Prokaryotic and eukaryotic cells respond to environmental stress by the induction of a variety of stress-related proteins. In mammalian cells, the most well-characterized group of stress proteins are induced by hyperthermia. Transcription of heat shock proteins increases markedly after hyperthermia, and several of these genes have been cloned from human and mouse cells in other laboratories. It is likely that transcription of other genes is also induced in mammalian cells since approximately 10-20 genes are induced in prokaryotes and lower eukaryotes. We have isolated cDNA clones coding for more than 15 different heat shock-induced RNA in Chinese hamster cells by differential hybridization screening of a cDNA library which we constructed from heat shocktreated cells. Based on homology to human heat shock cDNA clones, some of our cDNA clones were found to code for hsp7O, hsp27, hsp6O, hsp89-alpha, and hsp89-beta. Except for hsp6O, all these genes were coordinately induced by heat shock. By DNA sequencing the 2 most abundant isolates were found to code for ubiquitin and B2 RNA polymerase HI transcripts. Ubiquitin plays many important roles in eukaryotic cells, including a prominent role in the removal of damaged proteins. Ubiquitin has been found to be induced by heat shock in yeast and chicken cells, but ours is the first demonstration in mammalian cells. We estimated that up to 10% of the mRNA in heat shock-treated cells were ubiquitin transcripts. We have found also that ubiquitin transcription was strongly induced in both rodent and human cells by alkylating agents which indicates a possible role for ubiquitin in the cellular response to such damage; in yeast, the RAD6 DNA repair gene is a ubiquitin-conjugating enzyme. These studies have been extended to other heat shock genes and we have found that hsp27, in particular, was induced by certain DNA-damaging agents. In the case of B2 RNA, we constructed size-selected libraries from untreated and heat shock-treated cells, and isolated 200 B2 cDNA clones. By DNA sequencing, the heat shock-inducible B2 transcripts consistently differed from those of unheated cells.
