Some effects of growth conditions on steady state and heat shock induced htpG gene expression in continuous cultures of Escherichia coli

RpoS regulation of gene expression during exponential growth of Escherichia coli K12

RpoS is a major regulator of genes required for adaptation to stationary phase in E. coli. However, the exponential phase expression of some genes is affected by rpoS mutation, suggesting RpoS may also have an important physiological role in growing cells. To test this hypothesis, we examined the regulatory role of RpoS in exponential phase using both genomic and biochemical approaches. Microarray expression data revealed that, in the rpoS mutant, the expression of 268 genes was attenuated while the expression of 24 genes was enhanced. Genes responsible for carbon source transport (the mal operon for maltose), protein folding (dnaK and mopAB), and iron acquisition (fepBD, entCBA, fecI, and exbBD) were positively controlled by RpoS. The importance of RpoS-mediated control of iron acquisition was confirmed by cellular metal analysis which revealed that the intracellular iron content of wild type cells was two-fold higher than in rpoS mutant cells. Surprisingly, many previously identified RpoS stationary-phase dependent genes were not controlled by RpoS in exponential phase and several genes were RpoS-regulated only in exponential phase, suggesting the involvement of other regulators. The expression of RpoS-dependent genes osmY, tnaA and malK was controlled by Crl, a transcriptional regulator that modulates RpoS activity. In summary, the identification of a group of exponential phase genes controlled by RpoS reveals a novel aspect of RpoS function.

Expression of 17 genes in Clostridium thermocellum ATCC 27405 during fermentation of cellulose or cellobiose in continuous culture

Clostridium thermocellum is a thermophilic, anaerobic, cellulolytic bacterium that produces ethanol and acetic acid as major fermentation end products. The effect of growth conditions on gene expression in C. thermocellum ATCC 27405 was studied using cells grown in continuous culture under cellobiose or cellulose limitation over a approximately 10-fold range of dilution rates (0.013 to 0.16 h(-1)). Fermentation product distribution displayed similar patterns in cellobiose- or cellulose-grown cultures, including substantial shifts in the proportion of ethanol and acetic acid with changes in growth rate. Expression of 17 genes involved or potentially involved in cellulose degradation, intracellular phosphorylation, catabolite repression, and fermentation end product formation was quantified by real-time PCR, with normalization to two calibrator genes (recA and the 16S rRNA gene) to determine relative expression. Thirteen genes displayed modest (fivefold or less) differences in expression with growth rate or substrate type: sdbA (cellulosomal scaffoldin-dockerin binding protein), cdp (cellodextrin phosphorylase), cbp (cellobiose phosphorylase), hydA (hydrogenase), ldh (lactate dehydrogenase), ack (acetate kinase), one putative type IV alcohol dehydrogenase, two putative cyclic AMP binding proteins, three putative Hpr-like proteins, and a putative Hpr serine kinase. By contrast, four genes displayed >10-fold-reduced levels of expression when grown on cellobiose at dilution rates of >0.05 h(-1): cipA (cellulosomal scaffolding protein), celS (exoglucanase), manA (mannanase), and a second type IV alcohol dehydrogenase. The data suggest that at least some cellulosomal components are transcriptionally regulated but that differences in expression with growth rate or among substrates do not directly account for observed changes in fermentation end product distribution.

Hsp90: chaperoning signal transduction

Hsp90 is an ATP dependent molecular chaperone involved in the folding and activation of an unknown number of substrate proteins. These substrate proteins include protein kinases and transcription factors. Consistent with this task, Hsp90 is an essential protein in all eucaryotes. The interaction of Hsp90 with its substrate proteins involves the transient formation of multiprotein complexes with a set of highly conserved partner proteins. The specific function of each component in the processing of substrates is still unknown. Large ATP-dependent conformational changes of Hsp90 occur during the hydrolysis reaction and these changes are thought to drive the chaperone cycle. Natural inhibitors of the ATPase activity, like geldanamycin and radicicol, block the processing of Hsp90 substrate proteins. As many of these substrates are critical elements in signal transduction, Hsp90 seems to introduce an additional level of regulation.

