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Origin and direction of replication in mitochondrial plasmid DNAs of broad bean, Vicia faba
J A Wahleithner1, D R Wolstenholme
1Department of Biology, University of Utah, Salt Lake City 84112.
Abstract:
Broad bean (Vicia faba) mitochondrial DNA (mtDNA) includes three circular plasmids: mt-plasmid 1 (1,704 ntp), mt-plasmid 2 (1,695 ntp) and mt-plasmid 3 (1,476 ntp). Partially replicated circular forms of these mt-plasmids have been observed in electron microscope preparations. Restriction enzymes that cleave either mt-plasmid 2 (but not mt-plasmids 1 and 3) or mt-plasmid 3 (but not mt-plasmids 1 and 2) were used to generate linear forms of partially replicated mt-plasmid 2 and mt-plasmid 3 molecules. Analyses of these linearized replicative intermediates, observed by electron microscopy, indicated that in both mt-plasmid 2 and mt-plasmid 3 replication originates at a specific location and proceeds in the same, single direction around the molecules. The replication origins of mt-plasmid 2 and mt-plasmid 3 map close to sequences that can fold into hairpin structures.
Insights
Broad bean mitochondrial DNA contains three circular plasmids. Replication in two of these plasmids initiates at a specific site and proceeds unidirectionally, near hairpin-forming sequences.
Area of Science:
- Plant molecular biology
- Mitochondrial genetics
- Molecular replication mechanisms
Background:
- Broad bean (Vicia faba) possesses mitochondrial DNA (mtDNA) featuring three distinct circular plasmids.
- Partially replicated forms of these plasmids have been previously observed.
Purpose of the Study:
- To investigate the replication mechanism of broad bean mitochondrial plasmids.
- To determine the origin and directionality of replication for mt-plasmid 2 and mt-plasmid 3.
Main Methods:
- Utilized restriction enzymes to linearize partially replicated mt-plasmid 2 and mt-plasmid 3.
- Employed electron microscopy to analyze the structure of linearized replicative intermediates.
Main Results:
- Replication in both mt-plasmid 2 and mt-plasmid 3 initiates at a specific origin.
- Replication proceeds in a single, defined direction around each plasmid molecule.
- Replication origins are located near sequences capable of forming hairpin structures.
Conclusions:
- Broad bean mitochondrial plasmids exhibit unidirectional replication from a specific origin.
- Hairpin-forming sequences may play a role in initiating plasmid replication.