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Lecture at NIVB Meeting 2023 The Mason-Pfizer Monkey Virus (M-PMV) is a member of the family Retroviridaeand genus Betaretrovirus. As all viruses, M-PMV relies on the host-cell protein machinery. The retroviral genome is represented by two RNA molecules with positive polarity interacting with each other through the region situated at 5 ́end, resulting in a dimer formation. The dimer is coated by nucleocapsid (NC) protein molecules, facilitating the most thermodynamically suitable conformation inside the virion. The ribonucleoprotein complex together with enzymes is encapsulated in the core composed of the capsid protein. The core is enveloped by abilayer of phospholipid membrane obtained during budding, and matrix protein bound in the inner layer of the membrane. During the genome replication of RNA viruses, the dissociation of ribonucleoprotein complex and unfolding of RNA secondary structures are crucial for proper replication. These processes are often facilitated by RNA helicases. Some RNA viruses encode the RNA helicase in their genome; nevertheless, others, such as retroviruses, utilize host-cell RNA helicase to promote their replication. RNA helicases can participate in reshaping viral ribonucleoproteins, splicing, nuclear export of unspliced viral RNA, transcription, translation, and viral RNA packaging (reviewed in1,2).As a simple betaretrovirus, M-PMV encodes its proteins only in three genes: gag, pro, pol. These are translated into three polyprotein precursors: Gag, Gag-Pro, Gag-Pro-Pol. At the C-terminus of Gag-Pro and N-terminus of Gag-Pro-Pol, ashort glycine-rich peptide (G-patch, GP) is localized as apart of the protease (PR) and reverse transcriptase (RT), respectively. The GP-containing eukaryotic proteins are often involved in the recruitment and subsequent activation of DEAH/RHA RNA helicases, such as DHX153(reviewed in4,5).The MS analysis of the host-cell proteins in released M-PMV virions revealed the presence of RNA helicase DHX15 packaged into M-PMV particles but not into ΔGP M-PMV particles. Expectedly, during cell-based and in vitroexperiments such as mutagenesis, mRNA silencing, photoactivatable ribonucleoside-enhanced crosslinking, immunoprecipitation assay, thermophoresis, and isothermal titration calorimetry, we observed that the DHX15 was involved in reverse transcription as the initial step of retroviral genome replication (Fig. 1). Surprisingly, it was also noticed that DHX15 acts during the packaging of retroviral genomic RNA (gRNA), although the Gag polyprotein was reported to be particularly responsible for that.