Heat shock regulatory gene rpoH mRNA level increases after heat shock in Escherichia coli

Abstract

The Escherichia coli rpoH gene product sigma 32 is essential for the increase in heat shock gene transcription found after exposure of the bacteria to a sudden temperature increase. It is not known how the concentration of active sigma 32 is modulated. We showed that rpoH transcript levels increased after heat shock and that the magnitude of the increase in the level of mRNA was correlated with the magnitude of the temperature shift. The increase in the level of rpoH mRNA was still found in rpoH mutants so the mechanism of induction differed from that of the set of previously identified heat shock genes. The increased concentration of rpoH mRNA should result in a higher level of sigma 32, which is likely to be important for increasing heat shock gene transcription.The bacterium Escherichia coli synthesizes a set of proteins at a higher rate after exposure to a sudden temperature increase (called heat shock). A similar response is thought to occur in all organisms (see references 9 and 24 for reviews). At least two of the E. coli heat shock proteins, DnaK and C62.5, ate related in amino acid sequence to eucaryotic heat shock proteins (2; J. Bardwell and E. Craig, personal communication), suggesting that a heat shock response may have evolved before the divergence of pro-and eucaryotes. Among the 17 or more E. coli heat shock proteins are the products of the dnaK, dnaJ, groES (mopB), groEL (mopA), lysU, grpE, lon, and rpoD genes (1, 3, 24, and references therein). These proteins are involved in transcription, translation, DNA synthesis, proteolysis, and bacteriophage morphogenesis. One effect of heat shock protein synthesis is to help protect bacteria from being killed by high temperatures (31). It has not been established which heat shock protein(s) contributes to protection from thermal killing, but other stresses, such as bacteriophage infection (10, 17) or treatment with ethanol (29) or DNA-damaging agents (18), induce the synthesis of many of the same proteins, so the proteins involved may protect cells from damage caused by several adverse conditions found in nature.Several of the steps in the response of E. coli to heat shock are now known. Less than 1 min after a temperature upshift, the transcription of heat shock genes begins to increase coordinately (30). The protein synthesis rates and transcript levels increase 5-to 50-fold (8, 26, 31). The synthesis rates peak around 5 to 8 min after the temperature shift and decline to new steady-state levels, which at 42°C are about two times the level at 30°C (15). Heat shock genes are defined by a requirement for the rpoH (previously known as htpR or hin) gene product for their transcription (22, 31). The rpoH gene product is called sigma 32 because it acts as an RNA polymerase sigma subunit, directing the enzyme to initiate transcription at heat shock gene promoters (14). The consensus sequence for heat shock promoters differs from that for promoters transcribed by RNA polymerase in combination with sigma 70, the normal sigma subunit (8 The mechanism by which tr...

The sigma B-dependent promoter of the Bacillus subtilis sigB operon is induced by heat shock

“…In E. coli the heat shock response is controlled by a secondary sigma factor (&2) which increases in abundance after a rise in temperature to direct RNA polymerase to the promoters of the principal heat shock genes (10,11,24,25 …”

Modulation of stability of the Escherichia coli heat shock regulatory factor sigma

“…We and others have shown that several mechanisms modulate the a32 concentration in the cell. The levels of the c32 transcripts increase after heat shock as a consequence of mRNA stabilization (11,12,36 (40).…”

“…The heat shock response must be tightly regulated in order to allow rapid changes in heat shock protein synthesis rates. Although the level of mRNA from the rpoH gene (which encodes U32) increases after heat shock (11,12,36 Technology, Cambridge, MA 02139. this increase is insufficient and too slow to be the sole explanation of the rapid effect of heat shock. In this paper, we show that the concentration of active a32 limits the expression of heat shock genes and that the stability of C32 varies in conditions in which heat shock gene expression is modulated.…”

Identification of the heat-shock sigma factor RpoH and a second RpoH-like protein in Sinorhizobium meliloti The GenBank accession numbers for the sequences reported in this paper are AF128845 (rpoH1) and AF149031 (rpoH2).

“…In response to a sudden increase in temperature or other stresses, the levels of σ$# rise transiently because of increased synthesis and protein stabilization. The induction of synthesis is mainly mediated by relief of translational repression due to a secondary structure in the mRNA (Morita et al, 1999 ;Nagai et al, 1991 ;Yuzawa et al, 1993), though the level of rpoH transcription also increases slightly (Erickson et al, 1987 ;Tilly et al, 1986). Stabilization occurs with the release of σ$# from a DnaK\DnaJ\GrpE chaperone complex as DnaK binds denatured proteins generated under stress conditions (Gamer et al, 1996).…”