Yes, some viruses have mRNA. Specifically, positive-sense (+) RNA viruses essentially are mRNA.
Viruses employ various strategies to replicate within a host cell. A key component of this process involves the production of messenger RNA (mRNA), which is used to direct the synthesis of viral proteins. Whether or not a virus has mRNA depends on the specific type of virus and its replication strategy. Here's a breakdown:
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Positive-Sense RNA Viruses (+RNA viruses): These viruses possess genomes that directly function as mRNA. Once inside the host cell, the viral RNA can be immediately translated by the host's ribosomes to produce viral proteins, including the RNA-dependent RNA polymerase (RdRp). This enzyme is essential for replicating the viral genome. An example includes the poliovirus.
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Negative-Sense RNA Viruses (-RNA viruses): These viruses have genomes that are complementary to mRNA. Therefore, the viral genome cannot be directly translated. Instead, the virus must first use its own RNA-dependent RNA polymerase (RdRp) to transcribe the negative-sense RNA into positive-sense mRNA. This mRNA can then be translated to produce viral proteins. An example is the influenza virus.
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DNA Viruses: These viruses use DNA as their genetic material. Some DNA viruses directly use the host cell's machinery to transcribe viral DNA into mRNA. Others, like retroviruses, use reverse transcriptase to convert their RNA genome into DNA, which is then integrated into the host's genome. The host cell then transcribes this DNA into mRNA.
In summary, while not all viruses carry mRNA within the virion, all viruses either utilize mRNA directly (like +RNA viruses) or create it during their replication cycle. The +RNA viruses effectively have mRNA because their genome is mRNA.
