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Rhodobacter sphaeroides swims by unidirectional rotation of a single medial flagellum, re-orienting randomly by Brownian motion when flagellar rotation tops and restarts. Previously we identified a mutant with a paralysed flagellum, which was complemented by a Rhodobacter gene that had homology to motB of Escherichia coli , a bacterium with bidirectional flagella. In the current work, interposon mutagenesis upstream of the Rhodobacter motB gene gave rise to another paralysed mutant, RED5. DNA sequence analysis of this upstream region showed one open reading frame, the predicted polypeptide sequence of which shows homology to the MotA protein of E. coli . MotA is thought to be a proton 'pore' involved in converting proton-motive force into flagellar rotation. Several potential proton-binding amino acids were conserved between putative membrane-spanning regions of R. sphaeroides and E. coli MotA sequences, along with a highly charged cytoplasmic linker region. Complementation studies with mutant RED5 showed the presence of an active promoter upstream from motA which was found to be necessary for expression of both motA and motB , Examination of the upstream DNA sequence showed only one putative promoter-like sequence which resembled a σ54- type promoter, including a potential enhancer binding site. The overall similarities between the R. sphaeroides MotA protein and those from other bacteria suggest that, despite the novel unidirectional rotation of he R. sphaeroides flagellum, the function of the MotA protein is similar to that in bacteria with bidirectional flagella.  相似文献   
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Growth and function of well differentiated FRTL-5 thyroid cells depend on thyrotropin as its main regulatory hormone. We demonstrate here that stable transfection of FRTL-5 cells with the human thyrotropin receptor cDNA results in cellular transformation of these cells with altered cell shape and loss of contact inhibition. The transformed cells replicate in soft agar and form invasive tumors when cell suspensions are implanted onto nude mice. They have lost their thyrotropin dependent growth and their ability to concentrate iodide and synthesize thyroglobulin. But they still express the rat thyrotropin receptor mRNA and accumulate cAMP in response to thyrotropin stimulation. However, although the full length human thyrotropin receptor cDNA is integrated into their genome, transformed cells do not express the human thyrotropin receptor mRNA.  相似文献   
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We have cloned a transient type K channel from rat heart (RH10) and coexpressed a metabotropic glutamate receptor (mGluR5) to study the functional modulation of RH10 coupled to the phosphatidylinositol (PI) hydrolysis. Stimulation of mGluR5 suppressed peak amplitude of RH10 current and affected voltage dependence of activation and inactivation of the channel.  相似文献   
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The synthesis and characterization of 2'-5'-oligothioadenylate by UO2(2+) ion catalyst are described. The polymerization of imidazole-activated thioadenylate or thioinosylate yielded oligomers containing mainly 2'-5'-internucleotidyl blond and Rp configuration at phosphorous atom.  相似文献   
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In this paper the electrostatic interactions between membrane-embedded ion-pumps and their consequences for the kinetics of pump-mediated transport processes have been examined. We show that the time course of an intrinsically monomolecular transport reaction can become distinctly nonexponential, if the reaction is associated with charge translocation and takes place in an aggregate of pump molecules. First we consider the electrostatic coupling of a single dimer of ion-pumps embedded in the membrane. Then we apply the treatment to the kinetic analysis of light-driven proton transport by bacteriorhodopsin which forms two-dimensional hexagonal lattices. Finally, for the case of nonordered molecules, we also consider a model in which the pumps are randomly distributed over the nodes of a lattice. Here the average distance is equal to that deduced experimentally and the elemental size of the lattice is the effective diameter of one single pump. This latter model is applied to an aggregate of membrane-embedded Na, K- and Ca-pumps. In all these cases the electrostatic potential considered is the exact solution calculated from the method of electrical images for a plane membrane of finite thickness immersed in an infinite aqueous solution environment. The distributions of charges (ions or charged binding sites) are considered homogeneous or discrete in the membrane and/or in the external solution. In the case of discrete distributions we compare the results from a mean field approximation and a stochastic simulation.  相似文献   
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