THE FREEZE-THAW STRESS-RESPONSE OF THE YEAST SACCHAROMYCES-CEREVISIAEIS GROWTH-PHASE SPECIFIC AND IS CONTROLLED BY NUTRITIONAL STATE VIA THE RAS-CYCLIC AMP SIGNAL

The ability of cells to survive freezing and thawing is expected to de pend on the physiological conditions experienced prior to freezing, We examined factors affecting yeast cell survival during freeze-thaw str ess, including those associated with growth phase, requirement for mit ochondrial functions, acid prior stress treatment(s), and the role pla yed by relevant signal transduction pathways, The yeast Saccharomyces cerevisiae was frozen at -20 degrees C for 2 h (cooling rate, less tha n 4 degrees C min(-1)) and thawed on ice for 40 min, Supercooling occu rred without reducing cell survival and was followed by freezing, Loss of viability was proportional to the freezing duration, indicating th at freezing is the main determinant of freeze-thaw damage, Regardless of the carbon source used, the wild type strain and an isogenic petite mutant ([rho(0)]) showed the same pattern of freeze-thaw tolerance th roughout growth, i.e., high resistance during lag phase and low resist ance during log phase, indicating that the response to freeze-thaw str ess is growth phase specific and not controlled by glucose repression, In addition, respiratory ability and functional mitochondria are nece ssary to confer full resistance to freeze-thaw stress, Both nitrogen a nd carbon source starvation led to freeze-thaw tolerance, The use of s trains affected in the RAS-cyclic AMP (RAS-cAMP) pathway or supplement ation of an real mutant (defective in the cAMP phosphodiesterase gene) with cAMP showed that the freeze-thaw response of yeast is under the control of the RAS-cAMP pathway, Yeast did not adapt to freeze-thaw st ress following repeated freeze-thaw treatment with or without a recove ry period between freeze-thaw cycles, nor could it adapt following pre treatment by cold shock However, freeze-thaw tolerance of yeast cells was induced during fermentative and respiratory growth by pretreatment with H2O2, cycloheximide, mild heat shock, or NaCl, indicating that c ross protection between freeze-thaw stress and a limited number of oth er types of stress exists.