Unique nucleotide differences in the conserved 3' termini of brome mosaic virus RNAs are maintained through their optimization of genome replication

Brome mosaic virus Infection of Rice Results in Decreased Accumulation of RNA1

Brome mosaic virus (BMV) (the Russian strain) infects monocot plants and has been studied extensively in barley and wheat. Here, we report BMV can systemically infect rice (Oryza sativa var. japonica), including cultivars in which the genomes have been determined. The BMV capsid protein can be found throughout the inoculated plants. However, infection in rice exhibits delayed symptom expression or no symptoms when compared with wheat (Triticum aestivum). The sequences of BMV RNAs isolated from rice did not reveal any nucleotide changes in RNA1 or RNA2, while RNA3 had only one synonymous nucleotide change from the inoculum sequence. Preparations of purified BMV virions contained RNA1 at a significantly reduced level relative to the other two RNAs. Analysis of BMV RNA replication in rice revealed that minus-strand RNA1 was replicated at a reduced rate when compared with RNA2. Thus, rice appears to either inhibit RNA1 replication or lacks a sufficient amount of a factor needed to support efficient RNA1 replication.

The brome mosaic virus 3' untranslated sequence regulates RNA replication, recombination, and virion assembly

The 3' untranslated region in each of the three genomic RNAs of Brome mosaic virus (BMV) is highly homologous and contains a sequence that folds into a tRNA-like structure (TLS). Experiments performed over the past four decades revealed that the BMV 3' TLS regulates many important steps in BMV infection. This review summarizes in vitro and in vivo studies of the roles of the BMV 3' TLS functioning as a minus-strand promoter, in RNA recombination, and to nucleate virion assembly.

Emergence of distinct brome mosaic virus recombinants is determined by the polarity of the inoculum RNA

Despite overwhelming interest in the impact exerted by recombination during evolution of RNA viruses, the relative contribution of the polarity of inoculum templates remains poorly understood. Here, by agroinfiltrating Nicotiana benthamiana leaves, we show that brome mosaic virus (BMV) replicase is competent to initiate positive-strand [(+)-strand] synthesis on an ectopically expressed RNA3 negative strand [(-) strand] and faithfully complete the replication cycle. Consequently, we sought to examine the role of RNA polarity in BMV recombination by expressing a series of replication-defective mutants of BMV RNA3 in (+) or (-) polarity. Temporal analysis of progeny sequences revealed that the genetic makeup of the primary recombinant pool is determined by the polarity of the inoculum template. When the polarity of the inoculum template was (+), the recombinant pool that accumulated during early phases of replication was a mixture of nonhomologous recombinants. These are longer than the inoculum template length, and a nascent 3' untranslated region (UTR) of wild-type (WT) RNA1 or RNA2 was added to the input mutant RNA3 3' UTR due to end-to-end template switching by BMV replicase during (-)-strand synthesis. In contrast, when the polarity of the inoculum was (-), the progeny contained a pool of native-length homologous recombinants generated by template switching of BMV replicase with a nascent UTR from WT RNA1 or RNA2 during (+)-strand synthesis. Repair of a point mutation caused by polymerase error occurred only when the polarity of the inoculum template was (+). These results contribute to the explanation of the functional role of RNA polarity in recombination mediated by copy choice mechanisms.

Quantitative determination of cucumber mosaic virus genome RNAs in virions by real-time reverse transcription-polymerase chain reaction

A real-time RT-PCR procedure using the green fluorescent dye SYBR Green I was developed for determining the absolute and relative copies of cucumber mosaic virus (CMV) genomic RNAs contained in purified virions. Primers specific to each CMV ORF were designed and selected. Sequences were then amplified with length varying from 61 to 153 bp. Using dilution series of CMV genome RNAs prepared by in vitro transcription as the standard samples, a good linear correlation was observed between their threshold cycle (Ct) values and the logarithms of the initial template amounts. The copies of genomic RNA 1, RNA 2, RNA 3 and the subgenomic RNA 4 in CMV virions were quantified by this method, and the ratios were about 1.00:1.17:3.58:5.81. These results were confirmed by Lab-on-a-chip and northern blot hybridization assays. Our work is the first report concerning the relative amounts of different RNA fragments in CMV virions as a virus with tripartite genome.

