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Meeting Report
Vth International Conference on the
Molecular Biology of Hydrogenases
by Alfred Hansel

Preceded by the futuristic sounds of Pink Floyd, Paulette Vignais welcomed about 150 scientists dedicated to the study of hydrogenases to the French town Albertville, 12 - 17 July 1997. There in the heart of the Savoy alpine region, the participants were pleased not only by the beautiful surroundings, the delicious meals, and the excellent French wines, but also by the very well organized meeting. The presentations gave a good overview of recent progress concerning the structure and function of hydrogenases.

Peter Lindblad (Uppsala University, Sweden), summarizing the current knowledge about hydrogenases in Nostoc PCC 73102, pointed out that cyanobacteria contain at least two different types of enzymes acting on hydrogen. Reversible hydrogenases catalyze both the uptake and evolution of hydrogen, whereas uptake hydrogenases are able only to consume hydrogen. Unlike Anabaena variabilis, Anabaena PCC 7120, and Nostoc muscorum, Nostoc PCC 73102 seems to possess only an uptake hydrogenase.

Fredrik Oxelfelt from Lindblad's group succeeded in cloning two genes, hupS and hupL encoding this hydrogenase. The deduced amino acid sequences of hupS and hupL, reveal an overall similarity of about 90% to the corresponding genes of Anabaena PCC 7120, which were recently cloned by Claudio Carrasco in Jim Golden's lab. As in Anabaena, hupS and hupL from Nostoc appears to form an operon. However, hupL from Nostoc differs from the gene in Anabaena in that it is not interrupted by a DNA element that is excised during heterocyst differentiation. The group is currently trying to knock out the hupL gene in Nostoc in order to study the influence of the uptake hydrogenase on the physiology of this strain.

Oliver Schmitz, while working in Hermann Bothe's lab in Cologne (he's now with Susan Golden), found that the organization of the genes encoding a reversible hydrogenase in A. variabilis differs somewhat from that in other bacteria. The genes hoxFUYH (encoding a functional reversible hydrogenase) form a cluster in this strain, whereas the hoxF gene is separated by about 6 kB from the contiguous hoxUYH cluster in the unicellular Synechococcus PCC 6301.

Gudrun Boison, also in Bothe's lab, not only cloned these genes from Synechococcus but also showed that a hoxH-negative mutant is unable to evolve hydrogen. Her results indicate the presence of another hydrogenase, putatively of the uptake type, in Synechococcus.

Jens Appel, working with Rüdiger Schulz (Marburg University, Germany), was able to clone genes encoding a reversible hydrogenase in Synechocystis PCC 6803. The gene cluster shows an organization different from the two clusters known in Anabaena variabilis and Synechococcus. Physiological studies of a hoxH mutant indicate a link of the action of reversible hydrogenase to photosynthesis. In this mutant the ratio of PSI to PSII is 1 instead of 2. This observation implies that hydrogenase could be connected to cyclic electron transport around PSI, when light reactions are faster than dark reactions, thus functioning as an electron valve. A scan of the complete sequence of Synechocystis could find no genes similar to those known to encode uptake hydrogenases.

The group of Yasuo Asada (Tsukuba, Japan) is trying to express foreign hydrogenases in cyanobacteria, which then might be grown for biological hydrogen production in bioreactors. Towards this end, Masato Miyake introduced hydrogenases from Clostridium pasteurianum and Thiocapsa roseopersicina directly into Synechococcus elongotus by electroporation. Both enzymes are stable in the presence of O2, so they might work in cyanobacteria. The presence of the enzymes in the cyanobacterium was shown by Western blot analyses.

As an alternate approach, Y. Koike, from the same group, altered the ribosome binding sequence preceding the gene from C. pasteurianum to one more similar to cyanobacterial sequences. After the construct was introduced into Synechococcus PCC 7942, he could demonstrate the presence of the hydrogenase. Enzyme activity is another matter, and it remains to be seen if transfer of the structural gene is sufficient to get a functional enzyme complex in such a distantly related organism.