Search

Benzothiadiazine inhibitors of the HCV NS5B RNA-dependent RNA polymerase are an important class of non-nucleoside inhibitors that have received considerable attention in the search for novel HCV therapeutics. Research in our laboratories has identified a novel series of tetracyclic benzothiadiazine inhibitors of HCV polymerase bearing a benzylamino substituent on the B-ring. Compounds in this series exhibit low-nanomolar activities in both genotypes 1a and 1b polymerase inhibition assays and subgenomic replicon assays. Optimization of pharmacokinetic properties in rat led to compound 30, which has good oral bioavailability (F = 56%) and a favorable tissue distribution drug profile, with high liver to plasma ratios. Compound 30 is a potent inhibitor in replicon assays, with EC(50) values of 10 and 6 nM against genotypes 1a and 1b, respectively.

The Hepatitis C (HCV) replicon system is a useful tool for the high-volume screening of inhibitors of HCV replication. In this report, a cell-based assay has been described, which monitors the inhibition of HCV genotypes 1a and 1b as well as cytotoxicity, from a single well of a 96-well plate. A mixture of two stable replicon cell lines was used: one containing a 1a-H77 replicon expressing a firefly luciferase reporter, and the other one containing a 1b-N replicon with a secreted alkaline phosphatase reporter, thus allowing us to monitor replication of two HCV genotypes in the same well. Cytotoxicity was measured using the Resazurin cytotoxicity assay. The assay was validated with known HCV inhibitors and showed that the antiviral activity and cytotoxicity of compounds were reproducibly measured under screening conditions. It was also showed that the assay's signal-to-noise ratio and Z' coefficient were suitable for high-throughput screening. A panel of HCV inhibitors showed a good correlation between EC(50) and TD(50) values for 1a and 1b replicon activity and cytotoxicity measured using either a single replicon format or mixed replicon format. Thus, the use of this mixed replicon format provides an economical method for simultaneous measurement of compound activity against two HCV genotypes as well as cytotoxicity, thereby reducing cost of reagents and labor as well as improving throughput.

A high-throughput and sensitive liquid chromatography/tandem mass spectrometry assay was established to detect total unlabeled hepatitis C virus inhibitor concentrations in replicon cells. The intracellular concentrations determined by this assay correlated well with concentrations obtained using radiolabeled compound. Some compounds accumulated inside the cells, with concentrations up to 300-fold higher than the input concentration. Confocal microscopic evaluation of two fluorescent-tagged inhibitors confirmed high accumulation inside the cells, sequestered inside vesicles within the cytoplasm. Incubation of cells with compound at 4 degrees C revealed that nonspecific binding to the outside of the cell membrane and to the cell culture plate occurred for some compounds. Therefore, the total concentration of compound extracted at 37 degrees C was reduced by the amount that was nonspecifically bound at 4 degrees C to yield the amount of compound inside the cells. A modification of the protocol was used for compounds with low intracellular concentrations in which cells were harvested with trypsin-EDTA prior to extraction. This eliminated the nonspecific binding to the cell culture plate and decreased the overall background of the assay. This assay was used to understand differences in cellular potency between compounds and the effects of serum proteins on the metabolic stability of compounds during incubation with cells.

With 170 million people infected worldwide and an inadequate current standard of care, hepatitis C virus (HCV) infection represents a major unmet medical need. Multiple companies are working on the discovery and development of specific HCV antiviral drugs, including inhibitors of HCV polymerase, protease and NS5A. Because of the error-prone nature of viral RNA replication, resistance mutants will develop that could present a potentially significant challenge to developing antiviral treatment regimens.Here, we review the major drug classes currently in preclinical and clinical development and the resistance mutations specific for each class that have been identified from cell culture and/or in vivo studies.We have analyzed currently available scientific literature to create a comprehensive review of the current state of the art in the field of HCV resistance to specific antiviral agents, in vitro and in vivo.Most specific HCV inhibitors described in the literature can select resistant viral variants in cell culture and in the clinic. Interplay of a mutant's fitness and its level of resistance will determine its clinical importance. Combinations of non-cross-resistant classes of drugs will be key to successful antiviral therapy. The number of drugs in a combination as well as the optimal duration of antiviral treatment, are important issues that need to be addressed in future studies.