Sensitivity of brome mosaic virus RNA1 replication to mutations in the 3' tRNA-like structure implies a requirement for sustained synthesis of replicase protein 1a

The replication competence of a series of brome mosaic virus (BMV) RNA1 variants with defined mutations in the 3' tRNA-like structure, previously characterized in vitro to be defective in minus-strand synthesis and several tRNA-associated functions, was analyzed in barley protoplasts. Inocula containing wild type RNAs2 and 3 and RNA1 bearing either Deltaknob or 5'Psk mutation failed to replicate. Two additional RNA1 variants, each bearing either M4 or 5'AGA mutation, resulted in detectable accumulation of progeny but are inhibitory to overall viral replication when supplied in high concentrations. Another aminoacylation-defective mutation Delta5' supported viral replication but did not interfere with viral replication even at higher concentrations. Coinoculation of replication-incompetent variants of RNAl with wt RNAs2 and 3 to Chenopodium hybridum plants resulted in the delayed development of local necrotic lesions characteristic of a wt infection. Sequence analysis of progeny RNA recovered from these lesions indicated that, in each case, a functional 3' noncoding sequence was restored due to homologous recombination with a corresponding sequence from wt RNA3. Taken together the results suggest that, unlike protein 2a which is required in catalytic amounts, the intrinsic involvement of protein 1a at various stages of virus infection cycle demands its sustained synthesis.

Replication-independent expression of genome components and capsid protein of brome mosaic virus in planta: a functional role for viral replicase in RNA packaging

To begin elucidation of the relationship between Brome mosaic virus (BMV) replication and encapsidation, we used a T-DNA-based Agrobacterium-mediated transient expression (agroinfiltration) system in Nicotiana benthamiana leaves to express either individual or desired pairs of the three genomic RNAs. The packaging competence of these RNAs into virions formed by the transiently expressed coat protein (CP) was analyzed. We found that in the absence of a functional replicase, assembled virions contained non-replicating viral RNAs (RNA1 or RNA2 or RNA3 or RNA1 + RNA3 or RNA2 + RNA3) as well as cellular RNAs. By contrast, virions assembled in the presence of a functional replicase contained only viral RNAs. To further elucidate the specificity exhibited by the functional viral replicase in RNA packaging, replication-defective RNA1 and RNA2 were constructed by deleting the 3' tRNA-like structure (3' TLS). Co-expression of TLS-less RNA1 and RNA2 with wt RNA3 resulted in efficient synthesis of subgenomic RNA4. Virions recovered from leaves co-expressing TLS-less RNA1 and RNA2 and either CP mRNA or wt RNA3 exclusively contained viral RNAs. These results demonstrated that packaging of BMV genomic RNAs is not replication dependent whereas expression of a functional viral replicase plays an active role in increasing specificity of RNA packaging.

Repair of the tRNA-like CCA sequence in a multipartite positive-strand RNA virus

The 3' portions of plus-strand brome mosaic virus (BMV) RNAs mimic cellular tRNAs. Nucleotide substitutions or deletions in the 3'CCA of the tRNA-like sequence (TLS) affect minus-strand initiation unless repaired. We observed that 2-nucleotide deletions involving the CCA 3' sequence in one or all BMV RNAs still allowed RNA accumulation in barley protoplasts at significant levels. Alterations of CCA to GGA in only BMV RNA3 also allowed RNA accumulation at wild-type levels. However, substitutions in all three BMV RNAs severely reduced RNA accumulation, demonstrating that substitutions have different repair requirements than do small deletions. Furthermore, wild-type BMV RNA1 was required for the repair and replication of RNAs with nucleotide substitutions. Results from sequencing of progeny viral RNA from mutant input RNAs demonstrated that RNA1 did not contribute its sequence to the mutant RNAs. Instead, the repaired ends were heterogeneous, with one-third having a restored CCA and others having sequences with the only commonality being the restoration of one cytidylate. The role of BMV RNA1 in increased repair was examined.

