Heat shock response of Neurospora crassa: protein synthesis and induced thermotolerance

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Abstract
At elevated temperatures, germinating conidiospores of N. crassa discontinue synthesis of most proteins and initiate synthesis of 3 dominant heat shock proteins of 98,000, 83,000, and 67,000 MW and one minor heat shock reaction of 30,000 MW. Postemergent spores produce, in addition to these, a 4th major heat shock protein of 38,000 MW and a minor heat shock protein of 34,000 MW. The 3 heat shock proteins of lower MW are associated with mitochondria. This exclusive synthesis of heat shock proteins is transient, and after 60 min of exposure to high temperatures, restoration of the normal pattern of protein synthesis is initiated. Despite the transiency of the heat shock response, spores incubated continuously at 45.degree. C germinate very slowly and do not grow beyond the formation of a germ tube. The temperature optimum for heat shock protein synthesis is 45.degree. C, but spores incubated at other temperatures from 40 through 47.degree. C synthesize heat shock proteins at lower rates. Survival was high for germinating spores exposed to temperatures up to 47.degree. C, but viability declined markedly at higher temperatures. Germinating spores survived exposure to the lethal temperature of 50.degree. C when they had been preexposed to 45.degree. C; this thermal protection depends on the synthesis of heat shock proteins, since protection was abolished by cycloheximide. During the heat shock response mitochondria also discontinue normal protein synthesis; synthesis of the mitochondria-encoded subunits of cytochrome c oxidase was as depressed as that of the nucleus-encoded subunits.