Analysis of the role of 7 kDa cold-shock proteins of Lactococcus lactis MG1363 in cryoprotection

Low-temperature adaptation and cryoprotection were studied in the lactic acid bacterium Lactococcus lactis MG1363. An approximately 100-fold increased survival after freezing was observed when cells were shocked to 10 °C for 4 h compared to mid-exponential-phase cells grown at 30 °C, indicating an active protection against freezing. Using two-dimensional gel electrophoresis a group of 7 kDa cold-induced proteins (CSPs) was identified that corresponds to a previously described family of csp genes of L. lactis MG1363. The 7 kDa CSPs appeared to be the most strongly induced proteins upon cold shock to 10 °C. Northern blotting and two-dimensional gel electrophoresis showed that the csp genes were maximally expressed at 10 °C, while induction was lower at 20 and 4 °C. However, pre-incubation at 20 and 4 °C, as well as stationary-phase conditions, also induced cryoprotection (approx. 30-, 130- and 20-fold, respectively, compared to 30 °C mid-exponential phase). For all treatments leading to an increased freeze survival (exposure to 4, 10 and 20 °C and stationary-phase conditions), increased levels of three proteins (26, 43 and 45 kDa) were observed for which a role in cryoprotection might be suggested. Increased freeze survival coincides with increased CSP expression, except for stationary-phase conditions. However, the level of observed freeze protection does not directly correlate with the csp gene expression levels. In addition, for the first time specific overproduction of a CSP in relation to freeze survival was studied. This revealed that L. lactis cells overproducing CspD at 30 °C show a 2–10-fold increased survival after freezing compared to control cells. This indicates that the 7 kDa cold-shock protein CspD may enhance the survival capacity after freezing but that other factors supply additional cryoprotection