Cucumoviruses

Research on the molecular biology of cucumoviruses and their plant-virus interactions has been very extensive in the last decade. Cucumovirus genome structures have been analyzed, giving new insights into their genetic variability, evolution, and taxonomy. A new viral gene has been discovered, and its role in promoting virus infection has been delineated. The localization and various functions of each viral-encoded gene product have been established. The particle structures of Cucumber mosaic virus (CMV) and Tomato aspermy virus have been determined. Pathogenicity domains have been mapped, and barriers to virus infection have been localized. The movement pathways of the viruses in some hosts have been discerned, and viral mutants affecting the movement processes have been identified. Host responses to viral infection have been characterized, both temporally and spatially. Progress has been made in determining the mechanisms of replication, gene expression, and transmission of CMV. The pathogenicity determinants of various satellite RNAs have been characterized, and the importance of secondary structure in satellite RNA-mediated interactions has been recognized. Novel plant genes specifying resistance to infection by CMV have been identified. In some cases, these genes have been mapped, and one resistance gene to CMV has been isolated and characterized. Pathogen-derived resistance has been demonstrated against CMV using various segments of the CMV genome, and the mechanisms of some of these forms of resistances have been analyzed. Finally, the nature of synergistic interactions between CMV and other viruses has been characterized. This review highlights these various achievements in the context of the previous work on the biology of cucumoviruses and their interactions with plants.

tRNA elements mediate the assembly of an icosahedral RNA virus

tRNAs, the adapter molecules in protein synthesis, also serve as metabolic cofactors and as primers for viral RNA-directed DNA synthesis. The genomic and subgenomic RNAs of some plant viruses have a 3'-terminal tRNA-like structure (TLS) that can accept a specific amino acid and serve as a site for initiation of replication and as a simple telomere. We report a previously undescribed role for the TLS of brome mosaic virus (BMV), and potentially for cellular tRNA, in mediating the assembly of its icosahedral virions. BMV genomic RNAs and subgenomic RNA lacking the TLS failed to assemble into virions when incubated with purified BMV coat protein. Assembly was restored by addition of a 201-nt RNA containing the BMV TLS. TLSs from two other plant viruses as well as tRNAs from wheat germ and yeast were similarly active in the BMV virion assembly reaction, but ribosomal RNA and polyadenylate did not facilitate assembly. Surprisingly, virions assembled from TLS-less BMV RNA in the presence of tRNAs or TLS-containing short RNA did not incorporate the latter molecules. Consistent with a critical role for the BMV TLS in virion assembly, mutations in the BMV genomic RNAs that were designed to disrupt the folding of the TLS also abolished virion assembly. We discuss the likely roles of the TLS in early stages of virion assembly.

A subpopulation of RNA 1 of Cucumber mosaic virus contains 3' termini originating from RNAs 2 or 3

Tobacco plants transgenic for RNA 1 of Cucumber mosaic virus and inoculated with transcript of RNAs 2 and 3 regenerated viral RNA 1 from the transgenic mRNA, and the plants became systemically infected by the reconstituted virus. cDNA fragments corresponding to the 3' non-coding region (NCR) of viral RNA 1 were amplified, cloned and sequenced. In some clones the termini of the 3' NCR corresponded to those of viral RNAs 2 or 3. This suggested that in some cases RNA 1 may have been regenerated during replication by a template switching mechanism between the inoculated transcript RNAs and the mRNA. However, encapsidated, recombinant RNA 1 with the 3' NCR ends originating from RNAs 2 or 3 also was found in virus samples that had been passaged exclusively through non-transgenic plants. Thus, these chimeras occur naturally due to recombination between wild-type viral RNAs, and they are found encapsidated in low, but detectable amounts.

Virus recovery is induced in Brome mosaic virus p2 transgenic plants showing synchronous complementation and RNA-2-specific silencing

Nicotiana benthamiana plants expressing Brome mosaic virus (BMV) p2 protein complemented replication of RNAs1 + 3 but, surprisingly, supported little or no replication of RNA-2. Despite this, the p2 transgenic plants were able to support systemic migration of RNAs-1 and -3. Kinetic analyses showed identical degradation rates for RNAs-2 and -3, greatly detracting from the concept of an induction of an RNA-2-specific degradation system. Deletion analysis identified a 200-nucleotide sequence that may contribute to silencing in a context-specific manner. When R1 progeny of a severely silencing p2 transgenic line were tested for virus resistance, three different classes of reactions were observed. In class 1 and class 3 plants, the virus moved systemically and showed various extents of RNA-2 silencing. However, in class 2 plants, there was a stochastic onset of post-transcriptional silencing in the systemic leaves that was reminiscent of virus recovery. Plants showing recovery tended to have a greater number of transgene loci than did those exhibiting component-specific silencing. The induction of silencing did not appear to be dependent solely on the combined steady state levels of the transgene and viral RNA. Some plants transformed with a p2 frameshift construct showed a complete silencing phenotype, but none showed RNA-2-specific silencing. While the relationship between the two types of silencing remains unclear, we speculate that our observations reflect early events in the induction of virus recovery.

