Viruses with RNA Genomes

RNA viruses are categorized into positive-strand, negative-strand, or double-stranded groups based on their genomic structure and replication mechanisms. This classification dictates how they exploit host cellular machinery for protein synthesis and replication. Some RNA viruses also utilize reverse transcription as part of their life cycle, further diversifying their replication strategies.

Positive-Strand RNA Viruses

Positive-strand RNA viruses have genomes that function directly as messenger RNA (mRNA) upon infection, enabling immediate translation of viral proteins.

Bacteriophage MS2, a well-known example, infects Escherichia coli by attaching specifically to F pili and injecting its RNA genome. The viral RNA serves as a template for direct translation of viral proteins, including RNA replicase, which is essential for genome amplification.

Similarly, poliovirus enters human cells and synthesizes a large polyprotein, which is subsequently cleaved by viral proteases into structural and enzymatic components necessary for virion assembly.

Coronaviruses, such as SARS-CoV-2, employ a more complex strategy by producing subgenomic RNAs, allowing for the selective translation of various viral proteins. This mechanism facilitates their sophisticated replication cycle and contributes to their ability to evade host immune responses.

Negative-Strand RNA Viruses

Negative-strand RNA viruses have genomes that cannot be directly translated; instead, they require transcription into mRNA by viral RNA-dependent RNA polymerases.

Rabies virus follows this mechanism, using its polymerase to generate mRNAs, which are then translated into viral proteins necessary for replication and virion assembly.

Influenza virus, another negative-strand RNA virus, possesses a segmented genome that enables genetic reassortment. When different influenza strains infect the same host cell, they can exchange genome segments, leading to antigenic shift—a process responsible for the emergence of novel influenza strains with pandemic potential. In contrast, antigenic drift results from the gradual accumulation of mutations, leading to seasonal influenza variations.

Double-Stranded RNA Viruses

Double-stranded RNA viruses, such as rotavirus from the Reoviridae family, replicate within the viral nucleocapsid. This sequestration shields viral RNA from host immune detection, aiding immune evasion.

During replication, the virus transcribes positive-strand RNA from its double-stranded genome, which serves as a template for protein synthesis and genome replication. This controlled replication strategy ensures efficient viral propagation while minimizing host immune responses.

Reverse-Transcribing Viruses

Some RNA viruses employ reverse transcription to replicate, converting RNA into DNA as part of their life cycle.

Retroviruses, such as HIV, use reverse transcriptase to synthesize complementary DNA (cDNA) from their RNA genome. This cDNA integrates into the host genome, enabling persistent infection and lifelong latency.

In contrast, hepadnaviruses, such as hepatitis B virus (HBV), follow a unique replication strategy. Although HBV is a DNA virus, it utilizes reverse transcription to convert an RNA intermediate into a partially double-stranded DNA genome. This mechanism differentiates it from classical DNA viruses, highlighting its distinct replication pathway.