Tunable, continuous-wave optical parametric oscillator with more than 1W output power in the orange visible spectrum.

We report on the implementation of an all-solid-state optical parametric oscillator (OPO) laser system, pumped by a fiber laser, and extended by intra-cavity sum frequency generation (SFG) to provide tunable radiation with output powers well beyond 1 W in the visible regime between 605 and 616 nm. We use periodically poled sections for quasi phase-matched OPO and SFG processes, implemented on a single MgO:PPLN crystal. A Pound-Drever-Hall frequency stabilization reduces the laser linewidth to the range of 100 kHz (FWHM), determined by measurements of spectral hole burning in a rare-earth ion doped crystal as well as analysis of side-of-fringe transmission in a low finesse Fabry-Perot resonator.

Mi2beta shows chromatin enzyme specificity by erasing a DNase I-hypersensitive site established by ACF.

ATP-dependent chromatin-remodeling enzymes are linked to changes in gene expression; however, it is not clear how the multiple remodeling enzymes found in eukaryotes differ in function and work together. In this report, we demonstrate that the ATP-dependent remodeling enzymes ACF and Mi2beta can direct consecutive, opposing chromatin-remodeling events, when recruited to chromatin by different transcription factors. In a cell-free system based on the immunoglobulin heavy chain gene enhancer, we show that TFE3 induces a DNase I-hypersensitive site in an ATP-dependent reaction that requires ACF following transcription factor binding to chromatin. In a second step, PU.1 directs Mi2beta to erase an established DNase I-hypersensitive site, in an ATP-dependent reaction subsequent to PU.1 binding to chromatin, whereas ACF will not support erasure. Erasure occurred without displacing the transcription factor that initiated the site. Other tested enzymes were unable to erase the DNase I-hypersensitive site. Establishing and erasing the DNase I-hypersensitive site required transcriptional activation domains from TFE3 and PU.1, respectively. Together, these results provide important new mechanistic insight into the combinatorial control of chromatin structure.

Spectral broadening and stimulated Raman conversion in a continuous-wave optical parametric oscillator.

We have confirmed that single-frequency oscillation of a continuous-wave singly resonant optical parametric oscillator is limited to operation below a critical value of the pumping ratio, as predicted by early theoretical treatments. We also report different regimes of spectral broadening as well as stimulated Raman conversion of the signal wave above this critical pump level. We show that spectral broadening may be eliminated by implementing output coupling of the signal wave and demonstrate 8.6 W of total signal and idler output with single-frequency spectra at both wavelengths for 14.5 W of pump power.

Low threshold, singly-resonant CW OPO pumped by an all-fiber pump source.

An oscillation threshold of 780mW has been demonstrated in a singly-resonant, continuous-wave optical parametric oscillator (CW SRO) using a fiber-amplified, distributed feedback (DFB) fiber laser as pump source. A linewidth of 1MHz was measured, and the idler frequency was fine-tuned by up to 130GHz by tuning the pump laser. To our knowledge, this is the first example of a single frequency CW SRO pumped by an all-fiber pump source, a reduction in threshold by a factor of three over previous 1- microm-pumped CW SROs, and a reduction by two orders of magnitude in the linewidth of CW SROs pumped by fiber pump sources.

Recruitment of SWI/SNF to the human immunodeficiency virus type 1 promoter.

Following human immunodeficiency virus type 1 (HIV-1) integration into the host cell's genome, the 5' long terminal repeat (LTR) is packaged into a highly specific chromatin structure comprised of an array of nucleosomes positioned with respect to important DNA sequence elements that regulate the transcriptional activity of the provirus. While several host cell factors have been shown to be important for chromatin remodeling and/or basal transcription, no specific mechanism that relieves the transcriptional repression imposed by nuc-1, a positioned nucleosome that impedes the start site of transcription, has been found. Since phorbol esters cause the rapid disruption of nuc-1 and markedly stimulate HIV-1 transcription, we looked for protein factors that associate with this region of the HIV-1 promoter in a phorbol-ester-dependent manner. We report here that ATF-3, JunB, and BRG-1 (the ATPase subunit of the 2-MDa human chromatin remodeling machine SWI/SNF) are recruited to the 3' boundary of nuc-1 following phorbol myristate acetate stimulation in Jurkat T cells. Analysis of the recruitment of BRG-1 in nuclear extracts prepared from Jurkat T cells and reconstitution of an in vitro system with purified components demonstrate that ATF-3 is responsible for targeting human SWI/SNF (hSWI/SNF) to the HIV-1 promoter. Importantly, this recruitment of hSWI/SNF required HMGA1 proteins. Further support for this conclusion comes from immunoprecipitation experiments showing that BRG-1 and ATF-3 can exist together in the same complex. Although ATF-3 clearly plays a role in the specific targeting of BRG-1 to the HIV-1 promoter, the maintenance of a stable association between BRG-1 and chromatin appears to be dependent upon histone acetylation. By adding BRG-1 back into a BRG-1-deficient cell line (C33A cells), we demonstrate that trichostatin A strongly induces the 5'-LTR-driven reporter transcription in a manner that is dependent upon BRG-1 recruitment.