A polycistronic mRNA specified by the coronavirus infectious bronchitis virus

Area of Science:

• Virology
• Molecular Biology
• Genetics

Background:

• The coronavirus infectious bronchitis virus (IBV) genome includes subgenomic mRNAs.
• IBV mRNA3 is the third largest and contains three open reading frames (ORFs): 3a, 3b, and 3c.
• The coding functions of ORFs 3a and 3b were previously unestablished, unlike ORF 3c.

Purpose of the Study:

• To investigate the functional polycistronic nature of IBV mRNA3.
• To determine if ORFs 3a, 3b, and 3c are translated into proteins.
• To confirm the conservation of these ORFs across different IBV strains.

Main Methods:

• Nucleotide sequence analysis to assess ORF conservation in various IBV strains.
• Preparation of monospecific antisera against bacterial fusion proteins to detect ORF products in infected cells.
• In vitro translation studies using synthetic mRNAs encoding the ORFs.
• Expression studies in intact cells.

Main Results:

• Nucleotide sequence data confirmed the conservation of ORFs 3a, 3b, and 3c across diverse IBV strains.
• IBV-infected cells showed the presence of small amounts of protein products from these ORFs.
• In vitro and in vivo expression studies strongly indicated that all three proteins can be translated from a single mRNA3 molecule.

Conclusions:

• IBV mRNA3 is functionally tricistronic, capable of translating three separate proteins from a single mRNA.
• The findings establish the coding potential of all three ORFs within IBV mRNA3.
• This research clarifies the translational mechanism of a key viral mRNA.