In addition to the heat shock response in E. coli mediated by 32, the E (o 24) regulon also plays an important role in pro- tecting cells against severe heat damage (Table 1). Whereas the o 2 regulon functions to cope with cytoplasmic protein damage the o E regulon plays a major role in protecting cells against ex- tracytoplasmic (periplasmic) stress or extreme Under extreme heat stress, most protein synthesis stopped due to in of o 70, and. synthesis of HSPs under the control of o 2 orot increased until E. coli produced other proteins (Blaszczak and others 1995). De Las Peas and others (1997) reported that the o b regulon (genes induced by oE) was also essential for the growth of E. coli at all temperatures. Therefore, the o E regulon appears to provide essential functions related to protein folding and turnover in the cell envelopes that are particularly important at extreme temperatures. A third heat shock regulon in E. coli is controlled by ON (o 54), an alternative sigma factor that requires an activator (PspF) for transcriptional activation (Model et al 199.