Subgenomic Defective Interfering RNA Research Articles

Dengue virus-free defective interfering particles have potent and broad anti-dengue virus activity

Dengue virus (DENV) is spread from human to human through the bite of the female Aedes aegypti mosquito and leads to about 100 million clinical infections yearly. Treatment options and vaccine availability for DENV are limited. Defective interfering particles (DIPs) are considered a promising antiviral approach but infectious virus contamination has limited their development. Here, a DENV-derived DIP production cell line was developed that continuously produced DENV-free DIPs. The DIPs contained and could deliver to cells a DENV serotype 2 subgenomic defective-interfering RNA, which was originally discovered in DENV infected patients. The DIPs released into cell culture supernatant were purified and could potently inhibit replication of all DENV serotypes in cells. Antiviral therapeutics are limited for many viral infection. The DIP system described could be re-purposed to make antiviral DIPs for many other RNA viruses such as SARS-CoV-2, yellow fever, West Nile and Zika viruses.

Coronavirus transcription early in infection.

We studied the accumulation kinetics of murine coronavirus mouse hepatitis virus (MHV) RNAs early in infection by using cloned MHV defective interfering (DI) RNA that contained an intergenic sequence from which subgenomic DI RNA is synthesized in MHV-infected cells. Genomic DI RNA and subgenomic DI RNA accumulated at a constant ratio from 3 to 11 h postinfection (p.i.) in the cells infected with MHV-containing DI particles. Earlier, at 1 h p.i., this ratio was not constant; only genomic DI RNA accumulated, indicating that MHV RNA replication, but not MHV RNA transcription, was active during the first hour of MHV infection. Negative-strand genomic DI RNA and negative-strand subgenomic DI RNA were first detectable at 1 and 3 h p.i., respectively, and the amounts of both RNAs increased gradually until 6 h p.i. These data showed that at 2 h p.i., subgenomic DI RNA was undergoing synthesis in the cells in which negative-strand subgenomic DI RNA was undetectable. These data, therefore, signify that negative-strand genomic DI RNA, but not negative-strand subgenomic DI RNA, was an active template for subgenomic DI RNA synthesis early in infection.

Importance of coronavirus negative-strand genomic RNA synthesis prior to subgenomic RNA transcription

The (−)-strand viral RNAs that result from after infection of cells with coronaviruses, which possess RNA genomes of message polarity, are genomic-sized and subgenomic-sized. Each of the (−)-strand subgenomic RNAs corresponds in size to each of the subgenomic mRNA species that are made in infected cells. We tested whether (−)-strand subgenomic RNAs might initially be synthesized from the input single-stranded (+)-strand genomic RNA prior to the production of subgenomic mRNAs. We used a mouse hepatitis virus (MHV) defective interfering (DI) RNA, from which subgenomic RNA was produced in DI RNA-replicating cells, because this DI RNA had a functional MHV intergenic region inserted in its interior. MHV samples containing the DI particles were irradiated with UV-light and then superinfected into cells that had been infected with MHV 4 h prior to superinfection. Northern blot analysis of intracellular RNAs that were extracted 3 h after superinfection showed that genomic DI RNA and subgenomic DI RNA had similar UV-target sizes, indicating that (−)-strand genomic DI RNA synthesis from input genomic DI RNA probably occurred prior to the subgenomic-size DI RNA synthesis. We discuss why, in the course of coronavirus transcription, (−)-strand genomic-length coronavirus RNA synthesis might occur before subgenomic-sized RNAs of either polarity are made.

Characterizations of Coronaviruscis-Acting RNA Elements and the Transcription Step Affecting Its Transcription Efficiency

