Representatives of the group
Negative-sense single-stranded RNA viruses include the causative agents of numerous significant infectious diseases in humans and animals. According to classification, this group comprises the following families:
- Arenaviruses
- Bornaviruses
- Rhabdoviruses
- Paramyxoviruses
- Orthomyxoviruses
- Filoviruses
Despite morphological differences, all these agents share a common genetic organization principle: their genome is represented by a negative, or "minus strand" RNA, which dictates a specific pattern of interaction with the infected cell.
Initiation of infection and entry
The reproductive cycle begins with the successful attachment of the virion to the target cell. This process can be divided into three consecutive stages:
- Adsorption. The virus recognizes specific receptors on the surface of a susceptible cell. Binding is mediated by surface glycoproteins located in the outer viral envelope.
- Penetration. Following successful adsorption, the virion envelope fuses with the plasma membrane (plasmalemma) of the cell.
- Uncoating. As a result of membrane fusion, the internal contents of the virus are released directly into the cytoplasm. The genomic minus-strand RNA is freed from protective protein structures, preparing for the next, most crucial step.
Genome expression: the role of RNA polymerase
The main challenge of negative-sense single-stranded viruses is that their genomic (-)RNA is non-coding. The host cell ribosomes cannot "read" it and synthesize protein. Therefore, a critically important condition is the presence of a preformed enzyme within the virion — virion RNA-dependent RNA polymerase.
It is this enzyme that carries out transcription — the transformation of the genomic "minus strand" into complementary "plus strands". During this process, two molecules with fundamentally different functions are formed:
- Incomplete copies (plus strands). They function as messenger RNA (mRNA). These shortened molecules travel to cellular ribosomes, where they serve as a template for translation — the synthesis of new virus-specific proteins.
- Complete copies (plus strands). They represent the full-length antigenome. Their task is to serve as an intermediate template for subsequent replication, i.e., mass-producing new generations of genomic RNA "minus strands".
Virion assembly and exit from the cell
When a sufficient amount of newly synthesized viral proteins and fresh copies of genomic (-)RNA accumulate in the cytoplasm, the process of forming new virions is triggered. Using paramyxoviruses as an example, this mechanism looks as follows:
- New genomic RNA strands combine with structural proteins to form a nucleocapsid.
- The formed nucleocapsid interacts with a specific matrix protein (M-protein), which acts as a linking element.
- The entire complex heads toward the inner side of the plasmalemma — specifically to those areas that have already been modified (embedded) by viral surface glycoproteins.
- Virion exit occurs via budding. The virus evaginates a region of the altered cell membrane, detaches from the cell, and thereby acquires its final outer lipid envelope.
Briefly: answers to topical questions
Which specific structural proteins bind to the genomic RNA during nucleocapsid formation?
The cited sources explicitly indicate the binding of genomic RNA to nucleoproteins:
- Paramyxoviruses — nucleoprotein N is bound to RNA.
- Filoviruses — NP is a nucleoprotein.
- Orthomyxoviruses — nucleoprotein NP is bound to RNA as part of the nucleocapsid.
The filovirus L-protein, paramyxovirus L-protein, paramyxovirus P-protein, and orthomyxovirus polymerase proteins are part of the replication-transcription complex or nucleocapsid; however, their direct binding to genomic RNA is not explicitly stated in the cited sources.
Which specific surface glycoproteins ensure the adsorption of orthomyxoviruses onto the cell?
The adsorption of orthomyxoviruses onto a susceptible cell is ensured by the specific surface glycoprotein hemagglutinin.
- Hemagglutinin (HA or H) is responsible for the primary attachment (adhesion) of the virion to receptors on the plasma membrane of respiratory epithelial cells.
The second surface glycoprotein, neuraminidase (NA or N), is not directly involved in primary fixation, but facilitates invasion by cleaving sialic acid and preparing receptors for binding with hemagglutinin.
Which cellular receptors do the surface glycoproteins of orthomyxoviruses bind to during adsorption?
During adsorption, influenza virus hemagglutinin binds to sialic acid, which is part of the glycoproteins and glycolipids of the respiratory epithelium. Neuraminidase cleaves sialic acid from sialylated glycolipids, thereby preparing receptors for binding with hemagglutinin.
The rule "Minus does not work without Plus" will help remember the essence of transcription: the initial minus strand is mute to ribosomes. The polymerase makes incomplete pluses (going to proteins) and complete pluses (going to new minuses) from it.
Class and exam questions
Because it is non-coding (antisense). Cellular ribosomes recognize only mRNA (plus strand), so the minus genome must first be transcribed into a plus strand.
The genome of such viruses is non-coding. Ribosomes are capable of reading information only from mRNA (plus strand), so before translation, the polymerase must transcribe the minus strand into a complementary plus strand.
Where does the virus get the RNA-dependent RNA polymerase if it has not yet synthesized proteins?
This enzyme is obligatorily included in the structure of the virion itself (brought along by the virus) and enters the cell at the moment of uncoating.
What is the difference between complete and incomplete plus strands?
Incomplete copies serve as messenger RNA (a template for protein assembly), while complete copies are the antigenome — an intermediate template for creating new genomic minus strands.
What function does the M-protein perform?
The matrix protein (M-protein) connects the formed nucleocapsid with areas of the cell's plasma membrane that already contain viral glycoproteins, ensuring proper assembly prior to budding.
What else to study on this topic
- Structural features of virion RNA-dependent RNA polymerase
- Specific envelope glycoproteins of paramyxoviruses and their role in adsorption
- Mechanisms of viral envelope fusion with the cell plasmalemma
- Differences in reproduction between orthomyxoviruses and filoviruses
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