presenting random sequences of auditory (brief noise bursts), visual (flashes), and audiovisual (simultaneous noise bursts and flashes) stimuli from a central location at irregular intervals between 600 and 800 ms. The subjects' task was to press a button to infrequent and unpredictable (P=0.15) target stimuli that could be either a more intense noise burst, a brighter flash, or a combination of the two. In accordance with previous studies, behavioral data showed that bimodal target stimuli were responded to much faster and were identified more accurately than the unimodal target stimuli. The neural basis of this crossmodal interaction was investigated by subtracting the ERPs to the auditory (A) and the visual (V) stimuli alone from the ERP to the combined audiovisual (AV) stimuli (i.e. interaction=AV-(A+V)). Using this approach, we replicated previous reports of both early (at around 40 ms) and late (after 100 ms) ERP interaction effects. However, it appears that the very early interaction effects can be largely accounted for by an anticipatory ERP that precedes both the unimodal and bimodal stimuli. In calculating the ERP interaction this slow shift is subtracted twice, resulting in an apparent shift of the opposite polarity that may be confounded with actual crossmodal interactions.

Electrophysiological analysis of cortical mechanisms of selective attention to high and low spatial frequencies

OBJECTIVES

This study investigated whether short-latency (<100ms) event-related potential (ERP) components were modulated during attention to spatial frequency (SF) cues.

METHODS

Sinusoidally modulated checkerboard stimuli having high (5 cycles per degree (cpd)) or low (0.8cpd) SF content were presented in random order at intervals of 400-650ms. Subjects attended to either the high or low SF stimuli, with the task of detecting targets of slightly higher or lower SF, respectively, than the above standards. ERPs were recorded from 42 scalp sites during task performance and spatio-temporal analyses were carried out on sensory-evoked and attention-related components.

RESULTS

Attended high SF stimuli elicited an early negative difference potential (ND120) starting at about 100ms, whereas attended low SF stimuli elicited a positivity (PD130) in the same latency range. The neural sources of both effects were estimated with dipole modeling to lie in dorsal, extrastriate occipital areas. Earlier evoked components evoked at 60-100ms that were modeled with striate and extrastriate cortical sources were not affected by attention to SF. Starting at 150ms, attended stimuli of both SFs elicited a broad selection negativity (SN) that was localized to ventral extrastriate visual cortex. The SN was larger over the left/right cerebral hemisphere for attended stimuli of high/low SF.

CONCLUSIONS

These results support the view that attention to SF does not involve a mechanism of amplitude modulation of early-evoked components prior to 100ms. Attention to high and low SF information involves qualitatively different and hemispherically specialized neural processing operations.

Automatic gain control contrast mechanisms are modulated by attention in humans: evidence from visual evoked potentials

This study investigated the effect of attention on the contrast response curves of steady-state visual evoked potentials (VEPs) to counter-phased sinusoidal gratings. The 1 cyc/deg gratings were modulated either in luminance or chromaticity (equiluminant red-green). The luminance grating counter-phased at 9 Hz (to favour activation of the magno-cellular system), and the chromatic grating at 2.5 Hz (to favour activation of the parvo-cellular system). Attention was directed towards the gratings (displayed in the left visual field) by requiring subjects to detect and respond to randomly occurring changes in contrast. In the control condition, attention towards the grating was minimised by requiring subjects to detect a target letter amongst distracters briefly flashed in the contra-lateral visual field. Attention increased VEP amplitudes for both luminance and chromatic stimuli, more so at high than at low contrasts, increasing the slope of the contrast amplitude curves (over the non-saturating range of contrasts). The estimates of contrast threshold from extrapolation of amplitudes were unaffected by attention. Attention also changed the VEP phases, but only for luminance gratings, where it acted to reduce the magnitude of phase advance with contrast. Attention had no effect on the average phases for chromatic gratings. The results are consistent with the notion that attention acts on cortical gain control mechanisms, which are known to be different for the magno- and parvo-cellular systems.