Heat-induced expression and chemically induced expression of the Escherichia coli stress protein HtpG are affected by the growth environment

Differences in expression of the Escherichia coli stress protein HtpG were found following exposure of exponentially growing cells to heat or chemical shock when cells were grown under different environmental conditions. With an htpG::lacZ reporter system, htpG expression increased in cells grown in a complex medium (Luria-Bertani [LB] broth) following a temperature shock at 45 degrees C. In contrast, no HtpG overexpression was detected in cells grown in a glucose minimal medium, despite a decrease in the growth rate. Similarly, in pyruvate-grown cells there was no heat shock induction of HtpG expression, eliminating the possibility that repression of HtpG in glucose-grown E. coli was due to catabolite repression. When 5 mM phenol was used as a chemical stress agent for cells growing in LB broth, expression of HtpG increased. However, when LB-grown cells were subjected to stress with 10 mM phenol and when both 5 and 10 mM phenol were added to glucose-grown cultures, repression of htpG expression was observed. 2-Chlorophenol stress resulted in overexpression of HtpG when cells were grown in complex medium but repression of HtpG synthesis when cells were grown in glucose. No induction of htpG expression was seen with 2, 4-dichlorophenol in cells grown with either complex medium or glucose. The results suggest that, when a large pool of amino acids and proteins is available, as in complex medium, a much stronger stress response is observed. In contrast, when cells are grown in a simple glucose mineral medium, htpG expression either is unaffected or is even repressed by imposition of a stress condition. The results demonstrate the importance of considering differences in growth environment in order to better understand the nature of the response to an imposed stress condition.

Specific and Quantitative Assessment of Naphthalene and Salicylate Bioavailability by Using a Bioluminescent Catabolic Reporter Bacterium

A bioassay was developed and standardized for the rapid, specific, and quantitative assessment of naphthalene and salicylate bioavailability by use of bioluminescence monitoring of catabolic gene expression. The bioluminescent reporter strain Pseudomonas fluorescens HK44, which carries a transcriptional nahG-luxCDABE fusion for naphthalene and salicylate catabolism, was used. The physiological state of the reporter cultures as well as the intrinsic regulatory properties of the naphthalene degradation operon must be taken into account to obtain a high specificity at low target substrate concentrations. Experiments have shown that the use of exponentially growing reporter cultures has advantages over the use of carbon-starved, resting cultures. In aqueous solutions for both substrates, naphthalene and salicylate, linear relationships between initial substrate concentration and bioluminescence response were found over concentration ranges of 1 to 2 orders of magnitude. Naphthalene could be detected at a concentration of 45 ppb. Studies conducted under defined conditions with extracts and slurries of experimentally contaminated sterile soils and identical uncontaminated soil controls demonstrated that this method can be used for specific and quantitative estimations of target pollutant presence and bioavailability in soil extracts and for specific and qualitative estimations of napthalene in soil slurries.

Heat shock gene expression in continuous cultures of Escherichia coli

Temperature inducible systems for the controlled expression of recombinant genes are finding increasing industrial applications. These involve either short or long term exposure of the process culture to superoptimum temperatures. It is well known that bacteria respond to a sudden increase in their environmental temperature with an immediate transient increase in the synthesis rates of specific heat shock proteins. The use of continuous flow processes for the production of recombinant proteins would allow higher productivity and smaller scale bioreactors. However, the induction patterns of heat shock proteins in continuous culture after defined heat shocks are not well defined despite a large amount of information which is now available concerning heat shock protein induction in batch cultures. An overview of this information is presented to enable a better understanding of the response in continuous cultures. The latter was investigated by monitoring the transient expression of a representative heat shock gene, htpG, in E. coli in continuous culture. The relative magnitude of the response was found to be both temperature and exposure time dependent, but growth rate independent. Changing medium composition resulted in both different steady and transient state expression levels.