RESUMO
The sigma(E) (extracytoplasmic stress response sigma factor in Escherichia coli) signaling system of Gram-negative bacteria plays an essential role in the maintenance of the extracytoplasmic compartment. Upon induction of this system, approximately 100 genes are up-regulated. The majority of these genes encode proteins that participate in the synthesis, assembly, and homeostasis of outer membrane proteins and lipopolysaccharides. A second aspect of the sigma(E) response is a regulatory loop that prevents expression of the major porins. Misfolding or overproduction of most of these porins is sufficient to trigger the envelope stress response. Recent work indicates that small Hfq-binding RNAs play a major role in maintaining envelope homeostasis and, so far, two sigma(E)-dependent small noncoding RNAs (sRNAs), MicA and RybB, have been shown to facilitate rapid removal of multiple omp transcripts in response to elevated activity of the alternative sigma factor. Here we report the identification of the sRNA (CyaR, cyclic AMP-activated RNA) that promotes decay of the ompX mRNA. The transcription of the cyaR gene is stringently controlled by cAMP-cAMP receptor protein and, unexpectedly, cyaR expression is also up-regulated, directly or indirectly, by sigma(E). In addition, this work identified MicA as a factor that cooperates in the negative control of ompX expression. The conservation of CyaR, MicA, RybB, and their targets suggests that the omp mRNA-sRNA regulatory network is an integral part of the envelope stress response in many enterobacteria.
Assuntos
Proteínas da Membrana Bacteriana Externa/genética , Proteínas da Membrana Bacteriana Externa/metabolismo , Proteína Receptora de AMP Cíclico/genética , Proteína Receptora de AMP Cíclico/metabolismo , Proteínas de Escherichia coli/genética , Proteínas de Escherichia coli/metabolismo , Hidrolases/genética , Hidrolases/metabolismo , RNA Bacteriano/genética , RNA Bacteriano/metabolismo , RNA não Traduzido/genética , RNA não Traduzido/metabolismo , Fator sigma/genética , Fator sigma/metabolismo , Sequência de Bases , AMP Cíclico/metabolismo , DNA Glicosilases/genética , DNA Glicosilases/metabolismo , DNA Bacteriano/genética , Regulação para Baixo , Escherichia coli/genética , Escherichia coli/metabolismo , Genes Bacterianos , Fator Proteico 1 do Hospedeiro/genética , Fator Proteico 1 do Hospedeiro/metabolismo , Modelos Biológicos , Dados de Sequência Molecular , Regiões Promotoras Genéticas , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Homologia de Sequência do Ácido Nucleico , Transdução de SinaisRESUMO
The Escherichia coli ompA mRNA, encoding a highly abundant outer membrane protein, has served as a model for regulated mRNA decay in bacteria. The half-life of this transcript correlates inversely with the bacterial growth rate and is growth stage-dependent. The stability of the messenger is determined by the 5'-untranslated region which possesses cleavage sites for RNase E. Hfq binds to this region, is essential for controlling the stability and has been suggested to directly regulate ompA mRNA decay. Here we report that the 78 nucleotide SraD RNA, which is highly conserved among Enterobacteriaceae, acts in destabilizing the ompA transcript when rapidly grown cells enter the stationary phase of growth. During this growth-stage the expression of SraD RNA becomes strongly increased. The SraD-mediated decay of ompA mRNA depends on Hfq and in vitro studies revealed that Hfq facilitates binding of the regulatory RNA to the translational initiation region of the messenger. Deletion of sraD, however, does not significantly affect the stability of the ompA mRNA in slowly growing cells. Our results indicate that distinct regulatory circuits are responsible for growth phase- and growth rate-dependent control of the ompA mRNA stability.
Assuntos
Proteínas da Membrana Bacteriana Externa/metabolismo , Escherichia coli K12/crescimento & desenvolvimento , Regulação Bacteriana da Expressão Gênica , Estabilidade de RNA , RNA Bacteriano/metabolismo , RNA Mensageiro/metabolismo , RNA não Traduzido/metabolismo , Regiões 5' não Traduzidas/química , Regiões 5' não Traduzidas/genética , Proteínas da Membrana Bacteriana Externa/química , Proteínas da Membrana Bacteriana Externa/genética , Sequência de Bases , Escherichia coli K12/genética , Escherichia coli K12/metabolismo , Proteínas de Escherichia coli/genética , Proteínas de Escherichia coli/metabolismo , Meia-Vida , Fator Proteico 1 do Hospedeiro/genética , Fator Proteico 1 do Hospedeiro/metabolismo , Dados de Sequência Molecular , RNA Bacteriano/química , RNA Bacteriano/genética , RNA Mensageiro/química , RNA Mensageiro/genética , RNA não Traduzido/química , RNA não Traduzido/genéticaRESUMO
The conserved RNA-binding protein Hfq, originally discovered in Escherichia coli as a host factor for Qbeta replicase, has emerged as a pleiotropic regulator that modulates the stability or the translation of an increasing number of mRNAs. During the past 5 years, Hfq-mediated control has been an area of increasing focus because the protein has been linked to the action of many versatile RNA-based regulators that use basepairing interactions to regulate the expression of target mRNAs. The recent findings that Hfq assists in bimolecular RNA-RNA interactions and is similar structurally and functionally to eukaryotic Sm proteins have further fueled interest in this important post-transcriptional regulator. Here, we summarize the history of Hfq and highlight results that have led to an important gain in insight into the physiology, biochemistry and evolution of Hfq and its homologues.