An engineered closterovirus RNA replicon and analysis of heterologous terminal sequences for replication

Citrus tristeza virus (CTV) populations in citrus trees are unusually complex mixtures of viral genotypes and defective RNAs developed during the long-term vegetative propagation of the virus and by additional mixing by aphid transmission. The viral replication process allows the maintenance of minor amounts of disparate genotypes and defective RNAs in these populations. CTV is a member of the Closteroviridae possessing a positive-stranded RNA genome of approximately 20 kilobases that expresses the replicase-associated genes as an approximately 400-kDa polyprotein and the remaining 10 3' genes through subgenomic mRNAs. A full-length cDNA clone of CTV was generated from which RNA transcripts capable of replication in protoplasts were derived. The large size of cDNA hampered its use as a genetic system. Deletion of 10 3' genes resulted in an efficient RNA replicon that was easy to manipulate. To investigate the origin and maintenance of the genotypes in CTV populations, we tested the CTV replicase for its acceptance of divergent sequences by creating chimeric replicons with heterologous termini and examining their ability to replicate. Exchange of the similar 3' termini resulted in efficient replication whereas substitution of the divergent (up to 58% difference in sequence) 5' termini resulted in reduced but significant replication, generally in proportion to the extent of sequence divergence.

Sequence of the genomic RNA of nudaurelia beta virus (Tetraviridae) defines a novel virus genome organization

The monopartite genome of Nudaurelia beta virus, the type species of the Betatetravirus genus of the family Tetraviridae, consists of a single-stranded positive-sense RNA (ss+RNA) of 6625 nucleotides containing two open reading frames (ORFs). The 5' proximal ORF of 5778 nucleotides encodes a protein of 215 kDa containing three functional domains characteristic of RNA-dependent RNA polymerases of ss+RNA viruses. The 3' proximal ORF of 1836 nucleotides, which encodes the 66-kDa capsid precursor protein, overlaps the replicase gene by more than 99% (1827 nucleotides) and is in the +1 reading frame relative to the replicase reading frame. This capsid precursor is expressed via a 2656-nucleotide subgenomic RNA. The 3' terminus of the genome can be folded into a tRNA-like secondary structure that has a valine anticodon; the tRNA-like structure lacks a pseudoknot in the aminoacyl stem, a feature common to both genera of tetraviruses. Comparison of the sequences of Nudaurelia beta virus and another member of the Tetraviridae, Helicoverpa armigera stunt virus, which is in the genus Omegatetravirus, shows identities of 31.6% for the replicase and 24.5% for the capsid protein. The viruses in the genera Betatetravirus and Omegatetravirus of the Tetraviridae are clearly related but show significant differences in their genome organization. It is concluded that the ancestral virus with a bipartite genome, as found in the genus Omegatetravirus, likely evolved from a virus with an unsegmented genome, as found in the genus Betatetravirus, through evolution of the subgenomic RNA into a separate genomic component, with the accompanying loss of the capsid gene from the longer genomic RNA.

Sequences 5' of the conserved tRNA-like promoter modulate the initiation of minus-strand synthesis by the brome mosaic virus RNA-dependent RNA polymerase

Each of the brome mosaic virus (BMV) genomic RNAs contains a conserved tRNA-like structure that is sufficient to direct minus-strand RNA synthesis in vitro. The tRNA-like promoters, tB1 and tB3, direct approximately equal amounts of synthesis in vitro. However, 5' sequences were found to affect the amount of minus-strand synthesis, suggesting that sequences beyond the tRNA-like structure are important in moderating minus-strand synthesis. Consistent with this, sequences upstream the tRNA-like structure are able to partially suppress mutations at or near the initiation site. This activity is observed in the 5' sequences of both BMV and CMV (cucumber mosaic virus) templates. However, a chimeric RNA containing the CMV tRNA-like promoter fused to the 5' sequences of BMV was not able to suppress mutations at the initiation site, suggesting that homologous 5' and 3' sequences are required to affect initiation. The ability to suppress mutations at the initiation site was correlated with a slight increase in the ability of the BMV RNA-dependent RNA polymerase to interact with the RNA.

