Changes in populations of cauliflower mosaic virus DNA and RNA forms during turnip callus proliferation

Covey, Simon N.; Turner, David

doi:10.1099/0022-1317-74-9-1887

Cited by 13 publications

(3 citation statements)

References 19 publications

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“…These gaps are normally repaired in a presumably sequential manner in the nucleus to produce supercoiled DNA (24), which is the substrate for transcription by host RNA polymerase II. Delayed repair of these gaps could conceivably extend the lifetime of open circular DNA and increase opportunities for recombination between free ends of TPV DNA at the gaps and plant sequences.…”

Section: Discussionmentioning

confidence: 99%

Integrated pararetroviral sequences define a unique class of dispersed repetitive DNA in plants

Jakowitsch

Mette

Winden

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

117

107

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Although integration of viral DNA into host chromosomes occurs regularly in bacteria and animals, there are few reported cases in plants, and these involve insertion at only one or a few sites. Here, we report that pararetrovirus-like sequences have integrated repeatedly into tobacco chromosomes, attaining a copy number of Ϸ10 3 . Insertion apparently occurred by illegitimate recombination. From the sequences of 22 independent insertions recovered from a healthy plant, an 8-kilobase genome encoding a previously uncharacterized pararetrovirus that does not contain an integrase function could be assembled. Preferred boundaries of the viral inserts may correspond to recombinogenic gaps in open circular viral DNA. An unusual feature of the integrated viral sequences is a variable tandem repeat cluster, which might reflect defective genomes that preferentially recombine into plant DNA. The recurrent invasion of pararetroviral DNA into tobacco chromosomes demonstrates that viral sequences can contribute significantly to plant genome evolution. M ost plant viruses have single-stranded RNA genomes. Only two classes of plant viruses, caulimoviruses and badnaviruses, contain genomes of double-stranded DNA (1). Because these double-stranded DNA viruses use a virally encoded reverse transcriptase to replicate their genomes, they, together with vertebrate hepadnaviruses, are classified as pararetroviruses to distinguish them from true retroviruses, which have RNA genomes. Retroviruses have not yet been conclusively identified in plants, although recent findings of retrotransposons that encode envelope-like proteins suggest that they might exist (2-4). Pararetrovirus replication in plants proceeds by nuclear transcription of a slightly greater than genome-length RNA with terminal repeats that is generated by the host RNA polymerase II. This is followed by reverse transcription in the cytoplasm of the terminally redundant RNA, which also serves as an mRNA for viral proteins (5). Although retrovirus DNA integrates into host chromosomes by means of a virally encoded integrase (6), pararetroviruses generally lack the gene for this enzyme, and integration is not required for virus replication. However, pararetroviral DNA can in principle integrate into host DNA, as exemplified by mammalian hepatitis B (hepadna)virus, which has been found integrated into host chromosomes in hepatic tissue, where it is associated with liver carcinomas (7). Until recently, there were no data suggesting comparable integration of pararetroviral sequences into plant DNA.In contrast to bacterial and animal viruses, plant viral sequences are generally thought to integrate rarely, if at all, into host genomes. One well characterized example concerns a single insertion of sequences related to a geminivirus, which has a single-stranded circular DNA genome, into tobacco nuclear DNA (8-10). Although the geminivirus case has been considered exceptional, several recent reports prompt a reconsideration of the possibility that plant pararetrovirus DNA might integr...

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Section: Discussionmentioning

confidence: 99%

Integrated pararetroviral sequences define a unique class of dispersed repetitive DNA in plants

Jakowitsch

Mette

Winden

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

117

107

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“…Dividing cells seem more attuned to initiation of silencing in the presence of CaMV This conclusion comes from our experiments with cultured leaf discs taken from CaMV-infected Brassica rapa leaves, and induced to form callus (Rollo and Covey, 1985;Covey and Turner, 1993). Antisilencing responses have not specifically been identified during CaMV infection.…”

Section: Gene Silencing and Counter-silencingmentioning

confidence: 94%

Plant Gene Silencing

Matzke¹,

Matzke²

2000

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“…Although it has become a widely held view that the 35S promoter is constitutively expressed because of its behavior in transgenic plants (33), the suggestion that it might be under differential regulation by the host during virus infection comes from our observations of several situations in which CaMV progresses to a postreplicative phase (9,36). For example, induction of callus from CaMV-infected leaves results in loss of viral transcripts and accumulation of apparently inactive minichromosome DNA (9,11,28) through release of an end product regulatory mechanism which, we have suggested, also involves the gene VI protein (10). Although an effect on RNA stability per se was not excluded (35), such observations suggest that regulation of the CaMV minichromosome through its promoters might not be the same as when the isolated elements are used to express reporter genes integrated as transgenes in non-host plants.…”

mentioning

confidence: 97%

Roles of the 35S promoter and multiple overlapping domains in the pathogenicity of the pararetrovirus cauliflower mosaic virus

Turner

McCallum

Covey

1996

J Virol

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Elements associated with the 35S promoter involved in generating the pregenomic RNA (35S RNA) of the pararetrovirus cauliflower mosaic virus have been extensively studied in heterologous systems, but little is known about their role in viral pathogenicity. To investigate these elements, premature termination codons were progressively inserted into the 3 end of the adjacent gene VI to dissect it from colinear 35S enhancer sequences. The ability to cause a systemic infection in plants was retained with loss of up to 40 amino acids from the gene VI polypeptide, but truncations into a putative zinc finger proved lethal. In the 35S promoter, removal of the TATA box also abolished infectivity. However, upstream deletions encompassing the 35S enhancer showed that the sequence between ؊207 and ؊56 from the cap site comprised nonessential elements, although complete removal of this fragment caused loss of infectivity even when domain spacing was restored by linker insertion. Two separate enhancer domains (؊207 to ؊150 and ؊95 to ؊56) were identified, of which either one or the other, but not both, was required for infectivity. Some mutations affected the cellular levels of viral RNAs in unexpected ways, as with removal of the as-1 enhancer element causing an increase in 35S RNA. Others altered the relative abundance of nuclear and cytoplasmic viral DNAs. Mutations in promoter domains thought to be involved in regulating tissue-specific expression did not significantly affect virus accumulation in leaves versus roots, whereas gene VI mutants showed reduced root accumulation. We conclude that elements associated with the cauliflower mosaic virus 35S promoter contain extensive nonessential regions that can behave differently in their proper context than as isolated elements.

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Changes in populations of cauliflower mosaic virus DNA and RNA forms during turnip callus proliferation

Cited by 13 publications

References 19 publications

Integrated pararetroviral sequences define a unique class of dispersed repetitive DNA in plants

Integrated pararetroviral sequences define a unique class of dispersed repetitive DNA in plants

Plant Gene Silencing

Roles of the 35S promoter and multiple overlapping domains in the pathogenicity of the pararetrovirus cauliflower mosaic virus

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