Two kinds of signal-dependent transcription termination and RNA release mechanisms have been established in prokaryotes in vitro by: (i) binding of Rho to cytidine-rich nascent RNA [Rho-dependent termination (RDT)], and (ii) the formation of a hairpin structure in the nascent RNA, ending predominantly with uridine residues [Rho-independent termination (RIT)]. As shown here, the two signals act independently of each other and can be regulated (suppressed) by translation–transcription coupling in vivo. When not suppressed, both RIT- and RDT-mediated transcription termination do occur, but ribonucleolytic processing generates defined new 3′ ends in the terminated RNA molecules. The actual termination events at the end of transcription units are masked by generation of new processed 3′ RNA ends; thus the in vivo 3′ ends do not define termination sites. We predict generation of 3′ ends of mRNA by processing is a common phenomenon in prokaryotes as is the case in eukaryotes.

The Escherichia coli gal operon has the structure Pgal-galE-galTgalK-galM. During early log growth, a gradient in gene expression, named type 2 polarity, is established, as follows: galE > galT > galK > galM. However, during late-log growth, type 1 polarity is established in which galK is greater than galT, as follows: galE > galK > galT > galM. We found that type 2 polarity occurs as a result of the downregulation of galK, which is caused by two different molecular mechanisms: Spot 42-mediated degradation of the galK-specific mRNA, mK2, and Spot 42-mediated Rho-dependent transcription termination at the end of galT. Because the concentration of Spot 42 drops during the transition period of the polarity type switch, these results demonstrate that type 1 polarity is the result of alleviation of Spot 42-mediated galK down-regulation. Because the Spot 42-binding site overlaps with a putative Rho-binding site, a molecular mechanism is proposed to explain how Spot 42, possibly with Hfq, enhances Rhomediated transcription termination at the end of galT.D uring the exponential growth phase of Escherichia coli cells, transcription termination at the end of each cistron of the gal operon operates with less than 100% efficiency; a certain proportion of the transcription initiated from the two promoters of the gal operon terminates at the end of each constituent gene, galE, galT, galK, and galM, of the operon (1). The termination efficiencies measured at the end of each cistron are 16%, 29%, 65%, and 71%, respectively (1). Transcription termination at the end of each cistron generates four mRNA species: mE1, mT1, mK1, and mM1 (Fig.

No abstract

The chemokine receptor CXCR7 has been suggested to play important roles in the progression of several types of cancers. However, few studies have investigated the biological roles of CXCR7 in head and neck squamous cell carcinoma (HNSCC). CXCR7 expression and its clinical implications were examined in 103 HNSCC tissues using immunohistochemistry (IHC). The biological roles and mechanisms of CXCR7-mediated signaling pathways were investigated in HNSCC cells through CXCR7 overexpression in vitro and in vivo. High expression of CXCR7 was significantly associated with tumor size (P = 0.007), lymph node metastasis (P = 0.004), and stage (P = 0.020) in HNSCC. Overexpression of CXCR7 in HNSCC cells enhanced cell migration and invasion in vitro and promoted lymph node metastasis in vivo. CXCR7 also induced epithelial-mesenchymal transition through PI3K/AKT. CXCR7 increased secretion of transforming growth factor-β1 (TGF-β1) and promoted EMT through phosphorylated Smad2/3. Taken together, our results provide functional and mechanistic roles of CXCR7 as a master regulator of oncogenic TGF-β1/Smad2/3 signaling in HNSCC, suggesting that CXCR7 might be a therapeutic target for the treatment of HNSCC.Head and neck squamous cell cancer (HNSCC) constitutes a heterogeneous group of cancers. HNSCC is an epithelial malignancy with primary sites in the lip, oral cavity, pharynx, larynx, and paranasal sinuses 1,2 . High cure rates are achieved for localized HNSCC using surgery, radiation, and chemoradiation. However, recurrence after curative resection is common, and survival rates for recurrent/metastatic disease remain poor, with a 10% 5-year overall survival rate 3 . Therefore, an understanding of the molecular mechanisms of cancer progression is necessary to advance the treatment of HNSCC.In the tumor microenvironment, chemokine signaling systems play critical roles in tumor progression, invasion, migration, and metastasis 4 . Chemokines and chemokine receptors are differentially expressed in various malignant tumors 5,6 . Growing evidence shows that CXCR7 plays a crucial role in the development of tumors 7 . Furthermore, upregulation of CXCR7 serves as an oncogene in various cancers, such as breast and lung cancer 5,8 .Tumors metastasize through decreased cell adhesion, basement membrane perforation, migration by circulation, immune escape, and formation of colonies at distant sites 9 . Epithelial-mesenchymal transition (EMT) is essential for initiation and progression of metastasis 10,11 . Transforming growth factor (TGF)-β signaling is known to induce EMT through various intracellular messengers. Recent studies have shown that TGF-β promotes tumor progression and metastasis by regulating chemokines or chemokine receptors in the tumor MethodsPatients and tumor samples. A total of 103 HNSCC tissues were recruited from the Chungnam National University Hospital. All samples used in the study were approved by the ethics committee of Chungnam National University Hospital. Written informed consent was obtained from all the patients. ...