Spatial anisotropy of saccadic latency in normal subjects and brain-damaged patients

In the present study, reaction time of oblique and orthogonal saccades was investigated in normal subjects and in two groups of patients with right (RBD) and left (LBD) vascular cerebral lesions and no signs of spatial neglect. Clear altitudinal effects were present in each group of subjects: saccadic latencies were longer in the lower than in the upper part of the visual field for both orthogonal and oblique saccades. Asymmetry along the horizontal meridian was present only in case of right hemisphere damage. This supports the view that a lesion in the right hemisphere causes a greater deficit of visual-spatial processing than a left hemisphere lesion. A cerebral lesion in the right and/or left hemisphere produces a general slowing in the saccadic latency and a general reduction in the accuracy of saccades with respect to normal subjects performance. Further, it seems that making saccades in oblique direction reduces the general saccade efficiency.

Putting spatial attention on the map: timing and localization of stimulus selection processes in striate and extrastriate visual areas

This study investigated the cortical mechanisms of visual-spatial attention in a task where subjects discriminated patterned targets in one visual field at a time. Functional magnetic imaging (fMRI) was used to localize attention-related changes in neural activity within specific retinotopic visual areas, while recordings of event-related brain potentials (ERPs) traced the time course of these changes. The earliest ERP components enhanced by attention occurred in the time range 70-130 ms post-stimulus onset, and their neural generators were estimated to lie in the dorsal and ventral extrastriate visual cortex. The anatomical areas activated by attention corresponded closely to those showing increased neural activity during passive visual stimulation. Enhanced neural activity was also observed in the primary visual cortex (area V1) with fMRI, but ERP recordings indicated that the initial sensory response at 50-90 ms that was localized to V1 was not modulated by attention. Modeling of ERP sources over an extended time range showed that attended stimuli elicited a long-latency (160-260 ms) negativity that was attributed to the dipolar source in area V1. This finding is in line with hypotheses that V1 activity may be modulated by delayed, reentrant feedback from higher visual areas.

Influence of the radial and vertical dimensions on lateral neglect

The influence of radial (near-far) and vertical (upper-lower) dimensions on lateral visuo-spatial neglect was studied using two horizontal line-bisection tasks (one motor and one perceptual). A group of 15 patients with neglect and a group of 14 right-brain damaged patients without neglect were examined. This latter group was used to define the range of variability in line-bisection performance that was independent of neglect. For the radial dimension, some neglect patients showed greater errors in far space than in near space (for both stimuli presented in the upper and lower space). Fewer patients showed the opposite pattern (i.e., greater errors for near-space stimuli). These near-far asymmetries were present for both the motor and perceptual conditions and showed a good degree of intra-individual consistency. This finding contradicts the hypothesis that the motor component is critical for yielding such asymmetry. For the vertical dimension, the results indicated that neglect patients make more bisection errors for lower-space stimuli than for upper-space stimuli. This vertical asymmetry was nearly always confined to stimuli in near space. Asymmetries along the vertical dimension were present for both perceptual and motor conditions, although intra-individual consistency was low. When perceptual and motor conditions were directly compared, several neglect patients showed greater errors in the perceptual than in the motor task.

Abnormal visual event-related potentials in obsessive-compulsive disorder without panic disorder or depression comorbidity

Visual event-related potentials and spline map topography during a discriminative response task (DRT) were studied in 8 obsessive-compulsive disorder (OCD) patients without comorbidity for panic disorder or depression and in 12 age-matched controls. In the DRT task (like in a go/no-go task) the subject had to press a button when the target stimuli appeared and had to retain the response when the non-target stimulus appeared (vertical bars were intermixed with an equal probability of horizontals). OC patients had greater N1 latency than controls and their N1 and P3 amplitude was larger for the target stimuli, but not for non-target stimuli. In the normals, non-target stimuli (no-go task) produced a larger activation than target stimuli (go task). In the OCD patients the target stimuli produced the same large activation as the non-target. These findings are consistent with theories that consider OCD to be an attentional disorder deriving from a misallocating of cognitive resources. Moreover, spline map topography confirmed that P3 hyperactivation is localised principally on the frontal lobes.