Lopinavir (LPV)-ritonavir has demonstrated durable antiviral activity in human immunodeficiency virus type 1 (HIV-1)-infected antiretroviral-naïve and protease inhibitor (PI)-experienced patients. However, information on LPV activity against HIV-2 and the patterns of mutations in HIV-2 in response to selection by LPV is limited. The activity of LPV against three strains of HIV-2 was assessed and compared to activity against a reference HIV-1 strain. LPV demonstrated activity similar to that observed against HIV-1 in two HIV-2 strains (HIV-2MSand HIV-2CBL-23) tested. On the other hand, approximately 10-fold-reduced susceptibility was observed with the third HIV-2 strain, HIV-2CDC310319. Passage of HIV-2MSwith increasing concentrations of LPV selected mutations V47A and D17N in the HIV-2 protease gene. The introduction of both 17N and 47A either individually or together into HIV-2RODmolecular infectious clones showed that the single V47A substitution in HIV-2 resulted in a substantial reduction in susceptibility to LPV. In contrast, this mutant retained wild-type susceptibility to other PIs and appeared to be hypersusceptible to atazanavir and saquinavir.

Hepatitis C virus (HCV) replication is highly dependent on host cell factors. Identification of these host factors not only facilitates understanding of the biology of HCV infection but also enables the discovery of novel targets for anti-HCV therapy. To identify host genes important for HCV RNA replication, we screened a library of small interfering RNA (siRNA) that targets approximately 4,000 human genes in Huh7-derived EN5-3 cells harboring an HCV subgenomic replicon with the nonstructural region NS3-NS5B from the 1b-N strain. Nine cellular genes that potentially regulate HCV replication were identified in this screen. Silencing of these genes resulted in inhibition of HCV replication by more than 60% and exhibited minimal toxicity. Knockdown of host gene expression by these siRNAs was confirmed at the RNA level and, in some instances, at the protein level. The level of siRNA silencing of these host genes correlated well with inhibition of HCV. These genes included those that encoded a G-protein coupled receptor (TBXA2R), a membrane protein (LTβ), an adapter protein (TRAF2), 2 transcription factors (RelA and NFκB2), 2 protein kinases (MKK7 and SNARK), and 2 closely related transporter proteins (SLC12A4 and SLC12A5). Of interest, some of these genes are members of the tumor necrosis factor/lymphotoxin signaling pathway. Conclusion: Findings of this study may provide important information for understanding HCV replication. In addition, these cellular genes may constitute a novel set of targets for HCV antiviral therapy. (HEPATOLOGY 2007.)

Compounds A-782759 (an N -1-aza-4-hydroxyquinolone benzothiadiazine) and BILN-2061 are specific anti-hepatitis C virus (HCV) agents that inhibit the RNA-dependent RNA polymerase and the NS3 serine protease, respectively. Both compounds display potent activity against HCV replicons in tissue culture. In order to characterize the development of resistance to these anti-HCV agents, HCV subgenomic 1b-N replicon cells were cultured with A-782759 alone or in combination with BILN-2061 at concentrations 10 times above their corresponding 50% inhibitory concentrations in the presence of neomycin. Single substitutions in the NS5B polymerase gene (H95Q, N411S, M414L, M414T, or Y448H) resulted in substantial decreases in susceptibility to A-782759. Similarly, replicons containing mutations in the NS5B polymerase gene (M414L or M414T), together with single mutations in the NS3 protease gene (A156V or D168V), conferred high levels of resistance to both A-782759 and BILN-2061. However, the A-782759-resistant mutants remained susceptible to nucleoside and two other classes of nonnucleoside NS5B polymerase inhibitors, as well as interferon. In addition, we found that the frequency of replicons resistant to both compounds was significantly lower than the frequency of resistance to the single compound. Furthermore, the dually resistant mutants displayed significantly reduced replication capacities compared to the wild-type replicon. These findings provide strategic guidance for the future treatment of HCV infection.