Seven to eight species of viral subgenomic mRNAs are produced in coronavirus-infected cells. These mRNAs are produced in different quantities, and their molar ratios remain constant during viral replication. We studied RNA elements that affect coronavirus transcription efficiency by characterizing a series of cloned coronavirus mouse hepatitis virus (MHV) defective interfering (DI) RNAs containing an inserted intergenic sequence, from which subgenomic DI RNA is transcribed in MHV-infected cells. Certain combinations of upstream and downstream flanking sequences of the intergenic sequence suppressed subgenomic DI RNA transcription, yet changing one of the flanking sequences to a different sequence eliminated transcription suppression. The suppressive effect of certain combinations of flanking sequences, but not all combinations, could be counteracted by altering the intergenic sequence. Thus, the combination of intergenic sequence and flanking sequence affected transcription efficiency. We also characterized another set of DI RNAs designed to clarify which transcription step determines the relative molar ratios of coronavirus mRNAs. Our study indicated that if subgenomic mRNAs were exclusively synthesized from negative-strand genomic RNA, then the relative molar ratios of coronavirus mRNAs were most likely determined after synthesis of the genomic-sized template RNA. If negative-strand subgenomic RNAs were templates for subgenomic mRNAs, then the relative molar ratios of coronavirus mRNAs probably were determined after synthesis of the genomic-sized template RNA used for subgenomic-sized RNA transcription but prior to the completion of the synthesis of subgenomic-sized RNAs containing the leader sequence. The relative molar ratios of coronavirus mRNAs, therefore, seem to have been established prior to a putative replicon-type amplification of subgenomic mRNAs.

Coronavirus Transcription Mediated by Sequences Flanking the Transcription Consensus Sequence

In our studies of murine coronavirus transcription, we continue to use defective interfering (DI) RNAs of mouse hepatitis virus (MHV) in which we insert a transcription consensus sequence in order to mimic subgenomic RNA synthesis from the nondefective genome. Using our subgenomic DI system, we have studied the effects of sequences flanking the MHV transcription consensus sequence on subgenomic RNA transcription. We obtained the following results. (i) Insertion of a 12-nucleotide-long sequence including the UCUAAAC transcription consensus sequence at different locations of the DI RNA resulted in different efficiencies of subgenomic DI RNA synthesis. (ii) Differences in the amount of subgenomic DI RNA were defined by the sequences that flanked the 12-nucleotide-long sequence and were not affected by the location of the 12-nucleotide-long sequence on the DI RNA. (iii) Naturally occurring flanking sequences of intergenic sequences at gene 6–7, but not at genes 1–2 and 2–3, contained a transcription suppressive element(s). (iv) Each of three naturally occurring flanking sequences of an MHV genomic cryptic transcription consensus sequence from MHV gene 1 also contained a transcription suppressive element(s). These data showed that sequences flanking the transcription consensus sequence affected MHV transcription.

The effect of two closely inserted transcription consensus sequences on coronavirus transcription.

Insertion of an intergenic region from the murine coronavirus mouse hepatitis virus into a mouse hepatitis virus defective interfering (DI) RNA led to transcription of subgenomic DI RNA in helper virus-infected cells. Using this system, we studied how two intergenic regions in close proximity affected subgenomic RNA synthesis. When two intergenic regions were separated by more than 100 nucleotides, slightly less of the larger subgenomic DI RNA (synthesized from the upstream intergenic region) was made; this difference was significant when the intergenic region separation was less than about 35 nucleotides. Deletion of sequences flanking the two intergenic regions inserted in close proximity did not affect transcription. No significant change in the ratio of the two subgenomic DI RNAs was observed when the sequence between the two intergenic regions was altered. Removal of the downstream intergenic region restored transcription of the larger subgenomic DI RNA. The UCUAAAC consensus sequence was needed for efficient suppression of the larger subgenomic DI RNA synthesis. These results demonstrated that the downstream intergenic sequence was suppressing subgenomic DI RNA synthesis from the upstream intergenic region. We discuss possible mechanisms to account for the regulation of this suppression of subgenomic DI RNA synthesis and the ways in which they relate to the general regulation of coronavirus transcription.

Analysis of coronavirus transcription regulation.

Insertion of an intergenic region from murine coronavirus mouse hepatitis virus (MHV) into an MHV defective interfering (DI) RNA led to transcription of subgenomic DI RNA in helper virus-infected cells. Using this system we studied how two intergenic regions positioned in close proximity affected subgenomic RNA synthesis. When two intergenic regions were separated by more than 100 nt, slightly less of the larger subgenomic DI RNA (synthesized from the upstream intergenic region) was made; this difference was significant when the intergenic region separation was less than about 35 nucleotides. Deletion of sequences flanking the two intergenic regions inserted in close proximity did not affect transcription. No significant change in the ratio of the two subgenomic DI RNAs was observed when the sequence between the two intergenic regions was altered. Removal of the downstream intergenic region restored transcription of the larger subgenomic DI RNA. These results demonstrated the downstream intergenic sequence was suppressing subgenomic DI RNA synthesis from the upstream intergenic region.

