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The molecular nonlinear photonic absorption processes of two nonlinear fiber core liquids are discussed in the context of nonlinear propagation and optical limiting of short pulses. These fiber arrays are capable of limiting threshold and clamped output below 1 micro J for picosecond and nanosecond pulses. We also discuss the observation of perhaps the largest optical nonlinearity in some dye-doped nematic liquid crystal films. These films will provide limiting action with a threshold power of 100 nWatt and limited transmission of << 1 microJoule for ms - cw laser.

The crystal structure of the delta' subunit of the clamp-loader complex of E. coli DNA polymerase III has been determined. Three consecutive domains in the structure are arranged in a C-shaped architecture. The N-terminal domain contains a nonfunctional nucleotide binding site. The catalytic component of the clamp-loader complex is the gamma subunit, which is homologous to delta'. A sequence-structure alignment suggests that nucleotides bind to gamma at an interdomain interface within the inner surface of the "C." The alignment is extended to other clamp-loader complexes and to the RuvB family of DNA helicases, and suggests that each of these is assembled from C-shaped components that can open and close the jaws of the "C" in response to ATP binding and hydrolysis.

Optical limiting of nanosecond and picosecond laser pulses through millimeter-length isotropic liquid-crystalcored fiber structures is reported. Low limiting threshold and clamped transmitted outputs are observed. The underlying nonlinear mechanisms are nonlinear photoabsorptions and scattering and lossy waveguiding caused by laser-induced thermal-density index fluctuations.

The crystal structure of the paired homeodomain bound to DNA as a cooperative dimer has been determined to 2.0 A resolution. Direct contacts between each homeodomain and the DNA are similar to those described previously. In addition, an extensive network of water molecules mediates contacts between the recognition helix and the DNA major groove. Several symmetrical contacts between the two homeodomains underlie the cooperative interaction, and deformations in the DNA structure are necessary for the establishment of these contacts. Comparison with structures of homeodomains bound monomerically to DNA suggests that the binding of a single paired homeodomain can introduce these DNA distortions, thus preparing a template for the cooperative interaction with a second homeodomain. This study shows how the paired (Pax) class homeodomains have achieved cooperativity in DNA binding without the assistance of other domains, thereby enabling the recognition of target sequences that are long enough to ensure specificity.

A new optical transducer for the detection of acoustic pressure in the diagnostic ultrasound frequency range is described. This transducer is based on the modulation of an evanescent light field by the incident acoustic energy. Theoretical design considerations are presented for the purpose of developing the most sensitive transducer. Based on these considerations an experimental transducer was constructed. Although less sensitive than predicted this device was capable of transducing ultrasonic pulses with a 1.0-MHz center frequency at diagnostic ultrasound amplitude levels. The techniques developed here are applicable for two-dimensional transduction and may prove a viable alternative to piezoelectric array transducers.

An example of two related enzymes that catalyse similar reactions but possess different active sites is provided by comparing the structure of Escherichia coli thioredoxin reductase with glutathione reductase. Both are dimeric enzymes that catalyse the reduction of disulphides by pyridine nucleotides through an enzyme disulphide and a flavin. Human glutathione reductase contains four structural domains within each molecule: the flavin-adenine dinucleotide (FAD)- and nicotinamide-adenine dinucleotide phosphate (NADPH)-binding domains, the 'central' domain and the C-terminal domain that provides the dimer interface and part of the active site. Although both enzymes share the same catalytic mechanism and similar tertiary structures, their active sites do not resemble each other. We have determined the crystal structure of E. coli thioredoxin reductase at 2 A resolution, and show that thioredoxin reductase lacks the domain that provides the dimer interface in glutathione reductase, and forms a completely different dimeric structure. The catalytically active disulphides are located in different domains on opposite sides of the flavin ring system. This suggests that these enzymes diverged from an ancestral nucleotide-binding protein and acquired their disulphide reductase activities independently.

Trypanothione reductase, a flavoprotein disulfide reductase specific to trypanosomatid parasites, has been crystallized by vapor diffusion of a protein solution (10 mg/ml) against 22% polyethylene glycol (average Mr 8000) containing 100 mM-ammonium sulfate. Crystals of a size suitable for structure determination by X-ray diffraction have been obtained by seeding protein solutions with smaller crystals. The space-group is P21 (a = 60.9 A, b = 161.8 A, c = 58.4 A, beta = 99.1 degrees). The molecular mass and volume of the unit cell suggest that there is a dimer of the enzyme in the asymmetric unit, and this is confirmed by self-rotation functions calculated using data to 4.5 A resolution. The crystals diffract to beyond 3 A resolution. Crystals of another P21 form (a = 91.3 A, b = 114.4 A, c = 92.0 A, beta = 141.3 degrees) are observed to grow under similar conditions.