Complex RNA chain elongation kinetics by wheat germ RNA polymerase II.

Kinetics of RNA chain elongation catalyzed by wheat germ RNA polymerase II have been studied using various synthetic DNA templates in the presence of excess dinucleotide monophosphate primers. With single- or double-stranded homopolymer templates, the double reciprocal plots 1/(velocity) as a function of 1/(nucleotide substrate) exhibit positive, negative or no curvature. With poly(dAT) as template, the mechanism of nucleoside monophosphate incorporation into RNA is not the ping-pong kinetic mechanism which was derived for E. coli RNA polymerase (6). Noncomplementary nucleoside triphosphates inhibit RNA transcription allosterically. Cordycepin triphosphate behaves as ATP, and not only inhibits AMP incorporation but also that of UMP and GMP on appropriate templates. The reason for this complex kinetic behavior is not yet understood. Possibilities are raised that there are several nucleoside triphosphate binding sites on wheat germ RNA polymerase II, that additional nucleoside triphosphate dependent enzymatic activities are required for reaction to occur or that the Km value for incorporation of a given nucleoside monophosphate into RNA is dependent on the length of the RNA chain and/or the nucleotide sequence surrounding the complementary base on the DNA template

Effect of salts on abortive and productive elongation catalysed by wheat germ RNA polymerase II.

Modification of the ionic conditions in reaction assays containing wheat germ RNA polymerase II and poly(dAT) as template markedly alters the catalytic properties of the transcription complexes. These effects have been studied by measuring the rate of abortive initiation and the extent of productive RNA synthesis. Using combinations of metal ions or various salts, a marked inhibition of abortive initiation was always associated with an increased length of RNA chains. These results are discussed in terms of modulation of the stability of transcription complexes induced by salts or divalent cations. The behavior exhibited by wheat germ RNA polymerase II is also discussed in comparison with previously reported results for procaryotic and eucaryotic RNA polymerases

Electron microscopic mapping of wheat germ RNA polymerase II binding sites on cloned CaMV DNA.

The binding sites of wheat germ RNA polymerase II were mapped on the cloned CaMV genome by observation of enzyme-linear DNA complexes by electron microscopy. Twelve sites are observed. Three of them are relatively stable in the presence of heparin and are found at positions 8-9, 21-23, and 41-44 map units on the physical map of the genome. These positions correspond to AT-rich regions of the viral genome which contain potential promoter sites. These results are discussed with reference to current information on the structure and expression of the CaMV genome

Optical properties of GaN epilayers and GaN/AlGaN quantum wells grown by molecular beam epitaxy on GaN(0001) single crystal substrate

GaN epilayers and GaN/AlGaN quantum wells (QWs) were grown by molecular beam epitaxy on GaN(0001) single crystal substrates. Transmission electron microscopy (TEM) was used to assess the crystal quality of the homoepitaxial layers. A dislocation density of less than 10(5) cm(-2) is deduced from TEM imaging. Low temperature (1.8 K) photoluminescence (PL) of homoepitaxial GaN reveals PL linewidths as low as 0.3 meV for bound excitons. The PL integrated intensity variation between 10 and 300 K is compared to that observed on a typical heteroepitaxial GaN/Al2O3 layer. A 2 nm thick GaN/Al0.1Ga0.9N QW has been studied by time-resolved and continuous wave PL. The decay time is close to a purely radiative decay, as expected for a low defect density. Finally, the built-in polarization field measured in a homoepitaxial QW is shown to be comparable to that measured on heteroepitaxial QWs grown either on sapphire or silicon substrates. (C) 2000 American Institute of Physics. [S0021-8979(00)07513- 7]