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Journal of Bacteriology, April 2004, p. 2340-2345, Vol. 186, No. 8
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.8.2340-2345.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Relationship of Critical Temperature to Macromolecular Synthesis and Growth Yield in Psychrobacter cryopegella

Center for Microbial Ecology, Michigan State University, East Lansing, Michigan 48824,¹ Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740²

Received 20 June 2003/ Accepted 8 January 2004

Most microorganisms isolated from low-temperature environments (below 4°C) are eury-, not steno-, psychrophiles. While psychrophiles maximize or maintain growth yield at low temperatures to compensate for low growth rate, the mechanisms involved remain unknown, as does the strategy used by eurypsychrophiles to survive wide ranges of temperatures that include subzero temperatures. Our studies involve the eurypsychrophilic bacterium Psychrobacter cryopegella, which was isolated from a briny water lens within Siberian permafrost, where the temperature is -12°C. P. cryopegella is capable of reproducing from -10 to 28°C, with its maximum growth rate at 22°C. We examined the temperature dependence of growth rate, growth yield, and macromolecular (DNA, RNA, and protein) synthesis rates for P. cryopegella. Below 22°C, the growth of P. cryopegella was separated into two domains at the critical temperature (T_critical = 4°C). RNA, protein, and DNA synthesis rates decreased exponentially with decreasing temperatures. Only the temperature dependence of the DNA synthesis rate changed at T_critical. When normalized to growth rate, RNA and protein synthesis reached a minimum at T_critical, while DNA synthesis remained constant over the entire temperature range. Growth yield peaked at about T_critical and declined rapidly as temperature decreased further. Similar to some stenopsychrophiles, P. cryopegella maximized growth yield at low temperatures and did so by streamlining growth processes at T_critical. Identifying the specific processes which result in T_critical will be vital to understanding both low-temperature growth and growth over a wide range of temperatures.

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