Spatial attention has different effects on the magno- and parvocellular pathways

Attention was directed to the left or to the right of the fixation point by the lateral presentation of a target on which the subject had to perform an attention demanding task. A (task-irrelevant) grating displayed in the left visual field was the visual evoked potential (VEP) stimulus. Gratings modulated either in luminance or colour contrast at various temporal frequencies were used in order to maximise the activation of magno- or parvocellular pathways. VEPs recorded in attended and unattended conditions were compared. For luminance stimuli, both latency and amplitude of VEPs were modified by attention. For chromatic stimuli, attention affected the amplitude but not the latency of VEPs. Spatial attention uses different mechanisms when magno- or parvocellular systems are involved.

Electrophysiological evidence for an early attentional mechanism in visual processing in humans

The effect of sustained spatial attention on a task-irrelevant grating displayed in the left visual field was studied by steady-state and transient visual evoked potentials (VEP). For the steady-state experiment, the task irrelevant grating was phase-reversed at different temporal frequencies. In the transient experiment the grating was reversed abruptly at low temporal frequency. In both conditions a target requiring visual attention was presented either in the left or in the right visual field, directing attention either to the left or to the right. VEPs amplitude enhancement by attention was observed on steady-state VEPs and on P100 and N140 transient VEPs components, confirming previous observations. Moreover, VEPs in the attended condition had shorter latency than VEPs in the unattended condition. The difference was about 15 ms with steady-state; with transient recording the lag was 7 ms for N60 and 10 ms for P100. The latency change of the N60 component, suggests that the modulation of attention on visual processing might be earlier than previously thought. A control experiment to assess the influence of eye movements on the test was also performed.

Mutations that alter the kinase and phosphatase activities of the two-component sensor EnvZ

EnvZ, a membrane receptor kinase-phosphatase, modulates porin expression in Escherichia coli in response to medium osmolarity. It shares its basic scheme of signal transduction with many other sensor-kinases, passing information from the amino-terminal, periplasmic, sensory domain via the transmembrane helices to the carboxy-terminal, cytoplasmic, catalytic domain. The native receptor can exist in two active but opposed signaling states, the OmpR kinase-dominant state (K+ P-) and the OmpR-P phosphatase-dominant state (K- P+). The balance between the two states determines the level of intracellular OmpR-P, which in turn determines the level of porin gene transcription. To study the structural requirements for these two states of EnvZ, mutational analysis was performed. Mutations that preferentially affect either the kinase or phosphatase have been identified and characterized both in vivo and in vitro. Most of these mapped to previously identified structural motifs, suggesting an important function for each of these conserved regions. In addition, we identified a novel motif that is weakly conserved among two-component sensors. Mutations that alter this motif, which is termed the X region, alter the confirmation of EnvZ and significantly reduce the phosphatase activity.

Visual evoked potentials are affected by trunk rotation in neglect patients

Steady-state visual evoked potentials (VEPs) were recorded in four patients with unilateral visuo-spatial neglect, stimulating either the left or the right hemifield. In the standard condition (head and body oriented straight ahead towards the stimulus) the left hemifield VEP was delayed. When the body was turned to the left, however, the two hemifield latencies were comparable. These results were confirmed with the transient VEP technique. No effect of trunk rotation was observed in a group of patients with left brain damage and without neglect. The results imply that the sensory afferents from neck muscles might restore the altered occipital activity and suggest that the same conditions which modulate neglect modulate VEPs latencies.

The essential tension: opposed reactions in bacterial two-component regulatory systems

In many different bacteria several sensory-response functions are controlled by systems of similar design. Most consist of two proteins, one of which regulates the phosphorylation of the other in response to an environmental stimulus. Regulation is achieved by balancing opposed phosphorylation and dephosphorylation reactions against each other. Remarkably, such a system can generate a signal whose strength is independent of the concentration of either component.