Subgenomic RNA synthesis directed by a synthetic defective interfering RNA of mouse hepatitis virus: a study of coronavirus transcription initiation.

We have used a full-length cDNA clone of a mouse hepatitis virus strain A59 defective interfering (DI) RNA, pMIDI-C, and cassette mutagenesis to study the mechanism of coronavirus subgenomic mRNA synthesis. Promoter sequences closely resembling those of subgenomic mRNAs 3 and 7 were inserted into MIDI-C. Both subgenomic RNA promoters gave rise to the synthesis of a subgenomic DI RNA in virus-infected and DI RNA-transfected cells. From a mutagenic analysis of the promoters we concluded the following. (i) The extent of base pairing between the leader RNA and the intergenic promoter sequence does not control subgenomic RNA abundance. (ii) Promoter recognition does not rely on base pairing only. Presumably, transcription initiation requires recognition of the promoter sequence by the transcriptase. (iii) Fusion of leader and body sequences takes place at multiple--possibly random--sites within the intergenic promoter sequence. A model is presented in which, prior to elongation, the leader RNA is trimmed by a processive 3'-->5' nuclease.

Evidence for coronavirus discontinuous transcription.

Coronavirus subgenomic mRNA possesses a 5'-end leader sequence which is derived from the 5' end of genomic RNA and is linked to the mRNA body sequence. This study examined whether coronavirus transcription involves a discontinuous transcription step; the possibility that a leader sequence from mouse hepatitis virus (MHV) genomic RNA could be used for MHV subgenomic defective interfering (DI) RNA transcription was examined. This was tested by using helper viruses and DI RNAs that were easily distinguishable. MHV JHM variant JHM(2), which synthesizes a subgenomic mRNA encoding the HE gene, and variant JHM(3-9), which does not synthesize this mRNA, were used. An MHV DI RNA, DI(J3-9), was constructed to contain a JHM(3-9)-derived leader sequence and an inserted intergenic region derived from the region preceding the MHV JHM HE gene. DI(J3-9) replicated efficiently in JHM(2)- or JHM(3-9)-infected cells, whereas synthesis of subgenomic DI RNAs was observed only in JHM(2)-infected cells. Sequence analyses demonstrated that the 5' regions of both helper virus genomic RNAs and genomic DI RNAs maintained their original sequences in DI RNA-replicating cells, indicating that the genomic leader sequences derived from JHM(2) functioned for subgenomic DI RNA transcription. Replication and transcription of DI(J3-9) were observed in cells infected with an MHV A59 strain whose leader sequence was similar to that of JHM(2), except for one nucleotide substitution within the leader sequence. The 5' region of the helper virus genomic RNA and that of the DI RNA were the same as their original structures in virus-infected cells, and the leader sequence of DI(J3-9) subgenomic DI RNA contained the MHV A59-derived leader sequence. The leader sequence of subgenomic DI RNA was derived from that of helper virus; therefore, the genomic leader sequence had a trans-acting property indicative of a discontinuous step in coronavirus transcription.

Effect of intergenic consensus sequence flanking sequences on coronavirus transcription.