Described herein is the development of a potent non-nucleoside, small molecule inhibitor of genotype 1 HCV NS5B Polymerase. A 23 μM inhibitor that was active against HCV polymerase was further elaborated into a potent single-digit nanomolar inhibitor of HCV NS5B polymerase by additional manipulation of the R and R1 substituents. Subsequent modifications to improve physical properties were made in an attempt to achieve an acceptable pharmacokinetic profile.

Interferon (IFN) in combination with ribavirin is the main treatment for hepatitis C virus (HCV) infection. The sensitivity or resistance of the virus to IFN has been linked to certain types of the interferon sensitivity determining region (ISDR) and PKR-eIF2alpha phosphorylation homology domain (PePHD) sequences in the NS5A and E2 regions of the viral genome, respectively. In search of the other potential mechanisms of HCV resistance to IFN, we tested the effect of IFN-alpha on translational activity of the HCV IRES in various cell types. Using bicistronic dual luciferase reporter RNAs in direct RNA transfection studies, we found that the cap-dependent translation was dramatically inhibited by IFN (5- to 16-fold), whereas HCV IRES translation was inhibited only marginally in two hepatoma cell lines, Huh7 and HepG2 cells. No difference in IFN sensitivity was observed among IRESs of genotypes 1a, 1b, and 2a. Translation under the control of encephalomyocarditis virus (EMCV) IRES was inhibited by IFN to the same extent as cap-dependent translation. In cells of nonhepatic origin (HeLa and Raji), however, HCV IRES-, EMCV IRES-, and cap-dependent translation were dramatically inhibited to similar levels. The PKR expression level was enhanced by IFN in all cells, but eIF2alpha phosphorylation level was not changed, probably due to the absence of double-stranded RNA species. There was also no evidence of RNase L activation. Therefore, inhibition of translation by IFN under these conditions was probably mediated by novel IFN-induced inhibitory pathways, independent of eIF2alpha phosphorylation, while HCV IRES was not subject to this inhibition in hepatoma cell lines. Thus, HCV IRES-driven translation was resistant to IFN-induced, eIF2alpha-independent inhibition in human hepatoma cells that are frequently used in studies on HCV replication. This may present a new potential mechanism of viral resistance to IFN treatment during the early steps of virus infection.

f b w t ( t g s p t n p Many RNA viruses encode more than one gene on a single genomic RNA. Yet only the first gene, or open reading frame (ORF), on a normal eukaryotic mRNA is translated. Thus, downstream genes on viral genomes are expressed either via novel translational events or, more commonly, by deployment of subgenomic mRNAs (sgRNAs). Subgenomic RNAs of positive-strand viruses have the same 39 ends as genomic RNA, but have deleions at the 59 ends to bring the 59 end of the RNA in proximity with the start codon of downstream (on genomic RNA) ORFs. Because replication is required for sgRNA synthesis, the RNA-dependent RNA polymerase (RdRp) is always translated first, directly from genomic RNA of positive-strand RNA viruses. sgRNAs express products needed during intermediate and late stages of infection, such as structural or movement proteins. Taxa of (1) sense viruses that produce sgRNAs inlude animal viruses in the order Nidovirales (Coronairidae and Arteriviridae families) and the families Togairidae, Caliciviridae, Nodaviridae, and Astroviridae, plant iruses of the Luteoviridae, Bromoviridae, Tombusviridae, nd Closteroviridae families, and the Tobravirus, Carlairus, Tymovirus, Potexvirus, Hordeivirus, Tobamovirus, obemovirus, and Furovirus genera. Subgenomic RNA ynthesis has been studied more in plant viruses than in nimal viruses. This is probably because a greater perentage of all plant viruses make sgRNAs and also ecause plant viruses, with smaller genomes and highly fficient replication, are often more amenable than anial viruses to studies of RNA replication mechanisms, specially in cell-free extracts. The similarities between

