Single-stranded genome

A. Positive-Stranded RNA Viruses - The positive-stranded RNA viruses are best represented by the picornaviruses of which poliovirus is a model virus. Poliovirus contains a single-stranded genome RNA, which has a MW of 2.5x10 (7700 nucleotides), and four viral structural proteins. Upon entrance into a cell, the genome RNA is translated into one large polypeptide which is cleaved proteolytically to produce all the viral proteins (see below). The 5 ends of picornavirus genome RNAs are unique as they contain a covalently linked protein. The protein (VPg) has a MW of 6-12,000, and has both a basic and hydrophobic nature. It is linked... 

Plus Strand In viral genomes, a nucleic add strand that can serve as mRNA or (for DNA strand) that is homologous to one that can as distinct from the complementary (minus) strand. Most viruses with single-strand genomes package only the plus or minus strand in virions the other strand is made transiently during replication. Compare Minus Strand. 

Certain eukaryotic RNA viruses (e.g., picornaviruses) contain a single-stranded genomic RNA (—7.5 kb) that functions as a polycistronic mRNA. The 5 ends of the picornaviral RNAs are not capped, but are covalently linked to a small viral protein, VPg. In contrast to the average of 55-nt-Iong 5 UTR in yeast and higher eukaryotic mRNAs, the 5 UTRs of picornaviruses are over 700 bases long and contain extensive stem—loop structures. The particular 5 UTR structures of picornaviruses by no means fit the widely accepted scanning mechanism of... 

The human immunodeficiency vims (HIV) is the causative agent of the acquired immunodeficiency syndrome (AIDS). HIV is a retrovirus, whose replication includes the transcription of the single-stranded RNA genome into double stranded DNA (reverse transcription) and the covalent insertion of the DNA... 

HCV is a single-stranded RNA virus of the Flaviviridae family. It has a 9.4-kb positive-sense genome encoding a polyprotein precursor of 3011 amino acids. Individual isolates of HCV consist of closely related yet heterogeneous populations... 

Virus particles vary widely in size and shape. As we have stated, some viruses contain RNA, others DNA. We have discussed nucleic acids in previous chapters and have noted that the DNA of the cell genome is in the double-stranded form. Some viruses have double-stranded DNA whereas others have single-stranded DNA (Figure 6.3). 

The virus genome We have stated that the virus genome consists of either DNA or RNA, never both. Viruses differ in size, amount, and character of their nucleic acid. Both single-stranded and doublestranded nucleic acid is found in viruses, and the amount of nucleic acid per virion may vary greatly from one virus type to another. In general, in enveloped viruses the nucleic acid constitutes only a small part of the mass of the virus particle (1-2 percent), whereas in nonenveloped viruses the percent of the particle which is nucleic acid is much larger, often 25-50 percent. 

Virus messenger RNA In order for the new virus-specific proteins to be made from the virus genome, it is necessary for new virus-specific RNA molecules to be made. Exactly how the virus brings about new mRNA synthesis depends upon the type of virus, and especially upon whether its genetic material is RNA or DNA, and whether it is single-stranded or double-stranded. Which copy is read into mRNA depends upon the location of the appropriate promoter, since the promoter points the direction that the RNA polymerase will follow. In cells (uninfected with virus) all mRNA is made on the DNA template, but with RNA viruses the situation is obviously different. 

A number of bacterial viruses have RNA genomes. The best-known bacterial RNA viruses have single-stranded RNA. Interestingly, the bacterial RNA viruses known in the enteric bacteria group infect only bacterial cells which behave as gene donors (males) in genetic recombination. This restriction to male bacterial cells arises because these viruses infect bacteria by attaching to male-specific pili. Since such pili are absent on female cells, these RNA viruses are unable to attach to the females, and hence do not initiate infection in females. 

This enzyme is associated with the virions of RNA tumor viruses such as the Rous sarcoma virus (RSV). The enzyme has remarkable enzymatic activity in that it can catalyze several seemingly diverse steps in the synthesis of double-stranded DNA from the single-stranded RNA viral genome. The enzyme uses a tRNA for tryp-tophan as a primer to make a copy of DNA that is complementary to the viral RNA. The resulting RNA-DNA hybrid is converted to a double-stranded DNA molecule by ribon-uclease (RNase)H and DNA-dependent DNA polymerase activities that are intrinsic to reverse transcriptase. 

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