A labile membrane domain, characterized by a sharp phase transition above physiological temperatures, has been identified in Azotobacter vinelandii, Escherichia coli and Bacillus subtilis which are genetically competent. The phase transition is observed by fluorescence emission spectroscopy with amphiphilic and hydrophobic probes located in all regions of the bilayer. Its intensity correlates closely with transformability and also with the number and size of semi-regular pebbled protein-free regions in the plasma membranes of A. vinelandii and E. coli as observed by freeze-fracture electron microscopy. There is substantive evidence that R-(-)-poly hydroxybutyrate (PHB) is a constituent of the domain. Ca2+ is required for the stability of the domain, and its uptake by cells developing genetic transformability is coincident with polyphosphate synthesis and with the formation of the domain. It is proposed that the domain structure is a PHB helix spanning the membrane cross-bridged by Ca2 to an inner helix of polyphosphate anion which neutralizes and delocalizes the cation charge and orients the structure perpendicular to the plane of the membrane. It is postulated to function as a Ca2+ and phosphate transporter in exponential-phase cells and as an importer of single-stranded DNA in competent cells. The participation of PHB in the domain will be further evaluated by studies of the rotational relaxation times of probes in model systems, membranes and intact cells. Intracellular free Ca2- and membrane Ca2+ will be measured as a function of the domain with Fura-2 and chlorotetracycline fluorescence, respectively. The factors influencing the synthesis of polyphosphate, PHB and their precursors during the competence protocols will be investigated. Complex formation between PHB and the polyanions: polyphosphate native DNA and ss-DNA will be studied to determine stability constants, cation selectivity sequences and rates of association and disassociation. PHB-Ca2+-polyphosphate complexes isolated from competent E. coli DH1 will be examined by bright field-dark field imaging, energy dispersive x-ray analysis, electron energy loss spectroscopy and microdiffraction with FE-STEM, and by optical rotatory dispersion, circular dichroism, infra-red dichroism and nuclear magnetic resonance spectroscopy. The ionophoretic activity of the complex will be examined in liposomes. Finally, the functioning of the domain will be studied in membrane vesicles and intact cells by examining its relationship to Ca2+ transport, polyphosphate transport and ss-DNA transport.
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