Host cell genomic mRNAs

These results show that the competence of HeLa cells for infection by viral RNA does not change as dramatically during growth as the competence of bacteria for transformation. Only the two initial stages in the interaction of nucleic acids with host cells are equal in both systems, i.e. adsorption and penetration. Thereafter, quite different nucleic acid-dependent events occur, i.e. transforming DNA is incorporated in the host cell genome, viral RNA has to act as mRNA to induce a cycle of virus replication with concomitant interference with the metabolism of the host cell. 

The life history of a retroviras is described in chapter 17 (see Figure 17.45). A summary is presented here. The genome of a retrovirus is composed of RNA not DNA but, when a retrovirus infects a host cell its RNA is transcribed into DNA, catalysed by the enzyme, reverse transcriptase. This DNA is then incorporated into the genome of the host. On transcription of the host DNA, during cell division, viral mRNA and viral genomic RNA are produced. The... 

When viral replication occurs, the DNA segment corresponding to the viral genome is first transcribed by host cell enzymes (6). This gives rise not only to viral ssRNA, but also to transcription of mRNAs for precursors of the viral proteins (7). These precursors are integrated into the plasma membrane (8, 9) before undergoing proteolytic modification (10). The cycle is completed by the release of new virus particles (11). 

Some E. coli bacteriophages, including f2, MS2, R17, and Qj8, as well as some eukaryotic viruses (including influenza and Sindbis viruses, the latter associated with a form of encephalitis) have RNA genomes. The single-stranded RNA chromosomes of these viruses, which also function as mRNAs for the synthesis of viral proteins, are replicated in the host cell by an RNA-dependent RNA polymerase (RNA replicase). All RNA viruses—with the exception of retroviruses—must encode a protein with RNA-dependent RNA polymerase activity because the host cells do not possess this enzyme. 

Coding versus Template Strands The RNA genome of phage Q/3 is the nontemplate or coding strand, and when introduced into the cell it functions as an mRNA. Suppose the RNA replicase of phage Q/3 synthesized primarily template-strand RNA and uniquely incorporated this, rather than nontemplate strands, into the viral particles. What would be the fate of the template strands when they entered a new cell What enzyme would such a template-strand virus need to include in the viral particles for successful invasion of a host cell ... 

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