Insertion of a region, including the 18-nucleotide-long intergenic sequence between genes 6 and 7 of mouse hepatitis virus (MHV) genomic RNA, into an MHV defective interfering (DI) RNA leads to transcription of subgenomic DI RNA in helper virus-infected cells (S. Makino, M. Joo, and J. K. Makino, J. Virol. 66:6031-6041, 1991). In this study, the subgenomic DI RNA system was used to determine how sequences flanking the intergenic region affect MHV RNA transcription and to identify the minimum intergenic sequence required for MHV transcription. DI cDNAs containing the intergenic region between genes 6 and 7, but with different lengths of upstream or downstream flanking sequences, were constructed. All DI cDNAs had an 18-nucleotide-long intergenic region that was identical to the 3' region of the genomic leader sequence, which contains two UCUAA repeat sequences. These constructs included 0 to 1,440 nucleotides of upstream flanking sequence and 0 to 1,671 nucleotides of downstream flanking sequence. An analysis of intracellular genomic DI RNA and subgenomic DI RNA species revealed that there were no significant differences in the ratios of subgenomic to genomic DI RNA for any of the DI RNA constructs. DI cDNAs which lacked the intergenic region flanking sequences and contained a series of deletions within the 18-nucleotide-long intergenic sequence were constructed to determine the minimum sequence necessary for subgenomic DI RNA transcription. Small amounts of subgenomic DI RNA were synthesized from genomic DI RNAs with the intergenic consensus sequences UCUAAAC and GCUAAAC, whereas no subgenomic DI RNA transcription was observed from DI RNAs containing UCUAAAG and GCTAAAG sequences. These analyses demonstrated that the sequences flanking the intergenic sequence between genes 6 and 7 did not play a role in subgenomic DI RNA transcription regulation and that the UCUAAAC consensus sequence was sufficient for subgenomic DI RNA transcription.

Mutagenic analysis of the coronavirus intergenic consensus sequence.

Previously, a system in which an intergenic region from mouse hepatitis virus (MHV) inserted into an MHV defective interfering (DI) RNA led to transcription of a subgenomic DI RNA in helper virus-infected cells was established. In the present study, a DI cDNA containing one UCUAAAC consensus sequence in the middle of the 0.3-kb-long intergenic region located between genes 6 and 7 was constructed. From this DI cDNA clone, 21 mutant DI RNAs were constructed so that each of the seven consensus sequence nucleotides was changed individually to the three alternative bases. These mutants were used to define how changes in the integrity of MHV transcription consensus sequence UCUAAAC affected mRNA transcription. Except for two mutants with the sequences UGUAAAC and UCGAAAC, all of the mutants supported efficient subgenomic DI RNA transcription. This indicated that MHV transcription regulation was sufficiently flexible to recognize altered consensus sequences. Next, these and other mutants were used to examine the leader-body fusion site on the subgenomic DI RNAs. Sequence analysis demonstrated that all subgenomic DI RNAs analyzed contained two pentanucleotide sequences; the first sequence seemed to be contributed by the leader, and the leader-body fusion most likely took place at either the first or the second nucleotide of the second sequence. This observation was not consistent with the proposed coronavirus transcription model (S. C. Baker and M. M. C. Lai, EMBO J. 9:4173-4179, 1990) which states that nucleotide mismatch can be corrected by RNA polymerase proofreading activity.

Mechanism of coronavirus transcription: duration of primary transcription initiation activity and effects of subgenomic RNA transcription on RNA replication.

Previously, we established a system whereby an intergenic region from mouse hepatitis virus (MHV) inserted into an MHV defective interfering (DI) RNA led to transcription of a subgenomic DI RNA in helper virus-infected cells. By using this system, the duration of a primary transcription initiation activity which transcribes subgenomic-size RNAs from the genomic-size RNA template in MHV-infected cells was examined. Efficient DI genomic and subgenomic RNA synthesis was observed when the DI RNA was transfected at 1, 3, 3.5, 5, and 6 h postinfection, indicating that all activities which are necessary for MHV RNA synthesis are present continuously during the first 6 h of infection. The effect of subgenomic DI RNA synthesis on DI genomic RNA replication was then examined. Replication efficiency of the DI genomic RNA which synthesized the subgenomic RNA was approximately 70% lower than that of DI genomic RNA which did not synthesize the subgenomic DI RNA in MHV-infected cells. Cotransfection of two different-size DI RNAs demonstrated that replication of the larger DI RNA was strongly inhibited by replication of the smaller genomic DI RNA. Cotransfection of two DI RNA species of the same length into MHV-infected cells demonstrated that reduced replication of the genomic DI RNA which synthesizes the subgenomic RNA did not affect the replication of cotransfected DI RNA, demonstrating that the reduction in DI genomic RNA replication works only in cis, not in trans. Therefore, the previously proposed hypothesis that coronavirus, subgenomic RNA synthesis may inhibit the replication of genomic RNA by competing for a limited amount of virus-derived factors seems unlikely. Possible mechanisms of coronavirus transcription are discussed.