Figure 4.

Single molecules in bacteria. (A) FP-labeled MreB, an actin homolog, shows treadmilling through short MreB filaments in a living C. crescentus cell. Directional motion of MreB–FP was measured by imaging single copies of MreB–FP. Single molecules trace out the filaments and the cytoskeletal structure, exhibiting direction and zig-zag motions (bottom left). The diagrams in the center depict the mechanism of treadmilling and motion of MreB monomers in filaments. The cells in the upper right represent several trajectories of the movements of single MreB–FP, tracing out filaments. The + (toward the so-called “stalked” pole of the cell) and – (toward the “swarmer” pole) signs indicate the direction of the movement. See reference ³⁵. (B) Gene expression visualized on the individual-cell and single-molecule scale. (top) Time-lapse movie of fluorescence images (yellow) overlaid with simultaneous white-light images (gray) show E. coli cells expressing single FP-labeled proteins (sporadic bursts of yellow). (bottom) Time traces of the expression of proteins along three particular cell lineages extracted from time-lapse fluorescence movies. The vertical axis is the number of protein molecules newly synthesized during the last three minutes. The dotted lines mark the cell division times. The time traces show that protein production occurs in random bursts, within which variable numbers of protein molecules are generated. From reference ⁹⁴. (C) Single PopZ–FP molecules in a living C. crescentus cell. (left) Time-lapse visualization of two molecules in a cell, with colored lines tracking the distance moved between frames. One molecule (red) remains localized to the pole, and the other (green) has increased mobility. The tracks are overlaid on a transmitted light image of the cell, outlined in black. (right) A representation of the data from the experiment, showing the distance of the molecules from one pole as a function of time. The black horizontal dotted line marks the opposite pole; the red and green lines follow the stationary and mobilized molecules, respectively. From reference ⁸⁶.