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The RpoS transcription factor (also called S or 38) is required for the expression of a number of stationary-phase and osmotically inducible genes in Escherichia coli. RpoS is also a virulence factor for several pathogenic bacteria, including Salmonella typhimurium. The activity of RpoS is regulated in response to several different signals, at the transcriptional and translational levels as well as by proteolysis. Here we report that host factor I (HF-I), the product of the hfq gene, is required for efficient expression of rpoS in S. typhimurium. HF-I is a small, heat-stable, site-specific RNA-binding protein originally characterized for its role in replication of the RNA bacteriophage Q␤ of E. coli. Its role in the uninfected bacterial cell has previously been unknown. Assays of ␤-galactosidase in strains with rpoS-lac fusions, Western blot (immunoblot) analysis, and pulse-labeling and immunoprecipitation of both fusion proteins and native RpoS show that an S. typhimurium hfq mutant has a four-to sevenfold reduction in expression of rpoS that is attributable primarily to a defect in translation. These results add a new level of complexity to the regulation of RpoS activity.

The RpoS sigma factor (also called S or 38 ) is known to regulate at least 50 genes in response to environmental sources of stress or during entry into stationary phase. Regulation of RpoS abundance and activity is complex, with many factors participating at multiple levels. One factor is the nutritional stress signal ppGpp. The absence of ppGpp blocks or delays the induction of rpoS during entry into stationary phase. Artificially inducing ppGpp, without starvation, is known to induce rpoS during the log phase 25-to 50-fold. Induction of ppGpp is found to have only minor effects on rpoS transcript abundance or on RpoS protein stability; instead, the efficiency of rpoS mRNA translation is increased by ppGpp as judged by both RpoS pulse-labeling and promoter-independent effects on lacZ fusions. DksA is found to affect RpoS abundance in a manner related to ppGpp. Deleting dksA blocks rpoS induction by ppGpp. Overproduction of DksA induces rpoS but not ppGpp. Deleting dksA neither alters regulation of ppGpp in response to amino acid starvation nor nullifies the inhibitory effects of ppGpp on stable RNA synthesis. Although this suggests that dksA is epistatic to ppGpp, inducing ppGpp does not induce DksA. A dksA deletion does display a subset of the same multiple-amino-acid requirements found for ppGpp 0 mutants, but overproducing DksA does not satisfy ppGpp 0 requirements. Sequenced spontaneous extragenic suppressors of dksA polyauxotrophy are frequently the same T563P rpoB allele that suppresses a ppGpp 0 phenotype. We propose that DksA functions downstream of ppGpp but indirectly regulates rpoS induction.Eubacteria have developed complex regulatory networks that recognize and respond to a variety of environmental sources of physiological stress. One element common to many such networks in gram-negative bacteria is RpoS (29), a regulator defined by sequence and functional studies as an alternative sigma subunit of RNA polymerase (38). Over the past decade, it has come to be appreciated that RpoS participates in the regulation of at least 50 genes and that RpoS is itself regulated by nearly half as many factors. RpoS has been referred to as "the master regulator of the general stress response in Escherichia coli" (6).Regulation of RpoS itself is arguably the most complicated system in bacteria. Regulation of RpoS involves transcription, mRNA turnover, translation initiation, and proteolysis. Reported transcription regulators include BarA (37), cyclic AMP/ cyclic AMP receptor protein (30), and ppGpp (27). Leader mRNA is a regulatory target affecting efficiency of translation initiation in different ways. With the rpoS transcript originating in nlpD and the initiating AUG at ϩ565 (50), a structure extending from ϩ458 to ϩ565 sequesters the ribosomal binding sequence through a much smaller cis-acting antisense element. One hypothesis is that translation initiation is positively regulated by hfq (7, 36) with HF-1 binding to leader RNA changing antisense element conformation (8). A small RNA, called DsrA, is normally ma...

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Limb position drift: implications for control of posture and movement. J Neurophysiol 90: 3105-3118, 2003; 10.1152/jn.00013.2003. In the absence of visual feedback, subject reports of hand location tend to drift over time. Such drift has been attributed to a gradual reduction in the usefulness of proprioception to signal limb position. If this account is correct, drift should degrade the accuracy of movement distance and direction over a series of movements made without visual feedback. To test this hypothesis, we asked participants to perform six series of 75 repetitive movements from a visible start location to a visible target, in time with a regular, audible tone. Fingertip position feedback was given by a cursor during the first five trials in the series. Feedback was then removed, and participants were to continue on pace for the next 70 trials. Movements were made in two directions (30°and 120°) from each of three start locations (initial shoulder angles of 30°, 40°, 50°, and initial elbow angles of 90°). Over the 70 trials, the start location of each movement drifted, on average, 8 cm away from the initial start location. This drift varied systematically with movement direction, indicating that drift is related to movement production. However, despite these dramatic changes in hand position and joint configuration, movement distance and direction remained relatively constant. Inverse dynamics analysis revealed that movement preservation was accompanied by substantial modification of joint muscle torque. These results suggest that proprioception continues to be a reliable source of limb position information after prolonged time without vision, but that this information is used differently for maintaining limb position and for specifying movement trajectory.

