. The related Drosophila Suppressor 2 of zeste [Su(z)2] and Posterior sex combs (Psc) proteins are both locus-specific chromosome binding proteins. They are found at many of the same polytene chromosome loci as other Polycomb-group proteins. The 1,365 amino acid Su(z)2 protein and the 1,603 amino acid Psc protein share a conserved 200 amino acid domain, the homology region (HR). To identify the protein domain responsible for locus-specific chromosome binding, we made a series of Hsp70 :cDNA deletion constructs of the Sz(z)2 gene and transformed these into flies. We found that the HR is necessary and sufficient for Su(z)2 locus-specific polytene chromosome binding. The murine Bmi-1 protein also shares the conserved HR domain. When expressed in flies, the Bmi-1 protein showed a locus-specific chromosome binding pattern similar to that of the Su(z)2 and Psc proteins. These results argue that a locus-specific chromosome binding function resides in the HR domain. Other results show that a second, low affinity, non-specific chromosome binding function is localized outside the HR in the Su(z)2 protein, and that the Su(z)2 protein contains at least two nuclear localization signals.

Usually edge enhancement of optical images is produced by introduction of loss into the low spatial frequency components of the image-bearing beam in the Fourier plane of a lens. We report on edge-enhanced phase-conjugate images from a mutually pumped conjugator accomplished by spatial filtering in the Fraunhofer diffraction region of the input beams. High-resolution (128-160 lines/mm) edge-enhanced images are obtained through traditional spatial filtering in the Fourier plane and in the Fraunhofer regime. Amplification of these edge-enhanced images is observed with some loss of high spatial resolution (50 lines/mm).

Cross talk is observed during the transient time of the photorefractive grating formation in a mutually pumped phase conjugator. We show that this feature can be used to transfer pictorial information from one location to another. The transfer is instantaneous and is demonstrated at a resolution of 6 lines/mm.

The ability of a chimeric HP1-Polycomb (Pc) protein to bind both to heterochromatin and to euchromatic sites of Pc protein binding was exploited to detect stable protein-protein interactions in vivo. Previously, we showed that endogenous Pc protein was recruited to ectopic heterochromatic binding sites by the chimeric protein. Here, we examine the association of other Pc group (Pc-G) proteins. We show that Posterior sex combs (Psc) protein also is recruited to heterochromatin by the chimeric protein, demonstrating that Psc protein participates in direct protein-protein interaction with Pc protein or Pc-associated protein. In flies carrying temperature-sensitive alleles of Enhancer of zeste[E(z)] the general decondensation of polytene chromosomes that occurs at the restrictive temperature is associated with loss of binding of endogenous Pc and chimeric HP1-Polycomb protein to euchromatin, but binding of HP1 and chimeric HP1-Polycomb protein to the heterochromatin is maintained. The E(z) mutation also results in the loss of chimera-dependent binding to heterochromatin by endogenous Pc and Psc proteins at the restrictive temperature, suggesting that interaction of these proteins is mediated by E(z) protein. A myc-tagged full-length Suppressor 2 of zeste [Su(z)2] protein interacts poorly or not at all with ectopic Pc-G complexes, but a truncated Su(z)2 protein is strongly recruited to all sites of chimeric protein binding. Trithorax protein is not recruited to the heterochromatin by the chimeric HP1-Polycomb protein, suggesting either that this protein does not interact directly with Pc-G complexes or that such interactions are regulated. Ectopic binding of chimeric chromosomal proteins provides a useful tool for distinguishing specific protein-protein interactions from specific protein-DNA interactions important for complex assembly in vivo.

Phase-conjugate images with a resolution greater than 250 lines/mm are obtained through the use of a bridge, double-pumped phase conjugator. We demonstrate that this conjugator can carry out imageprocessing tasks, such as the addition and subtraction of complex spatial distributions, with a spatial resolution of >100 lines/mm. These results represent a significant improvement over previously reported resolutions obtained from photorefractive mutually pumped phase conjugators and approach the theoretical limit imposed by the grating spacing and cross talk.

We observe dynamic ferroelectric domain gratings in strontium barium niobate (SBN) induced by photorefractive space charge fields. The optically induced modulation of the spontaneous polarization attains a maximum of 1%. Quasi-phase matched second harmonic enhancements are observed above the ferroelectric-paraelectric phase transition due to the glassy ferroelectric nature of SBN. We find that the second harmonic power is significantly enhanced by recording gratings in optically fatigued rather than electrically poled crystals.

Single laser light pulses were used to induce gratings in nominally undoped and iron-doped Sr0.60Ba0.40Nb2O6 (SBN:60) using, in separate experiments, both 22 ps (λ = 532 nm) and 400 fs (λ = 580 nm) pulses. In both cases the gratings appear to have at least two temporal components outside the cross correlation time scale of the pulses. The first component is shown to be associated with induced absorption at the probe wavelength while a photorefractive grating associated with the displacement of charge carriers explains the second. The two-photon absorption coefficient, β, was measured to be 2.3 cm/GW (λ = 532 nm) and 2.0 cm/GW (λ = 580 nm) for the undoped sample and 2.15 cm/GW (λ = 532 nm) for the iron-doped sample.

Focusing and defocusing of laser light has been observed for many years. Kerr type materials exhibit this effect but only for high intensities. We show experimental evidence that photorefractive materials can also produce dramatic focusing and defocusing. Whereas Kerr materials produce this effect for high intensities, photorefractive materials produce these effects independent of intensity indicating that this effect would be ideal for an optical limiter. We compare the characteristics of Kerr and photorefractive materials, discuss the physical models for both materials and present experimental evidence for photorefractive defocusing. Self-focusing and defocusing was observed for any incident polarization although the effect was more pronounced using extraordinary polarized light. In addition, self-focusing or defocusing could be observed depending on the direction of the applied electric field. When the applied field was in the same direction as the crystal spontaneous polarization, focusing was observed. When the applied field was opposite the material spontaneous polarization, the incident laser light was dramatically defocused.

We consider the application of photorefractive beam fanning as an optical limiter. Results indicate that by focusing the incident laser light into the photorefractive crystal and by applying electric fields that limiting is possible at the 10μJ range with an optical density or O. D. of 3. These results are independent of the incident optical pulse width.

We demonstrate a method of dynamic, tunable quasi-phase matched second-harmonic generation using optically induced polarization gratings with periods equal to twice the coherence length. These gratings increase the peak second-harmonic conversion efficiency by a factor of 17 above a poled strontium barium niobate crystal, to 0.01% for fundamental beam intensities of 0.8 MW cm−2. We generate quasi-phase matching spectral response peaks as narrow as 0.175 nm and tailor the response by writing an ensemble of gratings in the same volume, each of which enhances the second-harmonic generation at a predetermined wavelength.