cr ~ and r ~2 are two heat-and ethanol-inducible g-factors in Escherichia coli. The cr 32 regulon is also induced by unfolded and misfolded proteins in the cytoplasm, and the function of many of the proteins in the cr 32 regulon is to bind to cytoplasmic proteins and assist them in folding or unfolding. To further understand the function of the cr F regulon, we searched for mutants that affected cr E activity. Our results indicate that a signal generated by expression of outer membrane proteins modulates cr E activity. Specifically, r ~ activity is induced by increased expression of OMPs and is reduced by decreased expression of OMPs. In addition, mutations that cause misfolded OMPs induce cr E activity. This signal is generated after the fate of OMPs and periplasmic proteins diverge in the secretory pathway and is not the result of an accumulation of OMP precursors in the cytoplasm. Our results indicate that this effect of OMPs is specific to the r E regnlon, because none of the above mutations affect r 32 activity. We propose that the ~r ~ regnlon is involved in processes that occur in extracytoplasmic compartments and that these two heat-inducible regulons may have distinct but complementary roles of monitoring the state of proteins in the cytoplasm (or 32) and outer membrane ((]rE).[Key Words: (r-Factors; protein export; outer membrane proteins; heat shock; (rE] Received September 13, 1993; revised version accepted October 14, 1993.In bacterial cells the (r-subunit directs RNA polymerase to initiate transcription at promoter sites on the DNA (Burgess et al. 1969). The primary (r-factor in the cell is responsible for transcription of most genes during exponential growth. In addition, alternative (r-factors direct transcription of sets of genes whose products are needed for specific functions, such as sporulation, nitrogen fixation, or flagella synthesis {Gross et al. 1992). Alternative (r-factors are often activated by changes in environmental or cellular conditions that generate morphological and/or molecular cues, signaling the need for the gene products in the regulon under control of a particular (r-factor. Elucidation of these signal-transduction pathways provides insights about global control of gene activity in prokaryotic cells.The activity of two Escherichia coli alternative (r-factors, (r32 and (re ((r24), increases after temperature upshift or exposure to ethanol {Grossman et al. 1984;Erickson et al. 1987;Straus et al. 1987;Erickson and Gross 1989;Wang and Kaguni 1989}. RNA polymerase (E) containing o ~2 (E(r 32) transcribes the heat shock genes with products that consist primarily of chaperones and proteases.

Little is known about either the process of periplasmic protein folding or how information concerning the folding state in this compartment is communicated. We present evidence that SurA, a periplasmic protein with peptidyl-prolyl isomerase activity, is involved in the maturation and assembly of LamB. LamB is a trimeric outer membrane porin for maltodextrins as well as the bacteriophage g receptor in Escherichia coilWe demonstrate that SurA is involved in the conversion of unfolded monomers into a newly identified intermediate in LamB assembly, which behaves as a folded monomer. The absence of SurA blocks the assembly pathway and leads to accumulation of species prior to the folded monomer. These species also accumulate when the stress sigma factor cr E is induced by Lamb overexpression. We suggest that accumulation of species prior to the generation of folded monomer is a stress signal sensed by r ~.

In Escherichia coli, the heat shock response is under the control of two alternative sigma factors: sigma 32 and sigma E. The sigma 32‐regulated response is well understood, whereas little is known about that of sigma E, except that it responds to extracytoplasmic immature outer membrane proteins. To further understand this response, we located the rpoE gene at 55.5′ and analyzed the role of sigma E. sigma E is required at high temperature, and controls the transcription of at least 10 genes. Some of these might contribute to the integrity of the cell since delta rpoE cells are more sensitive to SDS plus EDTA and crystal violet. sigma E controls its own transcription from a sigma E‐dependent promoter, indicating that rpoE transcription plays a role in the regulation of E sigma E activity. Indeed, under steady‐state conditions, the transcription from this promoter mirrors the levels of E sigma E activity in the cell. However, it is unlikely that the rapid increase in E sigma E activity following induction can be accounted for solely by increased transcription of rpoE. Based upon homology arguments, we suggest that a gene encoding a negative regulator of sigma E activity is located immediately downstream of rpoE and may function as the target of the E sigma E inducing signal.

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