Personal Information

1969	B.Sc. Hebrew University, Jerusalem
1972	M.Sc Weizmann Institute, Rehovot, Israel
1977	Ph.D. Weizmann Institute, Rehovot, Israel
1977-80	Postdoctural Fellow, University of Chicago, Chicago, Illinois, USA

Academic Positions

1980-85	Lecturer, Dept. Microbiology and Immunology, TAU
1985-90	Senior Lecturer, Dept Microbiology and Immunology, TAU
1990-	Assoc. Professor, Dept of Mol. Microbiology & Biotechnology, TAU

Other Appointments

1986	Visiting Professor, Dept. Microbiology, University of Alberta, Edmonton, Canada
1991	Visiting scientist, Max-Planck Institute for Biochemistry, Martinsried, Germany
1997	Visiting scientist, Millennium Pharmaceuticals Inc., Cambridge, MA, USA

Research Interests

The mechanism of the natural genetic exchange system of Haloferax volcanii.

Understanding the mode of adaptation of halophilic enzyme to function at extremely high salt concentrations.

Study bacteriorhodopsin biogenesis.

Developing genetic tools for Hf. volcanii.

Quorum sensing in halophilic archaea.

The mechanism of the natural genetic
exchange system of Haloferax volcanii.

Background
The transfer of genetic information between organisms is one of the fundamental processes by which genetic variability is accomplished. In eukaryotes, sexual transmission occurs when two haploid cells fuse to form a diploid cell. In eubacteria, the horizontal transmission of genetic information involves either phage mediated transduction, assimilation of naked DNA acquired from the environment, or cell contact-dependent DNA transfer termed conjugation. In most cases conjugation is mediated by plasmids that are mobilized in the process. So far, genetic transfer was demonstrated only in two archaea. The first archaeon for which genetic transfer has been reported is the extremely halophilic Haloferax volcanii (Mevarech & Werczberger 1985) . It was recently shown that a very similar system exists also in the thermophilic archaeon Sulfolobus acidocaldarius (Grogan 1996) .

A Model for halophilic archaeal mating
Any model to describe the events involved in mating in H. volcanii must account for two important features: i) that the transfer of genetic material is bidirectional (Mevarech & Werczberger 1985) , namely it is impossible to distinguish donor cells from recipient cells; ii) that plasmids and chromosomal markers are transferred in tight linkage and at similar frequencies. These two properties of the genetic exchange are compatible with a model in which the H. volcanii mating system resembles the mating system of eukaryotic organisms where cells that are involved in mating fuse to form diploid cells.

The first step in the process is adherence of the cells (Stage I) . The second step in the process is the establishment of the cytoplasmic bridges (Stage II). However, under natural conditions only about 1% of the connected cells complete the fusion process (Stage III). Following fusion, the cellular cytoplasmic components, including chromosomes and some plasmids can move from one cell to the other, establishing a transient diploid stage at which recombination between chromosomes can occur (Rosenshine & Mevarech 1991) . If no selection pressure is applied, when the fused cell divides (Stage IV) the two chromosomes will segregate due to the lack of a mechanism that ensures the cosegregation of the two chromosomes to daughter cells (unlike the situation in diploid eukaryotic cells). The plasmids, on the other hand, will be inherited by the two daughter cells due to their high copy number.

In conclusion, the genetic transfer in H. volcanii involves cell fusion between cells that resembles mating in eukaryotes. However, whereas eukaryotic cells can be distinguished according to their mating types, in H. volcanii no mating type has been detected. Every H. volcanii cell appears competent to fuse with any other cell of the same genus. The nature of the cellular interactions and the basis for their specificity is unclear but seems to involve contacts between surface proteins. Moreover, conditions that promote fusion between cells exist within developing colonies. Thus, advantageous mutations in one cell within a colony might, therefore, be distributed at high frequency to other cells. Also, the fact that interspecies genetic transfer occurs at high frequency provides an effective mechanism for horizontal transfer of genes.

References

Grogan, D. W. 1996. Exchange of genetic markers at extremely high temperatures in the archaeon Sulfolobus acidocaldarius. J. Bacteriol. 178:3207-3211.

Mevarech, M., Werczberger, R. 1985. Genetic transfer system in Halobacterium volcanii. J. Bacteriol. 162:461-462.

Ortenberg, R. Tchlet, R., and Mevarech, M. 1998. A Model for the Genetic Exchange System of the Extremely Halophilic Archaeon Haloferax volcanii. In: Microbiology and Biogeochemistry of Hypersaline Environments. (A. Oren ed.) CRC Press. pp. 331-338

Rosenshine, I., Mevarech, M. 1991. The kinetic of the genetic exchange process in Halobacterium volcanii mating. In General and Applied Aspects of Halophilic Microorganisms, ed. F. Rodriguez-Valera. pp. 265-270. New York: Plenum Press.

