Desulfovibrio magneticus RS-1 contains an iron- and phosphorus-rich organelle distinct from its bullet-shaped magnetosomes.

Intracellular magnetite crystal formation by magnetotactic bacteria has emerged as a powerful model for investigating the cellular and molecular mechanisms of biomineralization, a process common to all branches of life. Although magnetotactic bacteria are phylogenetically diverse and their crystals morphologically diverse, studies to date have focused on a few, closely related species with similar crystal habits. Here, we investigate the process of magnetite biomineralization in Desulfovibrio magneticus sp. RS-1, the only reported species of cultured magnetotactic bacteria that is outside of the alpha-Proteobacteria and that forms bullet-shaped crystals. Using a variety of high-resolution imaging and analytical tools, we show that RS-1 cells form amorphous, noncrystalline granules containing iron and phosphorus before forming magnetite crystals. Using NanoSIMS (dynamic secondary ion mass spectroscopy), we show that the iron-phosphorus granules and the magnetite crystals are likely formed through separate cellular processes. Analysis of the cellular ultrastructure of RS-1 using cryo-ultramicrotomy, cryo-electron tomography, and tomography of ultrathin sections reveals that the magnetite crystals are not surrounded by membranes but that the iron-phosphorus granules are surrounded by membranous compartments. The varied cellular paths for the formation of these two minerals lead us to suggest that the iron-phosphorus granules constitute a distinct bacterial organelle.

Data reduction methods for application of fluorescence correlation spectroscopy to pharmaceutical drug discovery.

Fluorescence methods are commonly used in pharmaceutical drug discovery to assay the binding of drug-like compounds to signaling proteins and other bio-particles. For binding studies of non-fluorescent compounds, a competitive format may be used in which the binding of the compound results in displacement of another fluorescently labeled ligand. Highly-sensitive measurements within nano-liter sized open probe volumes can be accomplished using a confocal epi-illumination geometry and thus key tools for such drug-binding studies include fluorescence correlation spectroscopy (FCS) and its related techniques. This paper reviews the general protocol for application of FCS to biomolecular compound-binding assays and it focuses on methods for the reduction of experimental photon count data to obtain the normalized autocorrelation function (ACF), on theoretical models of the ACF, and on statistical and systematic errors in the experimental ACF. Results from a detailed Monte Carlo simulation of FCS, which are useful for testing theoretical models and validating short-duration assay capabilities, are discussed. An illustrative example is presented on the use of FCS to assay binding of Alexa-488-labeled Bak peptide with Bcl-x(L), which is an intracellular protein that acts to protect against programmed cell death.