RESUMO
Cells of almost any organism respond to a sudden up-shift of temperature and to several other stress conditions, by a transient increase in the cellular concentration of a set of proteins, the heat-shock proteins (HSPs). The main HSPs, chaperones and proteases, are constituents of the cellular machinery of protein folding, translocation, repair and degradation. The bacteria Escherichia coli has been a paradigm regarding heat shock gene expression in prokaryotes. In this bacterium, the expression of the HSPs is regulated at the transcriptional level. The approximately 40 genes that encode the HSPs define the heat-shock stimulon. Most of these genes, including the main chaperone and protease genes, are under the positive control of sigma32, encoded by rpoH, while approximately 10 genes, including rpoH and rpoE, are regulated by sigma(E) , encoded by rpoE. The cytoplasmic response to heat is regulated by sigma32, while that of the periplasm is regulated by sigma(E). The expression of both regulons is interconnected, since sigma(E) regulates the transcription of rpoH at high temperatures. The activity of these sigma factors, under non-stress and stress conditions, depends upon negative and positive regulatory mechanisms acting at different levels: transcription, translation, half-life and activity of the factors. Models for the regulation of the cytoplasmic and periplasmic response to heat in E. coli are presented.
Assuntos
Proteínas de Escherichia coli/genética , Escherichia coli/genética , Regulação Bacteriana da Expressão Gênica , Proteínas de Choque Térmico/genética , Bactérias/metabolismo , Sequência de Bases , Escherichia coli/fisiologia , Escherichia coli K12/genética , Escherichia coli K12/fisiologia , Proteínas de Escherichia coli/biossíntese , Proteínas de Escherichia coli/fisiologia , Genes Bacterianos , Proteínas de Choque Térmico/biossíntese , Proteínas de Choque Térmico/fisiologia , Temperatura Alta , Modelos Biológicos , Chaperonas Moleculares/biossíntese , Chaperonas Moleculares/genética , Chaperonas Moleculares/fisiologia , Dados de Sequência Molecular , Periplasma/metabolismo , Biossíntese de Proteínas , Dobramento de Proteína , Regulon/genética , Fator sigma/fisiologia , Fatores de Transcrição/fisiologia , Transcrição GênicaRESUMO
The excimer-forming fluorophore dipyrenylpropane has been used to measure the relative fluidity of total membranes isolated from Escherichia coli grown at 30 or 45 degrees C, or exposed to a heat-shock from 30 to 45 degrees C for various periods of time. Parallel experiments were performed using [35S]methionine pulse-labeling of cells, to study the induction of heat-shock proteins (HSPs) at different times after the sudden change in E. coli growth-temperature from 30 to 45 degrees C. Results suggest that upon an abrupt temperature upshift from 30 to 45 degrees C, membrane fluidity adjustment to the steady-state level at the high temperature, takes place during the E. coli heat-shock response.
Assuntos
Escherichia coli/fisiologia , Resposta ao Choque Térmico , Fluidez de Membrana , Corantes Fluorescentes , Proteínas de Choque Térmico/biossíntese , Indicadores e Reagentes , Pirenos , Fatores de TempoRESUMO
The chromosomal DNA of all cells is under helical tension or supercoiling. There are two classes of DNA supercoiling: plectonemic and toroidal. Plectonemic supercoiling is generated by the action of DNA topoisomerases, while toroidal supercoiling is generated by DNA-protein interactions and by topoisomerase activitities. DNA supercoiling plays an important role in replication, repair, recombination, transposition and transcription. DNA topoisomerases type I are ATP-independent enzymes that cut one DNA strand and relax supercoiled molecules. DNA topoisomerases type II requiere ATP, cut both DNA strands and supercoil relaxed molecules. All organisms have more than one topoisomerase of each, type I and type II. Escherichia coli has two topoisomerases type I: topoisomerase I and topoisomerase III and two topoisomerases type II: topoisomerase II or gyrase and topoisomerase IV. In this review we discuss the concept of DNA supercoiling and present current knowledge on E. coli DNA topoisomerases.
Assuntos
Proteínas de Bactérias/metabolismo , DNA Topoisomerases Tipo II/metabolismo , DNA Topoisomerases Tipo I/metabolismo , DNA Bacteriano/ultraestrutura , DNA Super-Helicoidal/ultraestrutura , Escherichia coli/genética , Proteínas de Bactérias/genética , Cromossomos Bacterianos/metabolismo , Cromossomos Bacterianos/ultraestrutura , Replicação do DNA , DNA Topoisomerase IV , DNA Topoisomerases Tipo I/genética , DNA Topoisomerases Tipo II/genética , DNA Bacteriano/genética , DNA Bacteriano/metabolismo , DNA Super-Helicoidal/genética , DNA Super-Helicoidal/metabolismo , Escherichia coli/metabolismo , Regulação Bacteriana da Expressão Gênica , Transcrição GênicaRESUMO
The bacterial genome is present in the cell within a complex structure, the nucleoid. The nucleoid contains the genomic DNA, and molecules of RNA and proteins. The main proteins of the nucleoid are: RNA polymerase, topoisomerases and the histone-like proteins: HU, H-NS (H1), H, HLP1, IHF and FIS. The DNA molecule in the nucleoid is under helical tension or supercoiling and is organized into 43 +/- 10 topodomains. DNA supercoiling is generated by the activity of the topoisomerases and by DNA-protein interactions. In this review, we analize current knowledge in Escherichia coli about genome organization and proteins of the nucleoid.