Numerous RNA viruses generate subgenomic mRNAs (sgRNAs) for expression of their 3*-proximal genes. A major step in control of viral gene expression is the regulation of sgRNA synthesis by specific promoter elements. We used barley yellow dwarf virus (BYDV) as a model system to study transcriptional control in a virus with multiple sgRNAs. BYDV generates three sgRNAs during infection. The sgRNA1 promoter has been mapped previously to a 98-nucleotide (nt) region which forms two stem-loop structures. It was determined that sgRNA1 is not required for BYDV RNA replication in oat protoplasts. In this study, we show that neither sgRNA2 nor sgRNA3 is required for BYDV RNA replication. The promoters for sgRNA2 and sgRNA3 synthesis were mapped by using deletion mutagenesis. The minimal sgRNA2 promoter is approximately 143 nt long (nt 4810 to 4952) and is located immediately downstream of the putative sgRNA2 start site (nt 4809). The minimal sgRNA3 core promoter is 44 nt long (nt 5345 to 5388), with most of the sequence located downstream of sgRNA3 start site (nt 5348). For both promoters, additional sequences upstream of the start site enhanced sgRNA promoter activity. These promoters contrast to the sgRNA1 promoter, in which almost all of the promoter is located upstream of the transcription initiation site. Comparison of RNA sequences and computerpredicted secondary structures revealed little or no homology between the three sgRNA promoter elements. Thus, a small RNA virus with multiple sgRNAs can have very different subgenomic promoters, which implies a complex system for promoter recognition and regulation of subgenomic RNA synthesis. Synthesis of subgenomic mRNAs (sgRNAs) is a common strategy used by positive-sense RNA viruses for expression of their 39-proximal genes. In combination with other strategies such as unconventional translational events and posttranslational proteolytic processing of precursor polyproteins, it allows efficient utilization of the viral genetic material. Synthesized later in infection, sgRNAs encode late viral genes whose products are required for pathogenesis and particle formation. Alphaviruses such as Sindbis virus and the alpha-like multipartite Bromoviridae produce one sgRNA for expression of the coat protein. Plant viruses that belong to such groups as potexvirus, tombusvirus, carmovirus, and tobamovirus, as well as some other alpha-like viruses, produce two or three sgRNAs for expression of the coat protein and movement proteins. RNA viruses with larger genomes, such as Closteroviridae (also alpha-like) and the Nidovirales, produce up to nine and seven sgRNAs, respectively (14, 23).

A series of gem -dialkyl naphthalenone derivatives with varied alkyl substitutions were synthesized and evaluated according to their structure–activity relationship. This investigation led to the discovery of potent inhibitors of the hepatitis C virus at low nanomolar concentrations in both enzymatic and cell-based HCV genotype 1a assays.

There is a need for interferon-free treatment regimens for hepatitis C virus (HCV) infection. The goal of this study was to evaluate ABT-450, a potent HCV NS3 protease inhibitor, combined with low-dose ritonavir (ABT-450/r), in addition to ABT-333, a nonnucleoside NS5B polymerase inhibitor, and ribavirin, for the treatment of HCV infection.We conducted a 12-week, phase 2a, open-label study involving patients who had HCV genotype 1 infection without cirrhosis. All patients received ABT-333 (400 mg twice daily) and ribavirin (1000 to 1200 mg per day) and one of two daily doses of ABT-450/r. Groups 1 and 2 included previously untreated patients; group 1 received 250 mg of ABT-450 and 100 mg of ritonavir, and group 2 received 150 mg and 100 mg, respectively. Group 3, which included patients who had had a null or partial response to previous therapy with peginterferon and ribavirin, received daily doses of 150 mg of ABT-450 and 100 mg of ritonavir. The primary end point was an undetectable level of HCV RNA from week 4 through week 12 (extended rapid virologic response).A total of 17 of the 19 patients in group 1 (89%) and 11 of the 14 in group 2 (79%) had an extended rapid virologic response; a sustained virologic response 12 weeks after the end of treatment was achieved in 95% and 93% of the patients, respectively. In group 3, 10 of 17 patients (59%) had an extended rapid virologic response, and 8 (47%) had a sustained virologic response 12 weeks after therapy; 6 patients had virologic breakthrough, and 3 had a relapse. Adverse events included abnormalities in liver-function tests, fatigue, nausea, headache, dizziness, insomnia, pruritus, rash, and vomiting.This preliminary study suggests that 12 weeks of therapy with a combination of a protease inhibitor, a nonnucleoside polymerase inhibitor, and ribavirin may be effective for treatment of HCV genotype 1 infection. (Funded by Abbott; ClinicalTrials.gov number, NCT01306617.).