Rosenshine, I., Tchelet, R., Mevarech, M. 1989. The mechanism of DNA transfer in the mating system of an archaebacterium. Science 245:1387-1389.

Tchelet, R., Mevarech, M. 1994. Interspecies genetic transfer in halophilic archaebacteria. System. Appl. Microbiol. 16:578-581.

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Understanding the mode of adaptation of halophilic enzyme to function at extremely high salt concentrations.

Halophilic enzymes, while performing identical enzymatic functions as their non-halophilic counterparts, have been shown to exhibit substantially different properties, among them the requirement for high salt concentrations, in the 1-4 M range, for activity and stability, and a high excess of acidic over basic amino residues. In collaboration with several laboratories we have combined biochemical, genetic and structural methods to study three halophilic proteins: 1) the enzyme malate dehydrogenase (hMDH) from the halophilic archaeon Haloarcula marismortui (whose structure was solved by Dym et al. 1995) ; 2) the 2Fe-2S ferredoxin of Haloarcula marismortui (whose structure was solved by Frolow et al. 1996); 3) the enzyme dihydrofolate reductase of the halophilic archaeon Haloferax volcanii (whose structure was solved by Pieper et al. 1998). These extensive have lead us to the conclusion that the high negative surface charge of halophilic proteins makes them more soluble and renders them more flexible at high salt concentrations, conditions under which non-halophilic proteins tend to aggregate and become rigid. This high surface charge is neutralized mainly by tightly bound water dipoles. The requirement of high salt concentration for stabilization of halophilic enzymes, on the other hand, is due to low affinity binding of the salt to specific sites on the surface of the folded polypeptide, thus stabilizing the active conformation of the protein.

References

Blecher, O., Goldman, S. and Mevarech, M. (1993) High expression in Escherichia coli of the gene coding for dihydrofolate reductase of the extremely halophilic archaebacterium Haloferax volcanii. Reconstitution of the active enzyme and mutation studies. Eur. J. Biochem. 216: 199-203.

Cendrin, F., Chroboczek, J., Zaccai, G., Eisenberg, H. and Mevarech, M. (1993) Cloning, Sequencing and Expression in Escherichia coli of the Gene Coding for Malate Dehydrogenase of the Extremely Halophilic Archaebacterium Haloarcula marismortui. Biochemistry 32: 4308-4313.

Dym, O., Mevarech, M. and Sussman, J. L. (1995) Structural Features that Stabilize Halophilic Malate Dehydrogenase from an Archaebacterium. Science 267: 1344-1346.

Frolow, F., Harel, M., Sussman, J.L., Mevarech, M. and Shoham, M. (1996) Protein adaptation to a saturated salt environment: Insights from the crystal structure of a halophilic 2Fe-2S ferredoxin. Nature Struc. Biol. 3:451-457.

Madern, D., Ebel, C., Mevarech, M., Richard, S.B., Pfister, C. and Zaccai, G. (2000) Insights into the molecular relationships between malate and lactate dehydrogenases: Structural and biochemical properties of monomeric and dimeric intermediates of a mutant of tetrameric L-[LDH-like] malate dehydrogenase from the halophilic archaeon Haloarcula marismortui. Biochemistry 39:1001-1010.

Pieper, U., Kapadia, G., Mevarech, M. and Herzberg, O. (1998) . Structural features of halophilicity derived from the crystal structure of dihydrofolate reductase from the Dead Sea halophilic archaeon, Haloferax volcanii. Structure 6: 75-88.

Zusman, T., Rosenshine,I., Boehm, G., Jaenicke, R. and Mevarech, M. (1989) Dihyrofolate reductase of the extremely halophilic archaebacterium Halobacterium volcanii . J. Biol. Chem. 264: 18878-18883.

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Study bacteriorhodopsin biogenesis.

Bacterioopsin (Bop) is an integral membrane protein composed of seven trans-membrane helices, that is produced by the halophilic archaeon Halobacterium salinarum and by some other halobacterial species (Oesterhelt, 1998). The complex of Bop with the chromophore, all-trans -retinal, forms bacteriorhodopsin which is organized in clusters that are termed 'purple membrane'. Bacteriorhodopsin functions as a light-driven proton pump that upon illumination generates a proton gradient, which is used to produce ATP. Despite the vast knowledge of the biophysical properties of bacteriorhodopsin, very little is known about the biogenesis of the purple membrane.