bThe gal operon of Escherichia coli has 4 cistrons, galE, galT, galK, and galM. In our previous report (H. J. Lee, H. J. Jeon, S. C. Ji, S. H. Yun, H. M. Lim, J. Mol. Biol. 378:318 -327, 2008), we identified 6 different mRNA species, mE1, mE2, mT1, mK1, mK2, and mM1, in the gal operon and mapped these mRNAs. The mRNA map suggests a gradient of gene expression known as natural polarity. In this study, we investigated how the mRNAs are generated to understand the cause of natural polarity. Results indicated that mE1, mT1, mK1, and mM1, whose 3= ends are located at the end of each cistron, are generated by transcription termination. Since each transcription termination is operating with a certain frequency and those 4 mRNAs have 5= ends at the transcription initiation site(s), these transcription terminations are the basic cause of natural polarity. Transcription terminations at galEgalT and galT-galK junctions, making mE1 and mT1, are Rho dependent. However, the terminations to make mK1 and mM1 are partially Rho dependent. The 5= ends of mK2 are generated by an endonucleolytic cleavage of a pre-mK2 by RNase P, and the 3= ends are generated by Rho termination 260 nucleotides before the end of the operon. The 5= portion of pre-mK2 is likely to become mE2. These results also suggested that galK expression could be regulated through mK2 production independent from natural polarity. P olycistronic operons in bacteria show a differential expression of the constituent cistrons (1). A Northern blot analysis showed that there are 6 different species of mRNA specific to the galactose operon in wild-type E. coli cells grown exponentially in the presence of galactose (2). Five of the 6 mRNA species, mE1, mE2, mT1, mK1, and mM1, have their 5= ends at the transcription initiation region, and their 3= ends at 5 different locations within the operon, four of which (all but mE2) are at the ends of the galE, galT, galK, and galM cistrons, respectively (Fig. 1A). There is one distinct mRNA species, designated mK2, that has 5= ends not at the promoter region but at the middle of galT. The existence of these mRNA species automatically establishes a gradient of gene expression, higher in the promoter-proximal region and lower in the promoter-distal region, which has been referred to as "natural polarity" (3). Natural polarity is intrinsically different from what has been known as polarity that is caused by a mutation (4), because it can be observed in cells harboring the wild-type operon (2, 5-9). The term "polarity" refers to the phenomenon in which a mutation in one gene of an operon decreases the expression of the subsequent genes of the operon. The cause for polarity is well established. The cessation of translation by a nonsense mutation uncouples transcription from translation, allowing the transcription termination factor, Rho, to bind to the nascent RNA and terminate transcription at the next available termination signal. This Rho-mediated transcription termination leaves the rest of the operon untranscribed, creating polarity ...