Functional equivalence of common and unique sequences in the 3' untranslated regions of alfalfa mosaic virus RNAs 1, 2, and 3

The 3' untranslated regions (UTRs) of alfalfa mosaic virus (AMV) RNAs 1, 2, and 3 consist of a common 3'-terminal sequence of 145 nucleotides (nt) and upstream sequences of 18 to 34 nt that are unique for each RNA. The common sequence can be folded into five stem-loop structures, A to E, despite the occurrence of 22 nt differences between the three RNAs in this region. Exchange of the common sequences or full-length UTRs between the three genomic RNAs did not affect the replication of these RNAs in vivo, indicating that the UTRs are functionally equivalent. Mutations that disturbed base pairing in the stem of hairpin E reduced or abolished RNA replication, whereas compensating mutations restored RNA replication. In vitro, the 3' UTRs of the three RNAs were recognized with similar efficiencies by the AMV RNA-dependent RNA polymerase (RdRp). A deletion analysis of template RNAs indicated that a 3'-terminal sequence of 127 nt in each of the three AMV RNAs was not sufficient for recognition by the RdRp. Previously, it has been shown that this 127-nt sequence is sufficient for coat protein binding. Apparently, sequences required for recognition of AMV RNAs by the RdRp are longer than sequences required for CP binding.

Comparison of the replication of positive-stranded RNA viruses of plants and animals

It is clear from the experimental data that there are some similarities in RNA replication for all eukaryotic positive-stranded RNA viruses—that is, the mechanism of polymerization of the nucleotides is probably similar for all. It is noteworthy that all mechanisms appear to utilize host membranes as a site of replication. Membranes appear to function not only as a way of compartmentalizing virus RNA replication but also appear to have a central role in the organization and functioning of the replication complex, and further studies in this area are needed. Within virus supergroups, similarities are evident between animal and plant viruses—for example, in the nature and arrangements of replication genes and in sequence similarities of functional domains. However, it is also clear that there has been considerable divergence, even within supergroups. For example, the animal alpha-viruses have evolved to encode proteinases which play a central controlling function in the replication cycle, whereas this is not common in the plant alpha-like viruses and even when it occurs, as in the tymoviruses, the strategies that have evolved appear to be significantly different. Some of the divergence could be host-dependent and the increasing interest in the role of host proteins in replication should be fruitful in revealing how different systems have evolved. Finally, there are virus supergroups that appear to have no close relatives between animals and plants, such as the animal coronavirus-like supergroup and the plant carmo-like supergroup.

Identification of domains in brome mosaic virus RNA-1 and coat protein necessary for specific interaction and encapsidation

Even though many single-stranded RNAs are present in the cytoplasm of infected cells, encapsidation by brome mosaic virus (BMV) coat protein is specific for BMV RNA. Although the highly conserved 3' region of each of the three BMV genomic RNAs is an attractive candidate for the site of recognition by the coat protein, band shift and UV cross-linking assays in the presence of specific and nonspecific competitors revealed only nonspecific interactions. However, BMV RNA-1 formed a retarded complex (complex I) with the coat protein in the absence of competitors, and two domains of RNA-1 that specifically bound coat protein in a small complex (complex II), presumably early in the encapsidation process, were identified. Strong nonspecific, cooperative binding was observed in the presence of high concentrations of coat protein, suggesting that this provides the mechanism leading to rapid encapsidation seen in vivo. In contrast, no binding to a coat protein mutant lacking the N-terminal 25 amino acids that has been shown to be incapable of encapsidation in vivo (R. Sacher and P. Ahlquist, J. Virol. 63:4545-4552, 1989) was detected in vitro. The use of deletion mutants of RNA-1 revealed the presence of domains within the coding region of protein 1a that formed complexes with purified coat protein. One deletion mutant (B1SX) lacking these domains was only slightly more effective in dissociating RNA-1-coat protein complexes than were nonspecific competitors, further suggesting that regions other than the 3' end can participate in the selective encapsidation of BMV RNAs.

