Distinct structural elements and internal entry of ribosomes in mRNA3 encoded by infectious bronchitis virus.

Infectious bronchitis virus (IBV) mRNA3 encodes three small proteins, 3a, 3b, and 3c, at its 5' end. Recently, it was demonstrated that initiation of protein 3c is dependent on the upstream sequence. Monte Carlo simulations of RNA folding in this tricistronic mRNA3 indicate that a highly significant folding region occurs prior to the initiator AUG of 3c. The unusual folding region (UFR) of 265 nucleotides (nt) contains the coding sequences of proteins 3a and 3b. Details of the structural analyses show that five highly significant RNA stem-loops in the UFR can be modeled into a compact superstructure by the interaction of two predicted pseudoknot structures. The folded superstructure comprising nt 44 to 330, with additional 22 nt downstream from this UFR, is suggested to serve as a ribosome landing pad (or an internal ribosomal entry site) in the cap-independent translation of the 3c of IBV. Intriguingly, the proposed structural motif of this coronavirus shares structural features similar to those proposed in a number of picornavirus mRNAs. Based on the common structural features, a plausible base pairing model between mRNA3 and 18 S rRNA is suggested, which is consistent with a general mechanism for regulation of internal initiation described in many picornaviruses.