Tunable φ Josephson junction ratchet.

We demonstrate experimentally the operation of a deterministic Josephson ratchet with tunable asymmetry. The ratchet is based on a φ Josephson junction with a ferromagnetic barrier operating in the underdamped regime. The system is probed also under the action of an additional dc current, which acts as a counterforce trying to stop the ratchet. Under these conditions the ratchet works against the counterforce, thus producing a nonzero output power. Finally, we estimate the efficiency of the φ Josephson junction ratchet.

Experimental evidence of a φ Josephson junction.

We demonstrate experimentally the existence of Josephson junctions having a doubly degenerate ground state with an average Josephson phase ψ=±φ. The value of φ can be chosen by design in the interval 0<φ<π. The junctions used in our experiments are fabricated as 0-π Josephson junctions of moderate normalized length with asymmetric 0 and π regions. We show that (a) these φ Josephson junctions have two critical currents, corresponding to the escape of the phase ψ from -φ and +φ states, (b) the phase ψ can be set to a particular state by tuning an external magnetic field, or (c) by using a proper bias current sweep sequence. The experimental observations are in agreement with previous theoretical predictions.

The molecular basis for the differential translation of TMV RNA in tobacco protoplasts and wheat germ extracts.

Translation of tobacco mosaic virus (TMV) RNA in tobacco protoplasts yields the 17.5-K coat protein, a 126-K protein and a 183-K protein which is generated by an efficient readthrough over the UAG termination codon at the end of the 126-K cistron. In wheat germ extracts, however, only the 5'-proximal 126-K cistron is translated whereas the 183-K readthrough protein is not synthesized. Purification and sequence analysis of the endogenous tyrosine tRNAs revealed that the uninfected tobacco plant contains two tRNAs, both with GPsiA anticodons which stimulate the UAG readthrough in vitro and presumably in vivo. In contrast, 85% of the tRNA from wheat germ contains a QPsiA anticodon and 15% has a GPsiA anticodon. Otherwise the sequences of tRNAs from wheat germ and tobacco are identical. UAG readthrough and hence synthesis of the 183-K protein is only stimulated by tRNA(GPsiA) and not at all by tRNA(QPsiA). The tRNAs from wheat leaves were also sequenced. This revealed that adult wheat contains tRNA(GPsiA) only. This is very much in contrast to the situation in animals, where Q-containing tRNAs are characteristic for adult tissues whereas Q deficiency is typical for the neoplastic and embryonic state.

Anaerobic growth of halobacteria.

An energy-transducing pathway in halobacteria is described. Arginine mediates substrate level phosphorylation and allows the cells to grow anaerobically. Bacteriorhodopsin plus light can function as an alternative energy source under these conditions, provided the cells contain the pigment when transferred to the anaerobic environment. Therefore the selection of mutants functionally defective in ATP synthase or bacteriorhodopsin becomes possible.

Malonyl and palmityl transferase-less mutants of the yeast fatty-acid-synthetase complex.

146 independently isolated mutants of the fatty acid synthetase gene locus fas 1 were subdivided into six different complementation groups. Three of these groups, Va, Vb and Vd, have not been described before. The mutant fatty acid synthetases isolated from representatives of complementation group Vb were specifically deficient in two component enzymes at the same time, the malonyl and palmityl transferases. Among more than 180 fas 1 and fas 2 mutants systematically screened for malonyl and palmityl transferase activities no mutant was found affected in only one of these two fatty acid synthetase component enzymes. From this it is concluded that both transfer reactions are catalyzed by the same enzyme. In any malonyl transferase-less fatty acid synthetase, neither of the two known malonyl binding sites, i.e. enzyme-bound pantetheine and the non-thiol binding site, accepts malonate. This indicates that malonate is transferred to both groups by the same enzyme. So far, no acetyl transferase-less fas mutants have been characterized. On the other hand, the mutants of two fas 1 complementation groups, Va and Vd, though negative in overall fatty acid synthetase activity had no deficiency in any of the known component enzymes which can be tested in vitro. A possible interrelationship between both findings is discussed.