Absolute frequency measurement of the magnesium intercombination transition 1S0→3P1^1S_0 \to ^3P_1

We report on a frequency measurement of the $(3s^2)^1S_0\to(3s3p)^3P_1$ clock transition of $^{24}$Mg on a thermal atomic beam. The intercombination transition has been referenced to a portable primary Cs frequency standard with the help of a femtosecond fiber laser frequency comb. The achieved uncertainty is $2.5\times10^{-12}$ which corresponds to an increase in accuracy of six orders of magnitude compared to previous results. The measured frequency value permits the calculation of several other optical transitions from $^1S_0$ to the $^3P_J$-level system for $^{24}$Mg, $^{25}$Mg and $^{26}$Mg. We describe in detail the components of our optical frequency standard like the stabilized spectroscopy laser, the atomic beam apparatus used for Ramsey-Bord\'e interferometry and the frequency comb generator and discuss the uncertainty contributions to our measurement including the first and second order Doppler effect. An upper limit of $3\times10^{-13}$ in one second for the short term instability of our optical frequency standard was determined by comparison with a GPS disciplined quartz oscillator.Comment: 8 pages, 8 figure

Sub-Doppler-Kühlung und magnetische Speicherung von Magnesiumatomen bei Temperaturen unter 1 mK

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Influenza A virus does not encode a tetherin antagonist with Vpu-like activity and induces IFN-dependent tetherin expression in infected cells.

The interferon-induced host cell factor tetherin inhibits release of human immunodeficiency virus (HIV) from the plasma membrane of infected cells and is counteracted by the HIV-1 protein Vpu. Influenza A virus (FLUAV) also buds from the plasma membrane and is not inhibited by tetherin. Here, we investigated if FLUAV encodes a functional equivalent of Vpu for tetherin antagonism. We found that expression of the FLUAV protein NS1, which antagonizes the interferon (IFN) response, did not block the tetherin-mediated restriction of HIV release, which was rescued by Vpu. Similarly, tetherin-mediated inhibition of HIV release was not rescued by FLUAV infection. In contrast, FLUAV infection induced tetherin expression on target cells in an IFN-dependent manner. These results suggest that FLUAV escapes the antiviral effects of tetherin without encoding a tetherin antagonist with Vpu-like activity

NS1 does not antagonize tetherin.

<p>(A) Plasmids encoding the indicated proteins or empty plasmid were transfected into 293T cells and expression of tetherin and β-actin in cell lysates was determined by Western blot. Similar results were obtained in two separate experiments. (B) The experiment was performed as described in (A) but cell surface expression of tetherin was determined by FACS. The average of 6 to 8 independent experiments is shown, error bars indicate standard error of the mean (SEM). (C) Plasmids encoding HIV-1 Gag, tetherin and the indicated viral proteins were cotransfected into 293T cells and the presence of Gag in cell lysates and cell culture supernatants was determined by Western blot. Expression of β-actin in cell lysates was assessed as loading control. The results were confirmed in three separate experiments.</p

Influenza A virus infection induces tetherin expression at the cell surface in an interferon-dependent manner.

<p>(A) A549 cells were treated with 10,000 U/ml IFNβ (left panel) or infected with the indicated viruses at an MOI of 1 (right panel). After 24 h, the expression of tetherin at the cell surface was determined by FACS. The average of two separate experiments is shown, error bars indicate SEM. Similar results were obtained in two separate experiments. (B) A549 and Vero E6 cells, which were not exposed to FLUAV or IFN, were mock treated (filled grey histogram) or incubated with isotype matched control antibody and secondary antibody (black line) or incubated with anti-tetherin antibody and secondary antibody (grey line). Subsequently, staining was analyzed by FACS. (C) The experiment was carried out as described in (A) but Vero E6 cells were used. The average ± SEM of two separate experiments is shown for IFN induction of tetherin expression (left panel), while the average ± SD of a single experiment performed in triplicates are shown for FLUAV infection. Similar results were obtained in a separate experiment. (D) Expression of hemagglutinin (HA) on the cells analyzed in (C) was determined by FACS. The average ± SEM of two separate experiments is shown.</p

Influenza A virus infection does not antagonize tetherin.

<p>Plasmids encoding HIV-1 Gag and tetherin were cotransfected into 293T cells and the cells were subsequently infected with A/WSN/33 at the indicated MOIs or mock infected. At 24 h post infection the presence of Gag in cell lysates and culture supernatants (sups) as well as the expression of tetherin, FLUAV antigens and β-actin in cell lysates was determined by Western blot. Similar results were obtained in four separate experiments conducted with MOIs of 0.03 and 0.3.</p