A series of quinoline derivatives was synthesized as potential bioisosteric replacements for the benzothiadiazine moiety of earlier Hepatitis C NS5B polymerase inhibitors. Several of these compounds exhibited potent activity in enzymatic and replicon assays.

The hepatitis C virus (HCV) NS5B polymerase is essential for viral replication and has been a prime target for drug discovery research. Our efforts directed toward the discovery of HCV polymerase inhibitors resulted in the identification of unsymmetrical dialkyl-hydroxynaphthalenoyl-benzothiadiazines 2 and 3. The most active compound displayed activity in genotypes 1a and 1b polymerase and replicon cell culture inhibition assays at subnanomolar and low nanomolar concentrations, respectively. It also displayed an excellent pharmacokinetic profile in rats, with a plasma elimination half-life after intravenous dosing of 4.5 h, oral bioavailability of 77%, and a peak liver concentration of 21.8 microg/mL.

In our program to discover non-nucleoside, small molecule inhibitors of genotype 1 HCV polymerase, we investigated a series of promising analogs based on a benzothiadiazine screening hit that contains an ABCD ring system. After demonstrating that a methylsulfonylamino D-ring substituent increased the enzyme potency into the low nanomolar range, we explored a minimum core required for activity by truncating to a three-ring system. Described herein are the syntheses and structure–activity relationship of a set of inhibitors lacking the A-ring of an ABCD ring system. We observed that small aromatic rings and alkenyl groups appended to the 5-position of the B-ring were optimal, resulting in inhibitors with low nanomolar potencies.

Compound A-837093, a non-nucleoside HCV RNA-dependent RNA polymerase inhibitor, displayed nanomolar potencies against HCV genotypes 1a and 1b replicons. It also exhibited an excellent metabolic profile and achieved high plasma and liver concentrations in animals. In order to characterize the development of resistance to this anti-HCV agent, HCV subgenomic 1b strain N replicon cells were cultured in the presence of A-837093 with G418. Mutations S368A, Y448H, G554D, Y555C, and D559G in the NS5B polymerase gene were identified that led to substantial decreases in the susceptibilities of 1b genotype replicons to the inhibitor A-837093. However, the resistant mutants remained susceptible to HCV protease inhibitor BILN-2061 and alpha interferon as well as to a different class of non-nucleoside HCV polymerase inhibitor. In addition, each single resistant mutation identified significantly reduced the replication capacity of mutant compared to wild-type replicon. These findings provide a strategic guide for the future development of non-nucleoside inhibitors of HCV NS5B polymerase.

Hepatitis C virus (HCV) replicon-based shuttle vectors that permit phenotypes of NS5B polymerase genes from a large number of patient isolates to be rapidly assessed when transiently expressed in cultured cells were designed. When used to test responses to an inhibitor of HCV RNA-dependent RNA polymerase, IC50 values for inhibition covered a several hundred-fold range among 47 patient samples tested. This observation highlights the variability that can be found by testing isolates derived from HCV-infected subjects. Partial suppression with a polymerase inhibitor of the most sensitive species permitted detection of minor quasispecies that were 7–200-fold more resistant than the bulk population in approximately half of the samples. Sequence analysis showed a wide range of amino acid changes not detected by conventional selection methods using laboratory-derived strains. This approach provides a means to assess variation in antiviral efficacy, and to predict possible responses in a clinical setting.

