Functional roles of the conserved aromatic amino acid residues at position 108 (motif IV) and position 196 (motif VIII) in base flipping and catalysis by the N6-adenine DNA methyltransferase from Thermus aquaticus

The DNA methyltransferase (Mtase) from Thermus aquaticus (M.TaqI) catalyzes the transfer of the activated methyl group of S-adenosyl-L-methionine to the N6 position of adenine within the double-stranded DNA sequence 5'-TCGA-3'. To achieve catalysis M.TaqI flips the target adenine out of the DNA helix. On the basis of the three-dimensional structure of M.TaqI in complex with the cofactor and its structural homology to the C5-cytosine DNA Mtase from Haemophilus haemolyticus, Tyr 108 and Phe 196 were suggested to interact with the extrahelical adenine. The functional roles of these two aromatic amino acid residues in M.TaqI were investigated by mutational analysis. The obtained mutant Mtases were analyzed in an improved kinetic assay, and their ability to flip the target base was studied in a fluorescence-based assay using a duplex oligodeoxynucleotide containing the fluorescent base analogue 2-aminopurine at the target position. While the mutant Mtases containing the aromatic amino acid Trp at position 108 or 196 (Y108W and F196W) showed almost wild-type catalytic activity, the mutant Mtases with the nonaromatic amino acid Ala (Y108A and F196A) had a strongly reduced catalytic constant. Y108A was still able to flip the target base, whereas F196A was strongly impaired in base flipping. These results indicate that Phe 196 is important for stabilizing the extrahelical target adenine and suggest that Tyr 108 is involved in placing the extrahelical target base in an optimal position for methyl group transfer. Since both aromatic amino acids belong to the conserved motifs IV and XIII found in N6-adenine and N4-cytosine DNA Mtases as well as in N6-adenine RNA Mtases, a similar function of aromatic amino acid residues within these motifs is expected for the different Mtases.     

An impedance-matching technique for increasing the bandwidth ofmicrostrip antennas

The nature of the inherent narrow bandwidth of conventional microstrip patch antennas is considered. It is observed that, except for single-feed circularly polarized elements, their bandwidth is limited only by the resonant behavior of the input impedance and not by radiation pattern or gain variations, which usually are negligible over a moderate 10 to 20% bandwidth. Therefore, broadband impedance matching is proposed as a natural to increase the bandwidth. The maximum obtainable bandwidth is calculated using Fano's broadband matching theory. It is found that by using an optimally designed impedance-matching network, the bandwidth can be increased by a factor of at least 3.9, the exact value depending on the degree of matching required. A transmission-line prototype for a proper matching network is developed. The translation of this prototype network into a practical structure (e.g. a microstrip or stripline circuit) is considered. Practical design examples and experimental results which clearly show the validity of the technique are given     

A surface integral equation approach to the scattering andabsorption of doubly periodic lossy structures

The scattering and absorption of a doubly periodic array of absorbers, either placed in free space, backed by a perfect conductor or by a half-infinite space with the same material properties as the elements forming the array, is analyzed with a surface integral equation approach (SIE). The use of a suitable periodic Green's function as kernel of the SIE reduces the formulation of the problem to a single absorber. A set of equivalent electric and magnetic currents on the surface of the absorber is discretised using Glisson functions and the SIE is solved with Galerkin's method. The validity and flexibility of the SIE approach is exemplified by comparing numerical results with measurement data for a family of commercially available absorbers     

Approximate formulas for frequency dependence of microstrip parameters

Some commonly used formulas for the frequency dependence of microstrip parameters have a number of shortcomings such as limited range of validity and faulty asymptotic behaviour. Therefore it is recommended to use the planar waveguide model with a recently published dispersion formula and a modified formula for the effective width.     

Integral equation modeling of the scattering and absorption ofmultilayered doubly-periodic lossy structures

Previously, the scattering and absorption of a doubly-periodic array of pyramidal absorbers were analyzed using a surface integral equation (SIE) approach. This method is now extended to multilayered absorbers with arbitrarily shaped surfaces between the layers. The developed method is verified on planar stratified absorbers, on wedge absorbers, and rectangular absorbers showing the capability of the method to handle such structures as well. It is also shown that convergence depends critically on the discretization of the equivalent electric and magnetic current