Contributions of the brome mosaic virus RNA-3 3'-nontranslated region to replication and translation

Sequences upstream of the 3'-terminal tRNA-like structure of brome mosaic virus RNAs have been predicted to fold into several stem-loop and pseudoknot structures. To elucidate the functional role of this upstream region, a series of deletions was made in cDNA clones of RNA-3, a genomic component not required for replication. These deletion mutants were transcribed in vitro and cotransfected with RNA-1 and RNA-2 into barley protoplasts. Deletion of single stem-loop structures gave progeny retaining near-wild-type accumulation levels. Constructions representing deletion of two or three stem-loops substantially lowered the accumulation of progeny RNA-3 relative to wild-type levels. RNA-3 mutants bearing deletions of longer sequences or of the entire region (delta PsKs RNA-3) replicated poorly, yielding no detectable RNA-3 or RNA-4 progeny. Levels of RNA-1 and RNA-2, in the presence of a mutant RNA-3, were found to increase relative to the accumulation observed in a complete wild-type transfection. The stability of delta PsKs RNA-3 in protoplasts was somewhat lower than that of wild-type RNA during the first 3 h postinoculation. Little difference in translatability in vitro of wild-type and RNA-3 constructs bearing deletions within the stem-loop region was observed, and Western immunoblot analysis of viral coat protein produced in transfected protoplasts showed that protein accumulation paralleled the amount of RNA-4 message produced from the various sequences evaluated. These results indicate that the RNA-3 pseudoknot region plays a minor role in translational control but contributes substantially to the overall replication of the brome mosaic virus genome.

Recombination and polymerase error facilitate restoration of infectivity in brome mosaic virus

The tRNA-like structure present in the 3' noncoding region of each of the four virion RNAs of brome mosaic virus possesses a conserved A-67-U-A-65 (67AUA65) sequence. Four mutations in this region (67UAA65, 67GAA65, and 67CAA65, each with a double base change, and 67GUA65, containing a single point mutation), previously shown in vitro to be defective in minus-strand promoter function, were introduced into full-length genomic RNAs 2 and 3, and their replicative competence was analyzed in barley protoplasts. All four RNA 3 mutants were capable of replication, although progeny plus-sense RNA 3 accumulation was only 12 to 42% of that of the wild type. Replication of RNA 2 transcripts bearing these mutations was even more severely debilitated; the accumulation of each mutant progeny plus-strand RNA 2 was < 10% of that of the wild type. Analysis of mutant RNA 3 progeny recovered from local lesions induced in Chenopodium hybridum and systemic infections in barley (Hordeum vulgare) plants revealed that the mutant base at position 67 from the 3' end had in each case been modified to an A. These changes generated RNAs with functional pseudorevertant (67AAA65 for mutants 67UAA65, 67GAA65, and 67CAA65) or revertant (67GUA65-->67AUA65) sequences. In most instances, the presence of internal markers permitted discrimination between polymerase error and RNA recombination as the process by which sequence restoration occurred. The pseudorevertant sequence was found to be capable of persistence during subsequent propagation in plants when present on RNA 3 but not when present on RNA 2. These data document the fluidity of the RNA genome and reveal situations in which polymerase error or recombination can function preferentially to restore an optimal sequence. They also support the concept that RNA viruses frequently exist as quasispecies and have implications concerning evolutionary strategies for positive-strand RNA viruses.

Replication of satellite RNA in vitro by homologous and heterologous cucumoviral RNA-dependent RNA polymerases

The RNA-dependent RNA polymerases (RdRp) of cucumber mosaic virus (CMV) and peanut stunt virus (PSV), members of the cucumovirus group, have been purified from virus infected plants and were used to study RNA synthesis in vitro using different viral RNAs, two cucumoviral satellites, and chimeric satellite cDNA clone transcripts as templates. The results show that solubilized RdRp preparations of CMV and PSV have a high degree of template dependency and catalyze (-) strand synthesis of the homologous cucumoviral RNAs with greater efficiency than the RNAs of heterologous cucumoviruses, although the PSV RdRp exhibits a lesser specificity than the CMV RdRp. On the other hand, both (-) and (+) strands of the satellite RNAs of CMV and PSV are synthesized by their homologous but not by the heterologous viral RdRps, indicating that recognition of satellites by the viral RdRp determines their replicative dependence upon specific helper viruses. Cucumoviral RdRp reactions using chimeric satellite transcripts suggest that the promoter structure for the satellite (-) strand synthesis resides in regions harboring the 3' termini of the two satellites.