In mammals and birds, sulfite oxidase (SO) is a homodimeric molybdenum enzyme consisting of an N-terminal heme domain and a C-terminal molybdenum domain (EC 1.8.3.1). In plants, the existence of SO has not yet been demonstrated, while sulfite reductase as part of sulfur assimilation is well characterized. Here we report the cloning of a plant sulfite oxidase gene from Arabidopsis thaliana and the biochemical characterization of the encoded protein (At-SO). At-SO is a molybdenum enzyme with molybdopterin as an organic component of the molybdenum cofactor. In contrast to homologous animal enzymes, At-SO lacks the heme domain, which is evident both from the amino acid sequence and from its enzymological and spectral properties. Thus, among eukaryotes, At-SO is the only molybdenum enzyme yet described possessing no redoxactive centers other than the molybdenum. UV-visible and EPR spectra as well as apparent K m values are presented and compared with the hepatic enzyme. Subcellular analysis of crude cell extracts showed that SO was mostly found in the peroxisomal fraction. In molybdenum cofactor mutants, the activity of SO was strongly reduced. Using antibodies directed against At-SO, we show that a cross-reacting protein of similar size occurs in a wide range of plant species, including both herbacious and woody plants.

A novel unipolar transistor device has been realized starting from two-dimensional electron systems (2DES) in modulation-doped AlGaAs/GaAs heterostructures. A 600-nm-wide 1D channel is insulated laterally from 2DES regimes by 700-nm-wide deep mesa etched trenches. The conductivity in the quasi-one-dimensional channel can be tuned via the in-plane lateral field effect of the adjacent 2DES gates where the vacuum (or air) in the etched trenches serves as the dielectric. Room-temperature operation is demonstrated yielding a 17 μS transconductance corresponding to 170 mS/mm 2D transconductance.<hedend>

The longitudinal magnetoresistance of a two-dimensional electron gas is shown to be strongly dependent on the presence d ohmic contacts between the vonage probes. By connecting or disconnecting contacts with additional gates across the potential probes it is possible to drastically change the amDlitude of Shubnikov-de Haas osciilations. In an early work of Palaanen er a1 [l], an anomalous suppression of the Shubnikov-de Haas (SDH) maxima was observed in a standard AlGaAs/GaAs heterostructure, but the interpretation remained obscure. In the last few years, the Landauer-Buttker (U) formalism [24] has established a description of magneto-quantum transport phenomena in terms of the edge channel picture [5]. Within this picture, ohmic contacts serve as energy dissipating, distribution equilibrating and phase randomizing electron resetvoirs. In real samples nonideal ohmic contacts may be present due to an imperfect alloying [6,7]. Such a disordered contact reflects

We demonstrate that quantum ballistic devices can be prepared by conventional photolithographic techniques. A split Schottky gate on top of a high mobility modulation doped AIGaA-GaAs heterostructure is produced by standard mask exposure methods. The geometrical width of the split in the metal gate is 500-1200 nm. The 'electrical' width of the constricted two-dimensional electronic system can be controlled via a negative gate voiiage applied between the gate and'the two-dimensional electron gas. Electrical transport measurements at low temperatures (50 mK) as a function of the gate voltage show that the four-terminal resistance of the structure is quantized in values of h/2e2.. Many recent publications show the large potential which lies in the progress of microfabrication technologies. The fundamental physics of transport in onedimensional (ID) systems, especially the discovery of the quantized conductance [1,2] has been intensively investigated and opens the way for new low-dimensional device concepts, e.g. ID transistors [3,4]. The challenge for practical devices is that one needs a technology which is reproducible, fast, controllable and not too expensive. In the majority of experiments to date ultra-small structures have been fabricated by direct electron or ion beam writing with the disadvantage of low throughput and high cost. It is therefore interesting to investigate the potential of conventional techniques which are available in most industrial laboratories, to prepare quantum ballistic devices.

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