We have attempted to approach the problem of biogenesis of the purple membrane by an indirect approach. In this approach the gene coding for bacterioopsin is expressed in the extremely halophilic archaeon Hf. volcanii that lack the genetic capacity to produce the purple membrane. It was reasoned that it will be possible to produce the purple membrane by importing the necessary genetic elements from H. salinarum. Using fusions of bacteriorhodopsin to the halophilic enzyme dihydrofolate reductase or to the cellulose binding domain of Clostridium thermocellum cellulosome in pulse-chase and deletion analysis experiments, we could show (Ortenberg and Mevarech, 2000) that membrane insertion of Bop in Hf. volcanii is mediated through a cytoplasmic intermediate and requires the presence of an intact seventh transmembrane helix.

References

Oesterhelt, D. (1998) The structure and mechanism of the family of retinal proteins from halophilic archaea. Curr Opin Struct Biol 8: 489-500.

Ortenberg, R. and Mevarech, M. (2000) Evidence for post-translational membrane insertion of the integral membrane protein bacterioopsin expressed in the heterologous halophilic archaeon Haloferax volcanii. J. Biol. Chem. In Press.

Selected Publications

Rosenbluh, A., Mevarech, M., Koltin, Y. and Gorman, J. (1985). Isolation and expression of genes of Candida albicans by complimentation in Sacchromyces cerevisiae. Mol. Gen. Genet. 200: 500-502

Mevarech, M. and Werczberger, R. (1985). Genetic transfer in Halobacterium volcanii. J. Bacteriol. 162: 461-452

Rosenshine, I., Zusman, T., Werczberger, R. and Mevarech, M. (1987). Amplification of specific DNA sequences correlates with resistance of the archaebacterium Halobacterium volcanii to the dihydrofolate reductase inhibitors trimethoprim and methotrexate. Mol. Gen. Genet. 208: 518-522

Leskiw, B.K., Aharonowitz, Y., Mevarech, M., Wolf, S., Vining, L.C., Westlake, D.W.S. and Jensen, S. (1988). Cloning and sequence determination of the isopenicillin N synthetase gene from Streptomyces clavuligerus. Gene 62: 187-196

Shiffman, D., Mevarech, M., Jensen, S.E., Cohen, G. and Aharonowitz, Y. (1988) Cloning and comparitive sequence analysis of the gene coding for isopenicillin N synthase in Streptomyces. Mol.Gen. Genet. 214: 562-573

Harel, M., Shoham, M., Frolow, F., Eisenberg, H., Mevarech, M., Yonath, A. and Sussman, J.L. (1988) Crystallization of halophilic malate dehydrogenase from Halobacterium marismortui. J. Mol. Biol. 200: 609-610.

Rosenshine, I. and Mevarech, M. (1989) Isolation and partial characterization of plasmids found in three Halobacterium volcanii isolates. Can. J. Microbiol. 35: 92-95.

Reddy, P.G., Allon, R., Mevarech, M., Mendelovitz, S., Sato, Y. and Gutnick D.L. (1989) Cloning and expression in E. coli of an esterase gene from the oil degrading bacterium Acinetobacter calcoaceticus RAG-1. Gene 76: 145-152.

Mevarech, M., Hirsh-Twizer, S., Goldman, S., Yakobson, E., Eisenberg, H. and Dennis, P.P. (1989) Isolation and characterization of the rRNA gene clusters of Halobacterium marismortui. J. Bacteriol. 171: 3479-3485.

Rosenshine, I., Tchelet, R. and Mevarech, M. (1989) The mechanism of DNA transfer in the mating system of an archaebacterium. Science 245: 1387-1389.

Zusman, T., Rosenshine,I., Boehm, G., Jaenicke, R. and Mevarech, M. (1989) Dihyrofolate reductase of the extremely halophilic archaebacterium Halobacterium volcanii. J. Biol. Chem. 264: 18878-18883.

Shiffman, D., Cohen, G., Aharonowitz, Y., Palisa, H., von Dohren, H., Kleinkauf, H., Mevarech, M. (1990) Nucleotide sequence of the isopenicillin N synthase gene (pcbC) of the gram negative Flavobacterium sp. SC 12,154. Nucleic Acids Research 18: 660.

Cohen, G., Shiffman, D., Mevarech, M. and Aharonowitz, Y. (1990) The microbial isopenicillin N synthase genes: structure, function,diversity and evolution. Tends in Biotechnology 8: 105-111.

Leskiw, B.K., Mevarech, M., Barrit, L.S., Jensen, S.E., Henderson, D.J., Hopwood, D.A., Bruton, C.J. and Chater, K.F. (1990) Discovery of an insertion sequence from Streptomyces clavuligerus and its relatedness to two transposable elements of Streptomyces coelicolor. J. Gen. Microbiol. 136: 1251-1258.