Mutations that affect separate functions of OmpR the phosphorylated regulator of porin transcription in Escherichia coli

OmpR is a member of a family of bacterial transcriptional regulators whose activity is controlled by phosphorylation. It regulates the transcription of two genes, serving as an activator of ompC, and as both an activator and a repressor of ompF. A previously isolated collection of ompR mutations was analyzed for the effect of each on the expression of both genes simultaneously. The results of this analysis indicate that the activation, repression, and DNA binding functions of OmpR can be disrupted independently, and that mutations interfering with each of these functions cluster within the sequence of the OmpR protein. The nature of these mutations is discussed in terms of the mechanisms by which OmpR regulates transcription, and potentially similar mechanisms operating within closely related response regulators.

Direct simulation of yeast 2-microns circle plasmid amplification

The 2-microns circle is a plasmid found in most strains of the yeast Saccharomyces cerevisiae at approximately 60-100 copies per cell. The plasmid possesses the novel capacity for replicative amplification induced by site-specific recombination. To address the question of whether the recombination model is adequate to account for observed rates of 2-microns circle amplification, we developed a direct computational simulation of the amplification system. Results of this simulation show that theoretically at least six copies per plasmid can be produced in each generation, and that previously unanticipated replication intermediates contribute largely to this degree of amplification.

Membrane-derived oligosaccharides affect porin osmoregulation only in media of low ionic strength

Gram-negative bacteria grown under conditions of low osmolarity accumulate significant amounts of periplasmic glucans, membrane-derived oligosaccharides (MDO) in Escherichia coli and cyclic glucans in members of the family Rhizobiaceae. It was reported previously (W. Fiedlder and H. Rotering, J. Biol. Chem. 263:14684-14689, 1988) that mdoA mutants unable to synthesize MDO show a number of altered phenotypes, among them a decreased expression of OmpF and an increased expression of OmpC, when grown in a Bacto Peptone medium of low osmolarity and low ionic strength. Although we confirm the findings of Fiedler and Rotering, we find that the regulation of OmpF and OmpC expression in mdoA mutants is normal in cells grown on other low-osmolarity media, eliminating the possibility that MDO itself might control porin expression. Our data suggest that a certain minimal ionic strength in the periplasm is needed for normal porin regulation. In media containing very low levels of salt, this may be contributed by anionic MDO.

EnvZ controls the concentration of phosphorylated OmpR to mediate osmoregulation of the porin genes

Osmoregulation of the bacterial porin genes ompF and ompC is controlled by a two-component regulatory system. EnvZ, the sensor component of this system, is capable both of phosphorylating and dephosphorylating OmpR, the effector component. Mutations were isolated in envZ that abolish the expression of both porin genes. These mutants appear to have lost the kinase activity of EnvZ while retaining their phosphatase activity, so that in their presence OmpR is completely unphosphorylated. The behavior of these mutants in haploid, and in diploid with other envZ alleles, is consistent with a model in which EnvZ mediates osmoregulation by controlling the concentration of a single species. OmpR-P.

Suppressor mutations in rpoA suggest that OmpR controls transcription by direct interaction with the alpha subunit of RNA polymerase

We have isolated mutations in rpoA, the gene encoding the alpha subunit of RNA polymerase, that specifically affect transcriptional control by OmpR and EnvZ, the two-component regulatory system that controls porin gene expression in Escherichia coli. Characterization of these mutations and a previously isolated rpoA allele suggests that both positive and negative regulation of porin gene transcription involves a direct interaction between OmpR and RNA polymerase through the alpha subunit. Several of the rpoA mutations cluster in the carboxy-terminal portion of the alpha protein, further suggesting that it is this domain of alpha that is involved in interaction with OmpR and perhaps other transcriptional regulators as well.