Enhancers, mRNA synthesis

Chang, C.C., Konno, S., Wu, J.M. (1991). Enhanced expression of heat shock protein and mRNA synthesis by type I interferon in human HL-60 leukemic cells. Biochem Inti. 24, 369-377. 

Drought also has a profound effect on protein synthesis. In many plant tissues, a reduced water potential causes a reduction of total protein synthesis and a rapid dissociation of polyribosomes. The latter has been shown not to be the consequence of increase in ribonuclease activity (Hsiao, 1973 Dhindsa Bewley, 1976). For a specific protein, Jacobsen, Hanson Chandler (1986) have shown in barley leaves that water stress enhances the synthesis of one of the a-amylase isozymes. Using a cDNA probe they found that water-stressed leaves contained much more a-amylase mRNA than unstressed plants. 

Endo and co-workers at Ehime University, Matsuyama, Japan, have led the development of the most promising eukaryotic cell-free system to date, based on wheat embryos. A significant advance made by this group was the development of pEU expression vectors that have overcome many of the difficulties associated with mRNA synthesis for translation in a eukaryotic system [8]. In addition to extensive optimization of reaction conditions that have seen improvements in protein synthesis rates, Endo and colleagues have improved wheat extract embryo preparation protocols to enhance the stability of these systems to a remarkable extent [9]. When coupled with the dialysis mode of reaction, Endo et al. were able to maintain translational activity in a coupled transcription/ translation wheat embryo reaction for 150 hours, producing 5 mg of enzymatically active protein per mb reaction mixture [10]. This again represents a serious alternative to in vivo methods of large-scale protein production. 

MECHANISMS OE ACTION AND RESISTANCE Ribavirin alters cellular nucleotide pools and inhibits viral mRNA synthesis. Intracellular phosphorylation to the mono-, di-, and triphosphate derivatives is mediated by host cell enzymes. In both uninfected and RSV-infected cells, the predominant derivative is the triphosphate, which has an intracellular of <2 hours. Ribavirin monophosphate competitively inhibits cellular inosine-5 -phosphate dehydrogenase and interferes with the synthesis of GTP and thus nucleic acid synthesis. Ribavirin triphosphate also competitively inhibits the GTP-dependent 5 capping of viral messenger RNA and specifically influenza virus transcriptase activity. Ribavirin has multiple sites of action, and some of these e.g., inhibition of GTP synthesis) may potentiate others e.g., inhibition of GTP-dependent enzymes). Ribavirin also may enhance viral mutagenesis such that some viruses may be inhibited in effective replication, so-caUed lethal mutagenesis. 

The interpretations on the controls of two other well-studied enzymes in tissue culture may be mentioned. Nebert and Gelboin [130] using hamster fibroblasts, and the induction by benzanthracene of microsomal aryl hydroxylase, found that the inducer appeared to cause an increase in mRNA synthesis of the hydroxylase even if protein synthesis was blocked by inhibitors it was therefore concluded that the induction occurred at the level of transcription. In contrast, Tomkins et al. [131], using rat hepatoma cells induced with dexamethasone phosphate to form tyrosine-x-KG-amino transferase (TAT), concluded that the inducer acted at the level of translation to antagonize the action of a repressor. This conclusion was based on the fact that, when actinomycin D at the very high concentration of 5 //g/ml was given several hours after the induction, an enhanced synthesis of the enzyme occurred. To... 

Very recently, a third hypothesis has been pubHshed. Morita and co-workers [47] have suggested that the rpoH mRNA secondary structure itself acts as a thermosensor. In the absence of heat stress, the rpoH mRNA is folded into a secondary structure that occludes the ribosome binding site and the initiation codon. Upon heat shock, this structure is unfolded allowing ribosome binding and enhanced synthesis. 

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