Presence of an encephalomyocarditis virus internal ribosome entry site sequence in avian infectious bronchitis virus defective RNAs abolishes rescue by helper virus

Hepatitis A virus (HAV) packaging size limit

Background

Hepatitis A virus (HAV), an atypical Picornaviridae that causes acute hepatitis in humans, grows poorly in cell culture and in general does not cause cytopathic effect. Foreign sequences have been inserted into different parts of the HAV genome. However, the packaging size limit of HAV has not been determined. The purpose of the present study is to investigate the maximum size of additional sequences that the HAV genome can tolerate without loosing infectivity.

Results

In vitro T7 polymerase transcripts of HAV constructs containing a 456-nt fragment coding for a blasticidin (Bsd) resistance gene, a 1,098-nt fragment coding for the same gene fused to GFP (GFP-Bsd), or a 1,032-nt fragment containing a hygromycin (Hyg) resistance gene cloned into the 2A-2B junction of the HAV genome were transfected into fetal Rhesus monkey kidney (FRhK4) cells. After antibiotic selection, cells transfected with the HAV construct containing the resistance gene for Bsd but not the GFP-Bsd or Hyg survived and formed colonies. To determine whether this size limitation was due to the position of the insertion, a 606 bp fragment coding for the Encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) sequence was cloned into the 5' nontranslated (NTR) region of HAV. The resulting HAV-IRES retained the EMCV IRES insertion for 1-2 passages. HAV constructs containing both the EMCV IRES at the 5' NTR and the Bsd-resistance gene at the 2A-2B junction could not be rescued in the presence of Bsd but, in the absence of antibiotic, the rescued viruses contained deletions in both inserted sequences.

Conclusion

HAV constructs containing insertions of approximately 500-600 nt but not 1,000 nt produced viable viruses, which indicated that the HAV particles can successfully package approximately 600 nt of additional sequences and maintain infectivity.

An infectious recombinant equine arteritis virus expressing green fluorescent protein from its replicase gene

Thus far, systems developed for heterologous gene expression from the genomes of nidoviruses (arteriviruses and coronaviruses) have relied mainly on the translation of foreign genes from subgenomic mRNAs, whose synthesis is a key feature of the nidovirus life cycle. In general, such expression vectors often suffered from relatively low and unpredictable expression levels, as well as genome instability. In an attempt to circumvent these disadvantages, the possibility to express a foreign gene [encoding enhanced green fluorescent protein (eGFP)] from within the nidovirus replicase gene, which encodes two large polyproteins that are processed proteolytically into the non-structural proteins (nsps) required for viral RNA synthesis, has now been explored. A viable recombinant of the arterivirus Equine arteritis virus, EAV-GFP2, was obtained, which contained the eGFP insert at the site specifying the junction between the two most N-proximal replicase-cleavage products, nsp1 and nsp2. EAV-GFP2 replication could be launched by transfection of cells with either in vitro-generated RNA transcripts or a DNA launch plasmid. EAV-GFP2 displayed growth characteristics similar to those of the wild-type virus and was found to maintain the insert stably for at least eight passages. It is proposed that EAV-GFP2 has potential for arterivirus vector development and as a tool in inhibitor screening. It can also be used for fundamental studies into EAV replication, which was illustrated by the fact that the eGFP signal of EAV-GFP2, which largely originated from an eGFP-nsp2 fusion protein, could be used to monitor the formation of the membrane-bound EAV replication complex in real time.

Rnall: an efficient algorithm for predicting RNA local secondary structural landscape in genomes

BACKGROUND

The information of RNA local secondary structures (LSSs) can help retrieve biologically important motifs and study functions of RNA molecules. Most of the current RNA secondary structure prediction tools are not suitable for RNA LSS prediction on the genome scale due to high computational complexity.

METHODS

We developed a new computer package Rnall based on a dynamic programming technique, which scans an RNA sequence with a sliding window and extracts all RNA LSSs with sizes no larger than the window size using the nearest neighbor thermodynamic parameters. The worst case running time of Rnall is O(W(3)L), where W is the window size and L is the query sequence length. In practice we observed a running time of O(W(2)L). We further introduced the concept of energy landscape for illustrating RNA LSS, which may facilitate RNA motif mining on the genomic scale.

RESULTS

Rnall shows better prediction accuracy than two other prediction tools Lfold and Quickfold. Rnall is also applied to scan for RNA LSSs in three genomes, and the prediction maps well with known RNA motifs.

CONCLUSIONS

Rnall is designed for RNA LSS prediction and together with the energy landscape, it has unique features that could be used for RNA structural motif mining. Rnall is freely available for download at http://digbio.missouri.edu/~wanx/Rnall or http://www.sysbio.muohio.edu/Rnall.

Neither the RNA nor the proteins of open reading frames 3a and 3b of the coronavirus infectious bronchitis virus are essential for replication

Gene 3 of infectious bronchitis virus is tricistronic; open reading frames (ORFs) 3a and 3b encode two small nonstructural (ns) proteins, 3a and 3b, of unknown function, and a third, structural protein E, is encoded by ORF 3c. To determine if either the 3a or the 3b protein is required for replication, we first modified their translation initiation codons to prevent translation of the 3a and 3b proteins from recombinant infectious bronchitis viruses (rIBVs). Replication in primary chick kidney (CK) cells and in chicken embryos was not affected. In chicken tracheal organ cultures (TOCs), the recombinant rIBVs reached titers similar to those of the wild-type virus, but in the case of viruses lacking the 3a protein, the titer declined reproducibly earlier. Translation of the IBV E protein is believed to be initiated by internal entry of ribosomes at a structure formed by the sequences corresponding to ORFs 3a and 3b. To assess the necessity of this mechanism, we deleted most of the sequence representing 3a and 3b to produce a gene in which ORF 3c (E) was adjacent to the gene 3 transcription-associated sequence. Western blot analysis revealed that the recombinant IBV produced fivefold less E protein. Nevertheless, titers produced in CK cells, embryos, and TOCs were similar to those of the wild-type virus, although they declined earlier in TOCs, probably due to the absence of the 3a protein. Thus, neither the tricistronic arrangement of gene 3, the internal initiation of translation of E protein, nor the 3a and 3b proteins are essential for replication per se, suggesting that these proteins are accessory proteins that may have roles in vivo.

Gene 5 of the avian coronavirus infectious bronchitis virus is not essential for replication

The avian coronavirus Infectious bronchitis virus (IBV), like other coronaviruses, expresses several small nonstructural (ns) proteins in addition to those from gene 1 (replicase) and the structural proteins. These coronavirus ns genes differ both in number and in amino acid similarity between the coronavirus groups but show some concordance within a group or subgroup. The functions and requirements of the small ns gene products remain to be elucidated. With the advent of reverse genetics for coronaviruses, the first steps in elucidating their role can be investigated. We have used our reverse genetics system for IBV (R. Casais, V. Thiel, S. G. Siddell, D. Cavanagh, and P. Britton, J. Virol. 75:12359-12369, 2001) to investigate the requirement of IBV gene 5 for replication in vivo, in ovo, and ex vivo. We produced a series of recombinant viruses, with an isogenic background, in which complete expression of gene 5 products was prevented by the inactivation of gene 5 following scrambling of the transcription-associated sequence, thereby preventing the expression of IBV subgenomic mRNA 5, or scrambling either separately or together of the translation initiation codons for the two gene 5 products. As all of the recombinant viruses replicated very similarly to the wild-type virus, Beau-R, we conclude that the IBV gene 5 products are not essential for IBV replication per se and that they are accessory proteins.