Overexpression of dnaK/dnaJ and groEL confers freeze tolerance to Escherichia coli.

Heat shock proteins not only can protect host cells against heat stress, they can also enable freeze tolerance as well. With respect to this unexpected feature, we are able to show that, at least in Escherichia coli, the heat shock proteins DnaK/DnaJ and GroEL play a very significant role. We found that the recovery rate of E. coli cultures that had been stored at -80 degreesC in the absence of any cryoprotectant was related to the abundance of these heat shock proteins accumulated before the freeze treatment. Before freezing, the DnaK in the bacterial cells was induced to accumulate to a level comparable to that produced in response to heat shock. After the freezing treatment, the recovery rate of the induced culture was very similar to that of the heat-shocked culture. Over production of GroEL was also protective but less effective. While freezing inevitably leads to protein denaturation, we propose that advance synthesis of DnaK/DnaJ and GroEL can accordingly prevent irreversible denaturation by chaperoning the unfolded polypeptides during freezing. Copyright 1998 Academic Press.