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Biomedical subjects

H C Berg

Publications and source records attributed to H C Berg.

At least 109 records · Page 6Linked to original sources
PubMed1974

Temporal stimulation of chemotaxis in Escherichia coli.

We used the tracking microscope to study the chemotactic responses of E. coli to temporal gradients of L-glutamate generated in isotropic solutions by the action of the enzyme alanine aminotransferase. Positive gradients suppress directional changes which occur spontaneously in the absence of a stimulus. Negative gradients have little effect. The data can be fit with a model in which the suppression is proportional to the time rate of change of the fractional amount of chemoreceptor bound. The model accounts for the behavior of individual cells and populations of cells in spatial gradients. A computer simulation of the motion in spatial gradients indicates that if the bacteria have a "memory," its decay time cannot be much longer than a few seconds. The relationship between the responses observed in these experiments and in experiments in which solutions of an attractant at different concentrations are mixed is discussed.

Chemotaxis↗
PubMed1965

Erythrocyte membrane: chemical modification.

Erythrocytes treated with 1-fluoro-2,4-dinitrobenzene become permeable to Na(+) and K(+), but not to small water-soluble nonelectrolytes hemoglobin, and eventually lyse inisotonic buffer. Erythrocytes treated with 1,5-difluoro-2,4-dinitrobenzene become permeable to Na(+) and K(+) but do not lyse in buffer or in water, even after extraction with lipid solvents. The difluoro compound crosslinks the membrane and increases its strength. Both reagents appear to remove the positive fixed charge responsible for the cation impermeability of the normal cell.

Alcohols↗
PubMedDate not supplied

Successive incorporation of force-generating units in the bacterial rotary motor.

Mot mutants of Escherichia coli are paralysed: their flagella appear to be intact but do not rotate. The motA and motB gene products are found in the cytoplasmic membrane; they do not co-purify with flagellar basal bodies isolated in neutral detergents. Silverman et al. found that mot mutants could be ' resurrected ' through protein synthesis directed by lambda transducing phages carrying the wild-type genes. Here, we have studied this activation at the level of a single flagellar motor. Cells of a motB strain carrying plasmids in which transcription of the wild-type motB gene was controlled by the lac promoter were tethered to a glass surface by a single flagellum. These cells began to spin within several minutes after the addition of a lac inducer, and their rotational speed changed in a series of equally spaced steps. As many as 7 steps were seen in individual cells and, from the final speeds attained, as many as 16 steps could be inferred. These experiments show that each flagellar motor contains several independent force-generating units comprised, at least in part, of motB protein.

Cell Movement↗