Host RNA

In addition, they assist in replication of RNA viruses, another process that requires RNA synthesis. Viruses sometimes make use of host RNA polymerases but often synthesize their own catalytic subunits. Bacteriophage T4 uses the E. coli RNA polymerase and o factors but modifies their action through the binding of several phage-encoded proteins.248 In contrast, phage T7 encodes its own relatively simple RNAP whose initiation complex (Section A,2)29 and elongation complexes have been studied 249-249b... 

Three strategies are used by different DNA viruses to accomplish transcription of viral DNA. The first type utilizes the host RNA polymerase, in some cases modifying it or... 

A third type of virus, exemplified by bacteriophage N4, carries a virus specific RNA polymerase in its virion. This polymerase enters the cell together with the viral DNA and transcribes some early viral genes. Some of these genes code for specificity factors that direct the host RNA polymerase to transcribe late genes. Vaccinia virus is another example of a virus that contains a virion-encapsulated RNA polymerase. 

Viruses sometimes use the host RNA polymerase in a modified form and sometimes synthesize their own RNA polymerase. 

In A the host RNA polymerase is used throughout. Regulation is achieved through a series of repressors... 

Transcription to yield many copies of retroviral RNA by host RNA polymerase... 

SCHWARTZ, L.B., LAWRENCE, C., THACH, R.E. and ROEDER, R.G. Encephalomyocarditis virus infection of mose plasmacytoma cells. III. Effect on host RNA synthesis and RNA polymerases. 

Under properly defined conditions picomaviruses interrupt host RNA and protein synthesis (1 ) and subvert the cellular machinery to production of viral protein and ENA. By feeding radiolabeled amino acids to virus-infected cells after cessation of host-protein synthesis, viral protein can be selectively labeled. In a pioneering study, which introduced the now widely used SDS-polyacrylamide gel electrophoresis technique. Summers et al. (2) identified some 14 different virus-specified polypeptides in extracts of poliovirus infected HeLa cells. The net mass of these polypeptides exceeded two-fold or more the known coding capacity of the viral genome. 

Gene expression. Synthesis of a functional protein depends on transcriptimi of the gene, translation of the mRNA, often processing of the mRNA, and often post-translational processing of the initial translation product (see Post-translational modification of proteins). TVanscription of a cloned insert requires the presence of a promoter which is recognized by the host RNA polymerase. Translation requires a ribo-... 

Infection of cells with DNA phage may lead to the production of a new, phage-spedfic RNA-P. (e.g. T7 phage RNA-P. is a si e-chtiin protein, M, 107,000), or the host RNA-P. may be modfied with the addition of new protein subunits encoded by the phage (e.g. T4 and k phage) in each case the resulting RNA-P. is specific for transcription of phage DNA. 

RNA expression Quantitative PCR Measure the messenger RNA (mRNA) level of genes of interest (host RNA or RNA from an infectious agent) or the whole transcrip-tome to view changes in mRNA levels corresponding to the desired phenotype [11a]... 

A block at the level of initiation of protein synthesis has also been suggested as the mechanism of shut-off by vesicular stomatitis virus (VSV Stanners et al., 1977 Jaye et al., 1982 Gillies and Stollar, 1982). As in poliovirus-induced shut-off, degradation of host mRNAs does not seem to play a role in VSV-induced shut-off since host mRNA can be extracted from infected cells and translated in vitro (Ehrenfeld and Lund, 1977). However, recent work has demonstrated that the synthesis of VSV leader RNA is directly related to inhibition of host RNA synthesis (Grinnell and Wagner, 1983). Unlike poliovirus mRNAs, VSV mRNAs are capped and require cap-binding protein for translation (Banerjee, 1980 Rose et al., 1978). The mechanism of VSV-induced shut-off is presently under active investigation to determine if competition between mRNAs (Lodish and Porter, 1980,... 

Herpesvirus-induced inhibition of host protein synthesis has been primarily associated to a viral component which is either synthesized during the replication of herpesvirus type 1 (Stenberg and Pizer, 1982 Hill et al., 1983) or is a component of the parental herpesvirus type 2 (Fenwick and Walker, 1978). In addition, herpesvirus type 1 causes the degradation of host cell mRNA (Nishioka and Sil-verstein, 1978 Inglis, 1982 Stenberg and Pizer, 1982). All the three virus-cell systems mentioned above also cause inhibition of host RNA and DNA synthesis during virus replication which may contribute to shut-off. 

A structural polypeptide in the form of surface tubules has also been implicated in shut-off (Mbuy et al., 1982). Exposure of HEp-2 cells to high concentrations of surface tubules (equivalent to 10 par-ticles/cell) resulted in a striking inhibition of protein synthesis. Under similar conditions of infection, host RNA and DNA synthesis were not inhibited. The translation of globin mRNA in a messenger-dependent reticulocyte lysate was also inhibited by these surface tubules. These workers performed a series of control experiments, including the use of specific antibodies for surface tubules, to show that shut-off by these tubules was specific. However, these tubules were obtained by extraction with a non-ionic detergent (Nonidet P40), and it is well known that this and similar detergents (Triton X) remain firmly bound to proteins and require extensive dialysis or other pro-... 

These data lead to the conclusion that inhibition of host cell RNA synthesis occurs concomitantly with the increase in virus-specific RNA synthesis. Further evidence for this conclusion was obtained from the following experiments (1) in interferon-treated CEF cells, SF virus-specific RNAs are not detected, and no inhibition of host cell RNA synthesis is observed (Taylor, 1965), and (2) ultraviolet-irradiated WEE virus loses the ability to inhibit host cellular RNA synthesis (M. Wagatsuma and B. Simizu, unpublished data). These data suggest that viral replication is necessary for the inhibition of host RNA synthesis, and that components of infecting virions are insufficient for this effect. 

Synthesis of phage-specific RNA in the case of phage 0X174 evidently takes place under the influence of DNA-dependent RNA-polymerase. No repressor of synthesis of host RNA and DNA appears at the beginning of infection, for this synthesis continues parallel to phage formation, almost until the phase of lysis (Rueckert and Zillig, 1962). 

Expression in E. coli of a cloned gene 43 under its own promoter ( 220-bp sequence upstream from the 5 end of the gene 43) does not provide a viable clone apparently due to the transcription of the gene by the host RNA polymerase and the production of T4 DNA Pol in an amount harmful to the cell. 

These in vitro data reflect the transcription pattern in vivo. The region up to 20% of the genome is transcribed efficiently by host RNA pol5nnerase, and, with reduced rate, RNA is synthesized up to a termination signal at about 30%. 

---