Page 155 - Avian Virology: Current Research and Future Trends

148 | Liu et al.

Figure 5.12 Infectious bronchitis virus (IBV) replicase gene and processing scheme of replicase protein products. Translation of the
replicase gene would begin once the viral genome is released into the host cytoplasm. Replicase 1a and 1b are the only gene
Figure 12 Infectious bronchitis virus (IBV) replicase gene and processing scheme of replicase protein products. Translation of the replicase gene would to be
begin once the viral genome is released into the host cytoplasm. Replicase 1a and 1b are the only gene to be translated at this step of the virus life cycle, forming
translated at this step of the virus life cycle, forming polyprotein (pp) 1a and 1ab via ribosomal frameshifting. Pp1a and pp1ab will then be
polyprotein (pp) 1a and 1ab via ribosomal frameshifting. Pp1a and pp1ab will then be autoproteolytically cleaved at cleavage sites by papain-like protease
autoproteolytically cleaved at cleavage sites by papain-like protease (PLpro) (in red triangles) and main protease (Mpro) (in brown triangles)
(PLpro) (in red triangles) and main protease (Mpro) (in brown triangles) into 15 non-structural proteins.
into 15 non-structural proteins.

Table 5.3 Cleavage site and size (amino acid number) of IBV (strain Beaudette) non-structural proteins

IBV strain Beaudette
non-structural Protein Size (amino acids) Start (nucleotide) Stop (nucleotide) References
2 673 529 2547 Liu et al. (1995a,b,c);
Lim and Liu (1998a)
3 1592 2548 7323 Lim et al. (2000)
4 514 7324 8865 Lim et al. (2000)
5 307 8866 9786 Liu et al. (1994);
Ng and Liu (2000)
6 293 9787 10665 Ng and Liu (2000)
7 83 10666 10914 Ng and Liu (2000)
8 210 10915 11544 Ng and Liu (1998)
9 111 11545 11877 Liu et al. (1997)
10 145 11878 12312 Ng and Liu (2002)
11 13 12313 12351 Fang et al. (2007)
12 932 12313 15131 Liu et al. (1994)
13 601 15132 16931 Liu et al. (1998a,b,c)
14 521 16932 18494 Liu et al. (1998a,b,c)
15 338 18495 19508 Liu et al. (1998a,b,c)
16 302 19509 20414 Liu et al. (1998a,b,c)

Replication and transcription The sgRNAs serve as mRNAs for various structural and accessory
Viral RNA synthesis quickly ensues following the formation of genes residing 3′ downstream of the replicase gene.
the RTC. In CoVs, viral RNA synthesis produces two types of Replication of viral RNAs are not without the cis-acting
RNAs, the genome-sized gRNAs and subgenome-sized RNAs sequences. Within the 5′ UTR of the CoV genome, there are
(sgRNAs), both generated through negative-strand interme- many stem–loop structures which extend into rep1a while the
diates. These negative-strand intermediates are only ≈ 1% as 3′ UTR also contains varied cis-acting elements from stem–
abundant as their positive-stranded counterparts, and they also loops, pseudoknots and HVRs (Raman et al., 2003; Brown et
contain anti-leader and poly-U sequences (Sethna et al., 1991). al., 2007). While exactly how each of these different structures