The combinations of Abbott Hepatitis C virus (HCV) polymerase A-782759 with either Boehringer Ingelheim HCV NS3 protease inhibitor BILN-2061 or interferon (IFN) displayed additive to synergistic relationships over a range of concentrations of two-drug combination. Treatment of HCV replicon with A-782759, IFN or BILN-2061 for about 16 days resulted in dramatic reductions in HCV RNA (5.1, 3.0 and 3.9 log10 RNA copies, respectively). However, none of the compounds tested alone lead to replicon RNA reduction to undetectable levels. Ongoing replication in the presence of A-782759 or BILN-2061 was associated with the appearance of resistant mutations M414T in NS5B and D168V in NS3, respectively. In contrast, a combination of A-782759 with BILN-2061 resulted in greater than 7 logs RNA reduction leading to undetectable replicon RNA after 16 days of treatment. Our findings suggest that a monotherapy with either drug alone is likely to result in development of resistant mutants. However, a combination therapy with polymerase inhibitor has the potential to improve the efficacy of IFN or a protease inhibitor alone in vivo, due to the lower likelihood of resistance development.

We determined the 3prime prime or minute-terminal primary and secondary structures required for replication of Barley yellow dwarf virus (BYDV) RNA in oat protoplasts. Computer predictions, nuclease probing, phylogenetic comparisons, and replication assays of specific mutants and chimeras revealed that the 3prime prime or minute-terminal 109 nucleotides (nt) form a structure with three to four stem-loops followed by a coaxially stacked helix incorporating the last four nt [(A/U)CCC]. Sequences upstream of the 109-nt region also contributed to RNA accumulation. The base-pairing but not the sequences or bulges in the stems were essential for replication, but any changes to the 3prime prime or minute-terminal helix destroyed replication. The two 3prime prime or minute-proximal tetraloops tolerated all changes, but the two 3prime prime or minute-distal tetraloops gave most efficient replication if they fit the GNRA consensus. A mutant lacking the 3prime prime or minute-proximal stem-loop produced elevated levels of less-than-full-length minus strands, and no (+) strand. We propose that a "pocket" structure is the origin of (minus sign)-strand synthesis, which is negatively regulated by the inaccessible conformation of the 3prime prime or minute terminus, thus favoring a high (+)/(minus sign) ratio. This 3prime prime or minute structure and the polymerase homologies suggest that genus Luteovirus is more closely related to the Tombusviridae family than to other Luteoviridae genera.

Barley yellow dwarf luteovirus (BYDV) generates three 3′-coterminal subgenomic RNAs (sgRNAs) in infected cells. The promoter of sgRNA1 is a putative hot spot for RNA recombination in luteovirus evolution. The sgRNA1 transcription start site was mapped previously to either nucleotide 2670 or nucleotide 2769 of BYDV genomic RNA (gRNA) in two independent studies. Our data support the former initiation site. The boundaries of the sgRNA1 promoter map between nucleotides 2595 and 2692 on genomic RNA. Computer prediction, phylogenetic comparison, and structural probing revealed two stem-loops (SL1 and SL2) in the sgRNA1 promoter region on the negative strand. Promoter function was analyzed by inoculating protoplasts with a full-length infectious clone of the BYDV genome containing mutations in the sgRNA promoter. Because the promoter is located in an essential coding region of the replicase gene, we duplicated it in a nonessential part of the genome from which a new sgRNA was expressed. Mutational analysis revealed that secondary structure, but not the nucleotide sequence, was important at the base of SL1. Regions with both RNA primary and secondary structural features that contributed to transcription initiation were found at the top of SL1. Primary sequence, but not the secondary structure, was required in SL2, which includes the initiation site. Disruption of base pairing near the sgRNA1 start site increased the level of transcription three- to fourfold. We propose that both primary and secondary structures of the sgRNA1 promoter of BYDV play unique roles in sgRNA1 promoter recognition and transcription initiation.

