Conformational Fluctuations of Chromosomal DNA in Escherichia Coli

Abstract

The cell is a very crowded structure, consisting of various organelles, proteins, nucleic acids, and cellular inclusions. It is the site of active, motor-driven processes far from thermodynamic equilibrium. Therefore, the intracellular dynamics are complex and subject to a multitude of constraints and forces. We study the conformational fluctuations of chromosomal DNA in vivo in live and dead Escherichia Coli cells by Fluorescence Correlation Spectroscopy (FCS). Conformational fluctuations of the DNA move the bound fluorophores stochastically into the diffraction-limited excitation volume of a focused laser beam in a confocal microscope. From the time correlation functions of the fluorescence intensity, we obtain the mean square displacement of the DNA on a time scale from microseconds to seconds. We see a substantial decrease in the power spectral density of the displacement fluctuations at frequencies below 10 Hz in the dead cells, compared to the live cells. The larger fluctuations in the living cells may indicate that the fluctuations on this time scale may be driven by active processes involving molecular motors that generate forces by ATP hydrolysis. On shorter time scales, we see little difference between live and dead cells, suggesting that processes on corresponding short length scales rely primarily on thermally-driven diffusive mechanisms.