No abstract

No abstract

The rate of protein diffusion in bacterial cytoplasm may constrain a variety of cellular functions and limit the rates of many biochemical reactions in vivo. In this paper, we report noninvasive measurements of the apparent diffusion coefficient of green fluorescent protein (GFP) in the cytoplasm of Escherichia coli. These measurements were made in two ways: by photobleaching of GFP fluorescence and by photoactivation of a red-emitting fluorescent state of GFP (M. B. Elowitz, M. G. Surette, P. E. Wolf, J. Stock, and S. Leibler, Curr. Biol. 7:809–812, 1997). The apparent diffusion coefficient,Da , of GFP in E. coli DH5α was found to be 7.7 ± 2.5 μm2/s. A 72-kDa fusion protein composed of GFP and a cytoplasmically localized maltose binding protein domain moves more slowly, with Da of 2.5 ± 0.6 μm2/s. In addition, GFP mobility can depend strongly on at least two factors: first,Da is reduced to 3.6 ± 0.7 μm2/s at high levels of GFP expression; second, the addition to GFP of a small tag consisting of six histidine residues reduces Da to 4.0 ± 2.0 μm2/s. Thus, a single effective cytoplasmic viscosity cannot explain all values of Da reported here. These measurements have implications for the understanding of intracellular biochemical networks.

No abstract

2014 Nous présentons une étude théorique de la rétrodiffusion cohérente de la lumière par un milieu désordonné dans diverses situations incluant les effets dépendant du temps, les milieux absorbants et les effets liés à la modulation d'amplitude de la lumière. Nous discutons tout particulièrement le cas de la diffusion anisotrope et les effets de la polarisation afin d'expliquer quantitativement les résultats expérimentaux. Nous donnons un calcul microscopique de l'albedo cohérent afin de justifier la relation heuristique précédemment établie. Nous prédisons aussi la forme de l'albedo cohérent d'un milieu fractal. Enfin, la validité des différentes approximations utilisées est discutée et quelques développements ultérieurs sont évoqués. Abstract. 2014 A theoretical study of the coherent backscattering effect of light from disordered semi-infinite media is presented for various situations including time-dependent effects as well as absorption and amplitude modulation. Particular attention is devoted to the case of anisotropic scattering and to polarization in order to explain quantitatively experimental results. A microscopic derivation of the coherent albedo is given which strongly supports the heuristic formula previously established. In addition the coherent albedo of a fractal system is predicted. The validity of the different approximations used are discussed and some further theoretical developments are presented.