The above model could not be supported without an in vitro assay. Although Simon's work is a promising early step in the dissection of recombination mechanisms, many questions remain. For example, what features on the donor template encourage dissociation of the replicase and nascent strand? Possibilities proposed for a variety of RNAs include a stable secondary structure, the 5′ end of the template itself or AU-rich regions that weaken the interaction between the nascent strand and the template. In coronaviruses, dissociation has been attributed to the nonprocessive nature of the replicase[2xLai, M.M.C. Semin. Virol. 1996; 7: 381–388Crossref | Scopus (45)See all References[2]. In BMV, the replication complex is initially highly abortive but then undergoes a modification allowing efficient elongation[13xSun, J-H. and Kao, C.C. Virology. 1997; 236: 348–353Crossref | PubMed | Scopus (31)See all References[13]. Perhaps, at some points of the RNA synthesis, the replication complex can revert to its loosely bound abortive state and subsequently dissociate.What is the nature of the protein that interacts with the bulged stem–loop? Presumably, it is a component of the viral replicase, but is it the virally encoded RNA-dependent RNA polymerase itself or a host factor that participates? How does acceptor recognition by the replication machinery compare with the specific recognition of the origins of replication at the 3′ ends of positive and negative strands and internally at subgenomic RNA promoters?More broadly speaking, how widely does this type of mechanism apply? Recognition of an internal structure on a negative strand, as described here, resembles subgenomic RNA synthesis. Subgenomic RNA synthesis may be primer dependent in coronaviruses[2xLai, M.M.C. Semin. Virol. 1996; 7: 381–388Crossref | Scopus (45)See all References[2]but is primer independent in plant viruses[14xMiller, W.A., Dreher, T.W., and Hall, T.C. Nature. 1985; 313: 68–70Crossref | PubMed | Scopus (181)See all References[14]. However, we propose that plant viral subgenomic RNA promoters can also be recognized by this donor-primed mechanism in the recombination events that generate variation in luteoviruses[5xMiller, W.A., Dinesh-Kumar, S.P., and Paul, C.P. Crit. Rev. Plant Sci. 1995; 14: 179–211Crossref | Scopus (118)See all References[5]. This may occur in other viruses that produce subgenomic mRNAs, including many plant viruses, coronaviruses and alphaviruses.In contrast to the above viruses, recombination has been detected in poliovirus at hot spots for which no replication origin or subgenomic promoter would normally be predicted[8xKirkegaard, K. and Baltimore, D. Cell. 1986; 47: 433–443Abstract | Full Text PDF | PubMed | Scopus (268)See all References, 15xDuggal, R. et al. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 13786–13791Crossref | PubMed | Scopus (37)See all References, 16xTang, R.S. et al. RNA. 1997; 3: 624–633PubMedSee all References]. What are the features of the RNA and replicase that promote picornaviral recombination at specific sites? The recent demonstration of poliovirus recombination in cell-free systems[15xDuggal, R. et al. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 13786–13791Crossref | PubMed | Scopus (37)See all References, 16xTang, R.S. et al. RNA. 1997; 3: 624–633PubMedSee all References]bodes well for elucidation of the mechanism in the near future. Now that recombination has been demonstrated in two such very different in vitro systems, TCV and poliovirus, and numerous other cell-free replication systems are available, the next few years should yield a bounty of detailed analyses of a variety of different RNA recombination mechanisms.

We determined the 3′-terminal primary and secondary structures required for replication of Barley yellow dwarf virus (BYDV) RNA in oat protoplasts. Computer predictions, nuclease probing, phylogenetic comparisons, and replication assays of specific mutants and chimeras revealed that the 3′-terminal 109 nucleotides (nt) form a structure with three to four stem-loops followed by a coaxially stacked helix incorporating the last four nt [(A/U)CCC]. Sequences upstream of the 109-nt region also contributed to RNA accumulation. The base-pairing but not the sequences or bulges in the stems were essential for replication, but any changes to the 3′-terminal helix destroyed replication. The two 3′-proximal tetraloops tolerated all changes, but the two 3′-distal tetraloops gave most efficient replication if they fit the GNRA consensus. A mutant lacking the 3′-proximal stem-loop produced elevated levels of less-than-full-length minus strands, and no (+) strand. We propose that a “pocket” structure is the origin of (−)-strand synthesis, which is negatively regulated by the inaccessible conformation of the 3′ terminus, thus favoring a high (+)/(−) ratio. This 3′ structure and the polymerase homologies suggest that genus Luteovirus is more closely related to the Tombusviridae family than to other Luteoviridae genera.