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Structural and functional analysis of mitochondrial plasmids in Claviceps purpurea
A Düvell1, H Hessberg-Stutzke, B Oeser
1Institut für Mikrobiologie, Universität Düsseldorf, Federal Republic of Germany.
Abstract:
Several strains of Claviceps purpurea, a phytopathogenic Ascomycete, contain mitochondrial (mt) plasmids in high molar excess relative to mtDNA. Comparative analysis of plasmids of four strains of different geographic origin revealed that all the plasmids are structurally related (size; linearity; restriction map; probably 5'-linked terminal protein; terminal inverted repeats, TIRs); two of them are even identical, indicating a possible mobility of these genetic entities. In strain K it was shown that plasmid titres are comparably high in axenic cultures and in parasitic structures (sclerotia). Detailed analysis of plasmid pClK1 proved the existence of a perfect TIR of 327 bp; the plasmid's structure and details of its nucleotide sequence indicate a replication modus comparable to that of adenoviruses. pClK1 is almost completely transcribed resulting in two major transcripts of 3.5 and 3.15 kb, respectively. In plasmid-free strains (cured by ethidium bromide treatment) these mRNAs are not detectable; nevertheless they show no significant difference in phenotype. As judged from their structural properties they could be derived from viral ancestors. In this context the plasmids' close relationship to mt plasmids of higher plants may be of special interest.
Insights
Claviceps purpurea mitochondrial plasmids are structurally related and potentially mobile genetic elements. Their replication and transcription suggest viral origins, with no significant phenotypic impact observed in plasmid-free strains.
Area of Science:
- Mycology
- Molecular Biology
- Genetics
Background:
- Claviceps purpurea, a phytopathogenic Ascomycete, harbors mitochondrial (mt) plasmids.
- These plasmids exist in high molar excess compared to mitochondrial DNA (mtDNA).
Purpose of the Study:
- To investigate the structural relationships and potential mobility of C. purpurea mt plasmids.
- To analyze the replication and transcription of a specific plasmid (pClK1).
- To assess the phenotypic impact of plasmid presence or absence.
Main Methods:
- Comparative analysis of plasmids from four geographically distinct strains.
- Restriction mapping and nucleotide sequencing of plasmid pClK1.
- Analysis of plasmid transcripts via Northern blotting.
- Phenotypic comparison of plasmid-containing and plasmid-free strains.
Main Results:
- All analyzed plasmids were structurally related, featuring terminal inverted repeats (TIRs), suggesting possible mobility.
- Plasmid pClK1 exhibits a perfect 327 bp TIR and its sequence indicates an adenovirus-like replication mechanism.
- pClK1 is highly transcribed into two major mRNAs (3.5 and 3.15 kb), which are absent in plasmid-free strains.
- Curing strains of plasmids did not result in significant phenotypic differences.
Conclusions:
- C. purpurea mt plasmids are structurally conserved and may be mobile genetic elements.
- The replication and transcription patterns of pClK1 suggest viral ancestry.
- The functional significance of these plasmids remains unclear as their absence does not alter the host phenotype.