Heat and cold shock protein synthesis in arctic and temperate strains of Rhizobia

Abstract

We compared heat shock proteins (HSPs) and cold shock proteins (CSPs) produced by different species of Rhizobium having different growth temperature ranges. Several HSPs and CSPs were induced when cells of three arctic (psychrotrophic) and three temperate (mesophilic) strains of rhizobia were shifted from their optimal growth temperatures (arctic, 25°C; temperate, 30°C) to shock temperatures outside their growth temperature ranges. At heat shock temperatures, three major HSPs of high molecular weight (106,900, 83,100, and 59,500) were present in all strains for all shock treatments (29, 32, 36.4, 38.4, 40.7, 41.4, and 46.4°C), with the exception of temperate strains exposed to 46.4°C, in which no protein synthesis was detected. Cell survival of arctic and temperate strains decreased markedly with the increase of shock temperature and was only 1% at 46.4°C. Under cold shock conditions, five proteins (52.0, 38.0, 23.4, 22.7, and 11.1 kDa) were always present for all treatments (-2, -5, and -10°C) in arctic strains. Among temperate strains, five CSPs (56.1, 37.1, 34.4, 17.3, and 11.1 kDa) were present at temperatures down to 0°C. The 34.4- and the 11.1-kDa components were present in all temperate strains at -5°C and in one strain at -10°C. Survival of all strains decreased with cold shock temperatures but was always higher than 50%. These results show that rhizobia can synthesize proteins at temperatures not permissive for growth. In all shock treatments, no correspondence between the number of HSPs or CSPs produced and rhizobial survival was found. In conclusion, cold-adapted arctic rhizobia synthesized HSPs at temperatures higher (46.4 versus 41.4°C) than temperate rhizobia and produced more CSPs under freezing conditions (-10°C). However, their cold adaptation does not seem to provide them with better survival at freezing temperatures.