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Lazlo N. Csonka

Lazlo N. Csonka Profile Picture

Associate Professor of Biological Sciences
Ph.D - 1975 - Harvard University

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One of the important physiological processes in all cells is the ability to maintain the internal osmolarity at a relatively invariant level in face of fluctuations in the osmolarity of the environment. Cells of most organisms adapt to increases in the external osmolarity by elevating the intracellular concentrations of a limited number of low molecular weight solutes. This response enables them to maintain the proper balance between the external and internal osmolality. Prominent among the solutes that are accumulated under conditions of high osmolarity are proline and glycine betaine. Our research aim is to contribute to the understanding of the adaptive responses to osmotic stress in bacteria and plants.

Induction of Increased Thermotolerance by High Osmolality in Salmonella. Exposure to moderate or high osmolality has been shown to confer increased thermotolerance in several species of food borne pathogenic bacteria. This finding is important for food microbiology, because supplementation of foods with high concentrations of salts or sugars as preservatives or flavor components might antagonize the bactericidal efficacy of heat treatment. The mechanism by which high osmolality provides protection against high temperature stress is not understood. As part of our research, we are studying this important problem in Salmonella typhimurium and Salmonella enteritidis. High osmolality, imposed by > 0.2 M NaCl or equivalent concentrations of sugars, increases the thermotolerance of Salmonella strains by two criteria: it stimulates their growth rate at non-lethal high temperatures (44o - 46oC) and enhances their viability (colony forming ability) at lethal high temperatures (52o - 55o). In order to gain insights into the regulation of thermotolerance by external osmolality, we isolated mutants in which high osmolality was no longer able to confer increased thermotolerance. One of these mutations proved to be in the pathway of synthesis of the disaccharide trehalose. This results suggest that trehalose, which has been observed to accumulate in desiccation tolerant plants and animals at elevated temperatures, contributes to thermotolerance in Salmonella. Our future efforts will be directed at elucidation of the biochemical role of trehalose in thermotolerance.

Regulation of proline synthesis in plants. Numerous higher plants accumulate proline in response to dehydration by osmotic stress or draught. We isolated two clones from tomato, specifying two different isoenzyme forms of D1-pyrroline-5-carboxylate synthetase, both of which catalyze the first and second steps of proline synthesis. The enzymes specified by both clones are sensitive to allosteric feedback inhibition by proline. We are conducting a genetic and biochemical analysis to define the domains of these enzymes which are involved in allosteric control.

Selected Publications:

Fletcher, S. and Csonka, L.N. (1998). ) Characterization of the induction of thermotolerance by osmotic stress in Salmonella typhimurium. Food Microbiology 15: 307-317.

Fujita, T., Maggio, A., García-Ríos, M., Bressan, R.A. and Csonka, L.N. 1998. Comparative analysis of the regulation of expression and structures of two evolutionarily divergent genes for D1-pyrroline-5-carboxylate synthetase from tomato. Plant Physiol. 118: 661-674.

Csonka, L.N. and Epstein, W. (1996). Osmoregulation. pp. 1210 - 1223. In F.C. Neidhardt, J.L. Ingraham, E.C.C. Lin, K.B. Low, B. Magasanik, W.S. Reznikoff, M. Riley, M. Schaechter, H.E. Umbarger eds.) Escherichia coli and Salmonella typhimurium: Cellular and Molecular Biology, 2nd edition, American Society for Microbiology, Washington D.C.

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