The RpoS transcription factor (also called S or 38 ) is required for the expression of a number of stationary-phase and osmotically inducible genes in enteric bacteria. RpoS is also a virulence factor for several pathogenic species, including Salmonella typhimurium. The activity of RpoS is regulated in response to many different signals, at the levels of both synthesis and proteolysis. Previous work with rpoS-lac protein fusions has suggested that translation of rpoS requires hfq function. The product of the hfq gene, host factor I (HF-I), is a ribosome-associated, site-specific RNA-binding protein originally characterized for its role in replication of the RNA bacteriophage Q␤ of Escherichia coli. In this study, the role of HF-I was explored by isolating suppressor mutations that map to the region directly upstream of rpoS. These mutations increase rpoS-lac expression in the absence of HF-I and also confer substantial independence from HF-I. DNA sequence analysis of the mutants suggests a model in which the RNA secondary structure near the ribosome binding site of the rpoS mRNA plays an important role in limiting expression in the wild type. Genetic tests of the model confirm its predictions, at least in part. It seems likely that the mutations analyzed here activate a suppression pathway that bypasses the normal HF-I-dependent route of rpoS expression; however, it is also possible that some of them identify a sequence element with an inhibitory function that is directly counteracted by HF-I.The rpoS gene encodes a specificity factor for RNA polymerase (44, 46, 59) which is required for the transcription of many genes expressed during the onset of stationary phase. RpoS-dependent adaptations to nutrient limitation and starvation identified so far in Escherichia coli include not only shifts in metabolic pathways but also resistance mechanisms protective against life-threatening stresses such as high osmolarity, heat shock, elevated H 2 O 2 , and UV light (reviewed in references 26 and 37). RpoS is also a virulence factor for Salmonella typhimurium (16) and other enteric bacteria. The regulation of this regulatory protein is itself complex: RpoS abundance can be increased by a variety of inducing treatments (37). Control of RpoS can occur both at the level of synthesis and by proteolysis (27,34,65); in this respect it is reminiscent of heat shock sigma factor (RpoH) regulation (66). It has also been demonstrated that RpoS abundance is positively regulated by ppGpp (23), and this may provide a unifying mechanism for the multitude of RpoS inducers. An hns mutant, lacking the abundant DNA-binding protein H-NS, has an increased level of RpoS in exponential phase, and this effect is remarkable because it occurs through increases in both translation and protein stability (2, 65). Genetic evidence suggests that RpoS is degraded by the energy-dependent ClpXP protease (52) with the help of other factors (42,48,58).The E. coli hfq gene product, host factor I (HF-I), was discovered through its role in the in vitro replicati...

Abstract& Neural representations of novel motor skills can be acquired through visual observation. We used repetitive transcranial magnetic stimulation (rTMS) to test the idea that this ''motor learning by observing'' is based on engagement of neural processes for learning in the primary motor cortex (M1). Human subjects who observed another person learning to reach in a novel force environment imposed by a robot arm performed better when later tested in the same environment than subjects who observed movements in a different environment. rTMS applied to M1 after observation reduced the beneficial effect of observing congruent forces, and eliminated the detrimental effect of observing incongruent forces. Stimulation of a control site in the frontal cortex had no effect on reaching. Our findings represent the first direct evidence that neural representations of motor skills in M1, a cortical region whose role has been firmly established for active motor learning, also underlie motor learning by observing. &

Anatomical and physiological evidence suggests that vision-for-perception and vision-for-action may be differently sensitive to increasingly peripheral stimuli, and to stimuli in the upper and lower visual fields (VF). We asked participants to fixate one of 24 randomly presented LED arranged radially in eight directions and at three eccentricities around a central target location. One of two (small, large) target objects was presented briefly, and participants responded in two ways. For the action task, they reached for and grasped the target. For the perception task, they estimated target height by adjusting thumb-finger separation. In a final set of trials for each task, participants knew that target size would remain constant. We found that peak aperture increased with eccentricity for grasping, but not for perceptual estimations of size. In addition, peak grip aperture, but not size-estimation aperture, was more variable when targets were viewed in the upper as opposed to the lower VF. A second experiment demonstrated that prior knowledge about object size significantly reduced the variability of perceptual estimates, but had no effect on the variability of grip aperture. Overall, these results support the claim that peripheral VF stimuli are processed differently for perception and action. Moreover, they support the idea that the lower VF is specialized for the control of manual prehension. Finally, the effect of prior knowledge about target size on performance substantiates claims that perception is more tightly linked to memory systems than action.