Goldman,S., Hecht, K., Eisenberg, H. and Mevarech, M. (1990) Extracellular Ca++ Dependent Inducible Alkaline Phosphatase from the Extreme Halophilic Archaebacterium Haloarcula marismortui. J. Bacteriol. 172: 7065-7070.

Cendrin, F., Chroboczek, J., Zaccai, G., Eisenberg, H. and Mevarech, M. (1993) Cloning, Sequencing and Expression in Escherichia coli of the Gene Coding for Malate Dehydrogenase of the Extremely Halophilic Archaebacterium Haloarcula marismortui. Biochemistry 32: 4308-4313. medline

Daniel, E., Azem, A., Shaked, I. and Mevarech, M. (1993) Subunit Structure of Halophilic Malate Dehydrogenase from Haloarcula marismortui. Comp. Biochem. Biophys. 106: 401-405.

Blecher, O., Goldman, S. and Mevarech, M. (1993) High expression in Escherichia coli of the gene coding for dihydrofolate reductase of the extremely halophilic archaebacterium Haloferax volcanii. Reconstitution of the active enzyme and mutation studies. Eur. J. Biochem. 216: 199-203. medline

Tchelet, R. and Mevarech, M. (1994) Interspecies Genetic Transfer in Halophilic Archaebacteria. Syst. Appl. Microbiol. 16: 578-581.

Dym, O., Mevarech, M. and Sussman, J. L. (1995) Structural Features that Stabilize Halophilic Malate Dehydrogenase from an Archaebacterium. Science 267: 1344-1346.

Frolow, F., Harel, M., Sussman, J.L., Mevarech, M. and Shoham, M. (1996) Protein adaptation to a saturated salt environment: Insights from the crystal structure of a halophilic 2Fe-2S ferredoxin. Nature Struc. Biol. 3:451-457. medline

Yaar, L., Mevarech, M. and Koltin, Y. (1997) A Candida albicans RAS related (CaRSR1) gene is involved in budding, cell morphogenesis, and hyphae development. Microbiology 143:3033-3044.

Pieper, U., Kapadia, G., Mevarech, M. and Herzberg, O. (1998) . Structural features of halophilicity derived from the crystal structure of dihydrofolate reductase from the Dead Sea halophilic archaeon, Haloferax volcanii. Structure 6: 75-88. medline

Nomura, S., Kajimura, N., Matoba,. K., Miyata, T., Ortenberg,. R., Mevarech, M., Kamikubo, H., Kataoka, M. and Harada, Y. (1999) Ordered Structure Formation of Bacteriorhodopsin-hDHFR in a Plasma Membrane. Langmuir 15: 214-220.

Madern, D., Ebel, C., Mevarech, M., Richard, S.B., Pfister, C. and Zaccai, G. (2000) Insights into the molecular relationships between malate and lactate dehydrogenases: Structural and biochemical properties of monomeric and dimeric intermediates of a mutant of tetrameric L-[LDH-like] malate dehydrogenase from the halophilic archaeon Haloarcula marismortui. Biochemistry 39:1001-1010. medline

Ortenberg, R., Rozenblatt-Rosen, O. and Mevarech, M. (2000) The extremely halophilic archaeon Haloferax volcanii has two very different dihydrofolate reductases. Molec. Microbiol. 35: 1493-1505. medline

Mevarech, M., Frolow, F. and Gloss, L.M. Halophilic enzymes: proteins with a grain of salt. Biophys. Chem., In Press.

Ortenberg, R. and Mevarech, M. Evidence for post-translational membrane insertion of the integral membrane protein bacterioopsin expressed in the heterologous halophilic archaeon Haloferax volcanii. J. Biol. Chem. In Press. medline

Chapters in books

Zusman, T. and Mevarech, M. (1991) Biochemical characterization of dihydrofolate reductase of Halobacterium volcanii. In: General and Applied Aspects of Halophilic Microorganisms. F. Rodriguez-Valera ed., pp. 181-187, Plenum Press, New York.

Rosenshine, I. and Mevarech, M. (1991) The kinetics of the genetic exchange process in Halobacterium volcanii mating. In: General and Applied Aspects of Halophilic Microorganisms. F. Rodriguez-Valera ed., pp. 265-270, Plenum Press, New York.

Eisenberg, H., Mevarech, M. and Zaccai, G. (1992) Biocemical, Structural and Molecular Genetic Aspects of Halophilism. Adv. Prot. Chem. 43, 1-61.

Ortenberg, R. Tchlet, R., and Mevarech, M. (1998) A Model for the Genetic Exchange System of the Extremely Halophilic Archaeon Haloferax volcanii. In: Microbiology and Biogeochemistry of Hypersaline Environments. (A. Oren ed.) CRC Press. pp. 331-338